雅鲁藏布江蛇绿岩的形成与日喀则弧前盆地沉积演化

雅鲁藏布江蛇绿岩被时代连续的日喀则群沉积覆盖及其形成时代(120-110Ma)与冈底斯弧开始发育的时代(115-100Ma)十分相近的事实使人们有理由提出:雅鲁藏布江蛇绿岩是否代表着印度板块与拉萨地块间的特提斯-喜玛拉雅洋残迹的疑问。根据近期的研究,笔者认为雅鲁藏布江蛇绿岩不是形成于三叠纪的特提斯-喜玛拉雅洋的残迹,而是特提斯-喜玛拉雅洋向拉萨地块俯冲的初期(阿普第-阿尔必期),由俯冲作用在冈底斯弧前地区引发的海底扩张作用形成的一种俯冲带上叠型蛇绿岩(supra-subduction zone ophiolites).至森诺曼期,弧前海底扩张作用停止,雅鲁藏布江蛇绿岩开始向南仰冲,在其南侧形成增生杂岩楔。仰起的蛇绿岩开始向日喀则弧前盆地提供蛇绿质碎屑,如冲堆组。森诺曼期-土仑期,盆地接受了一套深水复理石沉积,沉积物源部分来自南部边缘脊的蛇绿质碎屑,而大部分则来自北侧的弧火山岩和岩浆岩碎屑。森诺期-路坦丁期,盆地逐渐变浅,接受了浅海-滨海沉积,物源均来自北部的岩浆弧。至始新世末期,发育在盆地南侧的增生杂岩楔与印度板块发生碰撞,日喀则弧前盆地闭合。     

雅鲁藏布江周缘前陆盆地物源分析及构造演化

本文通过雅鲁藏布江缝合带南侧江孜和岗巴地区晚白垩世-古近纪沉积地层的碎屑岩岩石学、地球化学和铬尖晶石电子探针分析,揭示了碰撞前后沉积盆地的物源区变化,提供了盆地和造山带早期的演化历史.江孜地区上白垩统宗卓组属于弧-陆或陆-陆碰撞背景下的海沟沉积.日朗砾岩中的岩屑质长石砂岩地球化学特征反映有大洋岛弧物质的注入,物源区为大洋岛弧或增生楔.上古新统-下始新统甲查拉组长石质岩屑砂岩反映了冈底斯岛弧和再循环造山带物源区特征,是陆-陆碰撞背景下形成的周缘前陆盆地的前渊沉积.岗巴地区古新统基堵拉组石英砂岩表现为印度大陆内部物源区特征,而始新统遮普惹组岩屑砂岩为再循环造山带和冈底斯岛弧物源区.沉积特征和物源区综合研究表明,雅鲁藏布江周缘前陆盆地在古新世期间开始发育,它指示了印度与欧亚板块的初始碰撞时间.     

西藏札达地区上白垩统-下始新统达机翁组:对冈底斯弧前盆地演化的制约

冈底斯弧前区域地层沉积记录,对新特提斯洋消亡和印度-亚洲碰撞过程的研究具有十分重要的意义。位于西藏南部札达地区的达机翁组,北邻冈底斯岩浆弧,南靠雅鲁藏布江缝合带。岩石组成主要包括砾岩、岩屑砂岩、泥页岩和灰岩等。沉积环境分析认为达机翁组形成于扇三角洲相环境。火山灰锆石U-Pb定年、碎屑锆石最年轻年龄以及底栖有孔虫化石组合共同约束达机翁组的形成时代为晚白垩世-始新世早期(即ca.73～49Ma)。物源区分析结果表明达机翁组物源类似于区域上分布的日喀则弧前盆地沉积,直接以北侧冈底斯岩浆弧为主要物质源区。通过与区域弧前沉积对比,为冈底斯弧前盆地海相地层时代提供制约,结果显示新特提斯洋在亚洲大陆南缘的弧前海退存在东西方向上的穿时性,即海水自东向西逐渐退出,并最终在～49Ma退出冈底斯-拉达克弧前区域。     

