中国西南天山西域砾岩的磁性地层年代与地质意义  

MAGNETOCHRONOLOGY AND ITS IMPLICATIONS OF THE XIYU CONGLOMERATE IN THE SOUTHWESTERN CHINESE TIAN SHAN FORELAND

作　　者：陈杰[1] R.V.Heermance D.W.Burbank K.M.Scharer 王昌盛[1] 

机构地区：[1]中国地震局地质研究所地震动力学国家重点实验室 [2]Department of Earth Science,University of California,Santa Barbara,CA 93106,USA [3]Department of Geology,Appalachian State University,Boone,NC 28608,USA

出　　处：《第四纪研究》2007年第4期576-587,共12页Quaternary Sciences

基　　金：国家自然科学基金项目(批准号:40672109和40372081);美国国家自然科学基金项目(项目号:EAR-0230403)联合资助

摘　　要：西域组是我国西部一重要并广泛引用的晚新生代地层,关于其年代和成因至今尚存争议。在西南天山喀什远源盆地喀什-阿图什褶皱带不同构造部位选择有代表性的5～6个晚新生代地层剖面开展了详细的沉积学、磁性地层年代学对比研究,据此限定了不同构造的起始变形时间以及西域砾岩的沉积年代。西域砾岩并非一年代地层单位,作为一岩石地层单位,其底界具有穿时特征,从山体(北)向喀什前陆盆地(南)逐渐变新。其底界年龄在盆地北部近源区约为15·5Ma[1],在盆地中部中源区约为8·6Ma[1],在盆地南部远源区的阿图什背斜为1·9Ma,喀什背斜为1·6～0·7Ma。这一穿时的砾岩沉积楔体的起始堆积起因于盆地北部边界逆冲断层(KBT)的活动。构造变形是由北南脉冲式迁移扩展的,其速率是非均匀的,在约15·5Ma至4·0Ma期间为1·4～3·4mm/a,在约4·0Ma以来剧增至>10mm/a。西域砾岩沉积前缘向南进积速率与构造变形前缘迁移速率有很好的一致性,但在时间上较构造变形可能滞后2·0Ma。这表明构造变形前缘向南的脉冲式扩展是西域砾岩进积并发生侧向和垂向上岩相突变的主因。Magnetostratigraphy of the Kashi foreland basin along the southern margin of the Tian Shan in western China defines the chronology of both sedimentation and the structural evolution of this collisional mountain belt. Five magnetostratigraphic sections representing ca. 8.5km of distal basin strata provide a 2- and 3-dimensional record of continuous deposition since ca. 5Ma. The distinctive Xiyu conglomerate makes up the uppermost strata in 4 of 5 magnetostratigraphic sections within the foreland and forms a wedge that thins southward. Age of the basal Xiyu conglomerate varies from 15.5Ma at the northernmost part of the foreland, to 8.6Ma in the central （medial） part of the foreland , and to 1.9-0.1^0.05 Ma, 1.6 ± 0. 1 Ma, and 0.73 ± 0.05Ma along the southern deformation front of the foreland basin. These data indicate that the Xiyu conglomerate is highly timetransgressive and has prograded southward immediately after the initial uplift of the Kashi Basin Thrust （KBT）. This finding contrasts with previous studies which assumed that the Xiyu conglomerate is either Plio-Pleistocene in age or represents a chronostratigraphic unit. The southward gravel progradation occurred at an average rate of 2.5 ～ 3.5mm/a between 15.5Ma and 1.9Ma, before accelerating to ca. 10 ～ 11mm/a. Results indicate a stepped migration of deformation into the Kashi foreland at least at 16.3Ma, 13.5Ma,4. 0Ma, and 1.4Ma. Progressive reconstruction of a seismically controlled cross section through the foreland produces total shortening of 13～21km and migration of the deformation front at 2. 1～3.4mm/a between 16.3 ～ 13.5Ma, 1.4 ～ 1.6mm/a between 13.5 ～4. 0Ma, and 〉 10mm/a since 4.0Ma. Since 5Ma, increased sediment accumulation （ca. 0. 7mm/a） and gravel progradation rates （ca. 10mm/a） appear linked to higher deformation rates on the Keketamu, Atushi, and Kashi anticlines and increased subsidence due to loading from both the Tian Shan and Pamir ranges. The accelerated erosion was possibly caused by climate change

关 键 词：天山 前陆盆地 西域砾岩 磁性地层 砾岩进积 构造 气候 

分 类 号：P539.3[天文地球—古生物学与地层学] P546[天文地球—地质学]