雅鲁藏布江缝合带西段仲巴地区俯冲—增生过程

雅鲁藏布江缝合带是新特提斯洋俯冲消亡的残余,记录了新特提斯洋打开—闭合的全过程。本文以雅鲁藏布江缝合带西段仲巴地区南侧的纳久混杂岩为研究对象,进行了详细的放射虫年代学,砂岩碎屑锆石U-Pb同位素年代学以及碎屑组分统计研究。我们的数据表明,纳久混杂岩中硅质岩含有大量保存较好的放射虫化石,包含Pseudodictyomitra carpatica带典型分子,根据放射虫时代组合确定其时代为早Barremian阶;混杂岩中砂岩岩块主要为岩屑砂岩,不同样品碎屑锆石得出的最大沉积年龄介于95～73 Ma之间。碎屑锆石U-Pb年龄源区分析表明,碎屑物质来自北侧的冈底斯岩浆弧和拉萨地体。纳久混杂岩南侧的砂岩沉积时代为早白垩世,碎屑锆石U-Pb年龄源区表明具有典型的特提斯喜马拉雅特征。我们的数据表明,纳久混杂岩基质时代为早白垩世,砂岩岩块时代为晚白垩世,与北侧的早白垩世蛇绿岩共同组成了白垩纪的增生楔,随着印度与欧亚大陆的碰撞仰冲到特提斯喜马拉雅之上。     

冈底斯-喜马拉雅碰撞造山带前陆盆地系统及构造演化

碰撞带前陆盆地的建立是大陆碰撞的直接标志和随后造山带构造变形的忠实记录。本文对欧亚板块与印度板块碰撞前后发育在拉萨地块上的冈底斯弧背前陆盆地,同碰撞产生的雅鲁藏布江周缘前陆盆地,以及碰撞后陆内变形产生的喜马拉雅前陆盆地的沉积地层演化以及碎屑锆石物源特征等进行了系统分析,结合前人及我们近些年的研究成果,认为冈底斯岛弧北侧发育一个典型的弧背前陆盆地系统而不是以前普遍接受的伸展盆地。除传统认为的喜马拉雅前陆盆地系统外,在碰撞造山带中还发育一个雅鲁藏布江前陆盆地系统,它是欧亚板块与印度板块碰撞以后,欧亚板块加载到印度被动大陆边缘产生的典型周缘前陆盆地。上述2个造山带前陆盆地系统的识别,大大提高了对新特提斯洋俯冲、碰撞过程的认识。造山带前陆盆地证据指示,新特提斯洋至少于140 Ma以前就已开始俯冲, 110 Ma俯冲速度开始提高,在65 Ma前后印度大陆与欧亚大陆发生碰撞,喜马拉雅山于40 Ma开始隆升,其剥蚀物质大量堆积在喜马拉雅前陆盆地中。     

藏南古近纪甲查拉组物源分析及其对印度-欧亚大陆碰撞启动时间的约束

位于特提斯喜马拉雅北亚带的江孜地区古近纪甲查拉组角度不整合于晚白垩世宗卓组之上,系该地区最高(时代最晚)海相地层。运用岩石学和地球化学方法对其进行分析研究结果表明该组物源区主要为近源再旋回造山带,岩屑的母岩类型主要是岩浆弧成因的中性、中酸性安山质火山岩。新生代以前,特提斯喜马拉雅属于印度板块的被动大陆边缘,从特提斯喜马拉雅南亚带向北亚带显示了一种从浅水陆棚到深水盆地的变化,在侏罗-白垩纪时其陆源碎屑物主要是成熟度极高的石英砂岩,所以甲查拉组的碎屑物质只能来源于当时的冈底斯弧地区,所获有限的古水流证据也指示了这一点。从欧亚大陆侵蚀下来的碎屑物质被带到原印度大陆地区沉积,暗示该区的特提斯洋壳已经完全消失,印度与欧亚大陆在特提斯喜马拉雅中、东部产生了初始的陆-陆碰撞,其碰撞的启动时间为甲查拉组开始沉积的65 M a±。     

特提斯喜马拉雅带江孜地区古近纪地层源区分析——对造山带早期地壳加厚的制约

特提斯喜马拉雅北亚带江孜地区上古新统-下始新统甲查拉组记录了喜马拉雅碰撞造山带的早期地壳加厚和沉积历史。本文我们报道了甲查拉组详细的碎屑锆石U-Pb年龄和全岩Sm-Nd同位素数据。甲查拉组由青灰色厚层的岩屑砂岩夹泥岩组成,不整合覆盖在宗卓组之上,碎屑锆石主要的峰值介于350～80 Ma, 900～470 Ma以及1 300～950 Ma,次要的峰值介于2 800～1 500 Ma。全岩⁸⁷Sr/⁸⁶Sr介于0.707 505～0.713 174,¹⁴³Nd/¹⁴⁴Nd介于0.512 206～0.512 355,ε_Nd(0)介于-5.52～-8.43。甲查拉组物源区以再循环的日喀则弧前盆地和上三叠统郎杰学群为主,少量物质来自雅鲁藏布江缝合带。上述研究表明,甲查拉组沉积在周缘前陆盆地的背景下,且特提斯喜马拉雅北亚带在始新世期间经历了明显的地壳加厚。     

宁芜盆地中三叠统黄马青组是大别造山带最早剥露的记录

由于后造山期的构造运动改造和巨量剥蚀,大别造山带的初始剥露时间,即开始为周缘盆地提供物源的时间缺乏精确限定。文章对下扬子宁芜盆地中三叠统黄马青组三角洲相陆源碎屑岩进行野外调查,并开展了沉积岩石学和物源分析。结果显示,黄马青组以岩屑石英砂岩、长石岩屑石英砂岩为主,石英含量76%~84%,岩屑含量10%~14%,长石含量6%~11%。岩屑以富含石英的变质岩或变泥质岩岩屑为主,指示变质岩的特殊物源的存在;黄马青组砂岩碎屑锆石年龄主要分布在350~250 Ma、420~400 Ma、900~700 Ma、2000~1600 Ma、2500~2100 Ma等区间,反映了华南板块北缘、华北板块南缘的混合信号,其物源供应主要来自大别造山带南部的宿松杂岩带、华北板块南缘基底及沉积盖层物质;通过与大别造山带周缘中生代沉积记录进行对比,发现在中—晚三叠世各盆地具有物源相似性,认为大别造山带低级变质带的早期折返导致了造山带的初始抬升和剥露,宁芜盆地中三叠统黄马青组至少一部分沉积物来自造山带早期抬升与剥蚀。     

印度-亚洲大陆碰撞初期的海沟沉积:藏东南宗卓组沉积岩石学与物源分析

混杂岩是汇聚板块边缘的地质体,主要形成于俯冲和碰撞的背景下。本文对藏东南浪卡子地区宗卓组进行了详细的野外地质调查、岩石学研究和物源分析。野外调查表明,宗卓组与下伏特提斯喜马拉雅地层的原始接触关系为逐渐过渡的沉积接触,后期构造作用多呈断层接触。界线附近,宗卓组滑塌的岩块长轴沿页岩片理方向展布,显示沉积混杂的特征;宗卓组多数地层受后期构造作用的改造。宗卓组的混杂岩由岩块和"基质"组成,岩块包括砂岩、灰岩、硅质岩,"基质"以硅质页岩、泥岩为主。砂岩岩块碎屑成分多为火成岩岩屑和沉积岩岩屑;碎屑锆石U-Pb年龄主要分布在88~140 Ma,中生代的锆石εHf(t)值变化范围大(-20~17)。这些特征表明岩块的物源为亚洲活动大陆边缘。结合宗卓组的基底为印度大陆北缘的特提斯喜马拉雅地层,因此宗卓组沉积混杂岩为印度-亚洲大陆碰撞之后沉积。由于宗卓组砂岩岩块缺乏冈底斯弧中古近纪年轻(60 Ma)年龄,推测这些砂岩岩块的碎屑并非直接来自冈底斯弧及拉萨地体,而是来自洋壳俯冲时期形成的增生楔修康混杂岩。由此,宗卓组为印度-亚洲板块碰撞初期,深水环境下侧向搬运形成的一套沉积混杂岩,物源主要来自西侧的修康混杂岩。宗卓组代表了印度-亚洲大陆碰撞最早期的海沟沉积,其分布指示了印度-亚洲大陆碰撞初期的缝合带位置。     

古新世特提斯喜马拉雅南亚带石英砂岩成因及其构造意义:以藏南岗巴地区古新统基堵拉组为例

特提斯喜马拉雅南亚带作为印度被动大陆北缘的主要构造单元,相较于其他类似构造单元发育着多套特殊的石英砂岩,意味着被动大陆边缘物源区陆源碎屑供应能力的多次变化,而引起印度被动大陆北缘石英砂岩沉积的构造背景和构造意义尚不明确。以特提斯喜马拉雅南亚带的岗巴地区古新统基堵拉组石英砂岩为例,通过砂岩碎屑成分分析、古流向恢复、重矿物分析和碎屑锆石年代学方法,对基堵拉组石英砂岩的沉积学及物源区特征,以及蕴含的成因和构造意义进行探讨。从沉积相分析结果来看,在早古新世岗巴地区所属的板块为印度被动大陆边缘,处于新特提斯洋的海岸线附近,以滨岸相为主,显示了一种浅海陆棚到陆相的变化。从砂岩岩相学的结果分析,基堵拉组的陆源碎屑物主要是成熟度极高的石英砂岩,同时古水流近NNE方向。从碎屑锆石年代学数据分析结果可知,基堵拉组的碎屑锆石年龄特征与早白垩世德干高原地区相吻合。故认为基堵拉组石英砂岩的形成是由于印度北缘的陆源碎屑供应量突然增多与被动大陆边缘物源区构造抬升导致,而引起被动大陆边缘物源区构造抬升的原因主要与德干大火成岩省形成相关。最终认为石英砂岩的发育成因与印度大陆北缘德干大火成岩省形成时构造隆升所导致的稳定克拉通再活化有关。     

西藏日喀则弧前盆地彭措林斑岩型铜多金属矿点的发现意义和成岩成矿时代约束

在特殊的大地构造位置——西藏日喀则弧前盆地内新发现了彭措林斑岩型铜多金属矿点,从矿点成矿地质背景和地球化学特征推测,该矿点具有中型斑岩型铜多金属矿的找矿远景。与成矿关系密切的花岗闪长斑岩LA-ICP-MS锆石U-Pb法定年结果为(11.04±0.46)Ma,指示该矿点形成时代为晚喜马拉雅期,可能属于冈底斯斑岩铜矿带大规模成矿事件。该矿点的发现扩大了冈底斯斑岩铜矿带南成矿亚带的范围,使斑岩型铜矿今后的找矿空间一直向南扩大到日喀则弧前盆地。     

The evolution of Yarlung Tsangpo River: Constraints from the age and provenance of the Gangdese Conglomerates,southern Tibet

Cenozoic conglomerates are exposed discontinuously along the length of the Yarlung Tsangpo suture zone on the southern margin of the Gangdese arc. These conglomerates (the “Gangdese Conglomerates” herein) record a crucial stage in the uplift and erosion histories of the southern Tibet after the initial India–Asia collision. In the Mt. Kailas area, the Gangdese Conglomerates strata consist of multiple sedimentary cycles and each cycle is a fining-upward sequence that was deposited by alluvial fan, braided-river and delta systems. Whereas in the Xigaze area, the Gangdese Conglomerates strata comprise a coarsening-upward sequence that was deposited by delta, braided-river and alluvial fan systems. Based on the detrital and igneous zircon U–Pb ages, the depositional ages of the Gangdese Conglomerates are late Oligocene to early Pliocene (ca. 26–5 Ma) in the Mt. Kailas area, late Oligocene to middle Miocene (ca. 26–15 Ma) in the Xigaze area, and late Oligocene to early Miocene (ca. 26–19 Ma) in the Zedong area. Paleocurrent measurements and provenance data (i.e., conglomerate clast composition, sandstone petrology and detrital zircon age) indicate that the initial detritus of the Gangdese Conglomerates were entirely derived from the north (mainly from the Gangdese arc). Sediment resulting from denudation to the south (the Xigaze forearc basin, the Yarlung Tsangpo suture zone and the northern margin of the Indian plate) first appeared by the early Miocene (ca. 19 Ma) and subsequently increased in abundance gradually. Our new results, together with previous data from the Xigaze area, reveal 3 major stages in the evolution of the Yarlung Tsangpo River system: (1) the southward-flowing stage (ca. 26–19 Ma) featured southward-draining transverse rivers that transported materials from the Gangdese arc southward. Southward paleocurrents in the Gangdese Conglomerates indicate a northern source. (2) The westward-flowing stage (ca. 19–15 Ma) developed due to the uplift of the suture zone and Tethys Himalaya to the south. Northward-draining rivers began to develop, and lakes resembling a string of beads formed and finally connected together, initiating the westward-flowing paleo-Yarlung Tsangpo River. Westward paleoflows were recorded in the Gangdese Conglomerates. (3) The eastward-flowing stage (ca. 15 Ma–present) resulted from differential uplift and denudation of the southern Tibet, which reversed the direction of the young Yarlung Tsangpo River by ca. 15 Ma. The deposition of the Gangdese Conglomerates was controlled by eastward paleoflows. At this point, the modern eastward-flowing Yarlung Tsangpo River system was established.     

CHRONOSTRATIGRAPHY,SEDIMENTATION AND EVOLUTION OF THE XIGAZE FOREARC BASIN: IMPLICATIONS FOR DYNAMIC EVOLUTION OF THE YARLUNG ZANGBO SUTURE ZONE

CHRONOSTRATIGRAPHY,SEDIMENTATION AND EVOLUTION OF THE XIGAZE FOREARC BASIN: IMPLICATIONS FOR DYNAMIC EVOLUTION OF THE YARLUNG ZANGBO SUTURE ZONE1　All埁ｇｒｅＣＪ,CourtillotV ,TapponnierP ,etal.StructureandevolutionoftheHimalaya Tibetorogenicbelt[J] .Nature,1984 ,30 7:17～ 2 2 . 2　CoulonC ,MaluskiH ,BollingerC ,etal.MesozoicandCenozoicvolcanicrocksfromcentralandsouthernTibet:3 9Ar 40 Ardating ,petrotogicalcharacteristicsandgeodyn…     

日喀则弧前盆地碎屑铬尖晶石地球化学与物源判别

该工作在藏南日喀则弧前盆地砂岩中发现了大量碎屑铬尖晶石。电子探针化学成分分析显示这些铬尖晶石具有高铬(铬指数Cr#为0.52~0.89)、低Fe3+含量(Fe3+/ Fe2+<0.5)、低TiO2含量(多小于0.2%)的特征,指示这些铬尖晶石来源于与洋壳俯冲作用有关的橄榄岩和火成岩,因此弧火山岩和SSZ型蛇绿岩套是其最可能的物源。考虑到日喀则弧前盆地的碎屑物主要来源于拉萨地体,而拉萨地体北侧的班公-怒江缝合带的碎屑物质不可能穿过晚白垩世时期已经隆起的冈底斯岩浆弧。笔者推测,铬尖晶石可能来自于冈底斯弧和拉萨地体内部已经消失的古蛇绿岩套。     

New insights into the timing of the India–Asia collision from the Paleogene Quxia and Jialazi formations of the Xigaze forearc basin,South Tibet

The Xigaze forearc basin provides information on subduction evolution and magmatic growth of the Gangdese arc as well as on the India–Asia continental collision. Recently obtained sedimentological, biostratigraphic, petrographic, geochemical and geochronological data on Cretaceous to Paleogene strata in the Cuojiangding area (Zhongba county, south Tibet) shed new light on the tectonic evolution of the southern margin of the Lhasa Block during closure of Neotethys and initial collision with India. The uppermost Cretaceous Padana and Qubeiya formations, deposited in deltaic to inner shelf environments, and representing the final filling of the Xigaze forearc basin, were unconformably overlain by the Quxia and Jialazi formations, deposited in fan-delta environments during the Paleocene/earliest Eocene. Petrographic data and U–Pb ages of detrital zircons document the progressive unroofing of the Gangdese arc, which remained the dominant source of detritus throughout the Late Cretaceous to Paleogene. Detrital Cr-spinels in the Quxia and Jialazi formations are geochemically similar to those in Cretaceous Xigaze forearc strata but different from those hosted in Yarlung Zangbo ophiolites, suggesting that the latter were not exposed to erosion in the considered time window. Sandstone petrography, Cr-spinel-geochemistry, U–Pb age spectra and Hf isotopic ratios of detrital zircons in the Quxia and Jialazi formations match those in Paleogene sediments deposited on the distal (Sangdanlin and Zheya formations) and proximal Indian margin (Enba and Zhaguo formations), suggesting that the Quxia and Jialazi formations documents syncollisional fan-deltas deposited on top of the nascent Himalayan orogenic belt. In this scenario, the onset of the India–Asia collision predates deposition of the Quxia and Jialazi formations and is thus constrained as younger than 66 Ma and older than 58 Ma.     

Provenance of the Palaeocene-Eocene clastic rocks of the Komandorsky Islands

The Palaeocene–Eocene sequence on the Komandorsky Islands is represented by sediments and minor volcanics with a total thickness of more than 3000 m. Sandstone, siltstone, mudstone, siliceous rocks and conglomerate, which have accumulated in the upper and middle parts of a deep-water fan, prevail. All the debris was delivered from the north-east, from the side of the contemporary Komandorsky basin in the Bering Sea. Products of volcanic denudation and fresh tephra dominate its composition. However, at some levels the sandstone contains fragments of jasper, quartzite, slate, schist, gabbro and granite as well as non-volcanic quartz along with a volcanic component. The heavy minerals fraction of the sandstone contains considerable quantities of apatite, zircon, garnet and chromite, as well as pyroxenes, amphiboles and magnetite. The chemical composition of the garnet is specific to metamorphic rocks. A significant admixture of chromite is indicative of denudation from an ophiolite complex. The folded and partly metamorphosed pre-Palaeogenic rocks were a source of these fragments. Their most likely source terrane was the Shirshov-Bowers chain (Cretaceous palaeoarc). The Komandorsky block originally was the forearc part of the Aleutian arc where the latter was superimposed on the Shirshov-Bauers chain. A subsequent north-west displacement of forearc structures, in response to the transform fault at the boundary between the Pacific and North American plates, could have resulted in the separation of the Komandorsky block from its Early Palaeogene debris source area.     

Sedimentology,provenance and geochronology of the Miocene Qiuwu Formation: Implication for the uplift history of Southern Tibet

Located on the south of the Gangdese,the Qiuwu Formation has traditionally been considered as Eocene coal-bearing clastic sediments consisting of sandstone,mudstone and conglomerate,unconformably on top of Gangdese batholith.However,its precise age and depositional environment remain ambiguous.Here,we present a newly measured stratigraphic section near the Ngamring County,western Xigaze.Detrital zircon U-Pb ages were also applied to trace the provenance of sediments and to constrain the maximum depositional age of the Qiuwu Formation.Sedimentary facies analyses indicate subaqueous fan and alluvial fan depositional environments.Clast composition of the conglomerate is dominated by magmatic rocks at the lower part,while chert and mafic detritus occur in the upper part,suggesting a southern source.Sandstone modal analyses indicate that the compositions of quartz,feldspar and lithic grains changed from transitional arc to dissected arc,implying the unroofing of the Gangdese arc.Detrital zircon U-Pb ages of the Qiuwu Formation are compared with those from Gangdese magmatic rocks and Yarlung-Zangbo ophiolites,suggesting that the Gangdese arc is a main source of the Qiuwu detritus and that the southern source played a role during the later stage.The major peak of detrital zircon ages is at 45-55 Ma,which corresponds to Linzizong volcanic rocks in southern Gangdese arc.The weighted mean age of the five youngest zircons from the lower part of the section is 21.0 ± 2.2 Ma,suggesting that the Qiuwu Formation was deposited in early Miocene,coeval with other conglomerates exposed along the southern margin of Gangdese.Combining new observations with previously published data,we propose that the provenance of the Qiuwu Formation had shifted from a single northern source to double sources from both the north and the south.Activities of Great Counter Thrust were primarily responsible for the shift by making the south area a high elevation to provide sediments for the Qiuwu Formation.     

The Yarlung suture mélange,Lopu Range,southern Tibet: Provenance of sandstone blocks and transition from oceanic subduction to continental collision

With the aim of better understanding the history of ocean closure and suturing between India and Asia, we conducted a geologic investigation of a siliciclastic matrix tectonic mélange within the western Yarlung suture zone of southern Tibet (Lopu Range region, ~ 50 km northwest of Saga). The siliciclastic matrix mélange includes abundant blocks of ocean plate stratigraphy and sparse blocks of sandstone. Metapelite and metabasite blocks in the mélange exhibit lower greenschist facies mineral assemblages, indicating that they were not deeply subducted. We obtained detrital zircon U-Pb geochronologic and sandstone petrographic data from sandstone blocks in the mélange and sandstone beds from Tethyan Himalayan strata exposed to the south of the suture. The sandstones from both units are all similar in U-Pb detrital zircon age spectra and petrography to the nearby Tethyan Cretaceous–Paleocene Sangdanlin section, which records the earliest appearance (at ~ 59 Ma) of arc-affinity strata deposited conformably on Indian-affinity strata. Two Paleocene sandstones, one of which is a schistose block incorporated in the siliciclastic matrix mélange, yielded indistinguishable maximum depositional ages of ~ 59 Ma. Mesozoic Asian-affinity sandstone blocks previously documented in the siliciclastic matrix mélange 200–500 km along strike to the east are notably absent in the Lopu Range region. We documented a gradational transition in structural style from the block-in-matrix mélange in the northeast to the south-vergent Tethyan thrust belt in the southwest. Blocks of Tethyan Himalayan strata increase in size and the volumetric proportion of matrix decreases from northeast to southwest. We conclude that no arc-affinity sandstone blocks were incorporated into the subduction complex until India-Asia collision at ~ 59 Ma when the Xigaze forearc basin became overfilled and Tethyan Himalayan strata entered the trench. As collision progressed, there was a gradual transition in structural style from block-in-matrix mélange formation to imbricate-style thrust belt formation.