Gondwana Research 22 (2012) 415–433

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Revision of the Cretaceous–Paleogene stratigraphic framework, facies architecture and provenance of the Xigaze forearc basin along the Yarlung Zangbo suture zone

Chengshan Wang a,⁎, Xianghui Li b, Zhifei Liu c, Yalin Li a, Luba Jansa d, Jingen Dai a, Yushuai Wei a a State Key Laboratory of Biogeology and Environmental Geology, Research Center of Qinghai–Tibet Plateau Geology, China University of Geoscience (Beijing), Beijing 100083, China b State Key Laboratory of Mineral Deposit Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China c State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China d Department of Earth Sciences, Dalhousie University, Halifax, and Geologic Survey of Canada-Atlantic, Dartmouth, N.S., Canada article info abstract

Article history: New field investigation, structural analysis, review of biostratigraphy, stratigraphic framework, facies archi- Received 16 March 2011 tecture, provenance, and flow direction of the Cretaceous–Paleogene sedimentary strata in southern Tibet Received in revised form 22 August 2011 resulted in reinterpretation of the evolution of the Xigaze forearc basin (XFB). The XFB is comprised by Accepted 8 September 2011 flysch-dominant Xigaze Group (consisting of Ngamring, Chongdoi, and Sangzugang Formations) and of the Available online 17 October 2011 shallow marine Cuojiangding Group (Padana, Qubeiya, Quxia, and Gyalaze Formations), with their ages here- in revised as the Albian–Coniacian and the Santonian–Ypresian respectively. The deep-sea flysch represented Keywords: fi fi Cretaceous-Paleogene by the Ngamring Formation constitutes the early sedimentary ll of the XFB. It was subdivided into ve Xigaze Group megasequences MS1–MS5 in order: the early submarine fan facies stage, channel submarine fan stage, the Cuojiangding Group mature submarine fan stage, the highstand submarine fan stage, and outer fan-pelagic stage. The paleocur- Forearc basin rent flows were eastwards and westwards during the early and late stages of basin development and north- Yarlung Zangbo suture zone wards and southwards during its middle stage, indicating typical forearc basin sedimentation with longitudinal supply and lateral filling. Provenance studies point to multiple sources, with andesitic debris sourced from the Jurassic–Lower Cretaceous Yeba Formation and Sangri Group formed in the volcanic island arc chains in the southern Gangdese. The four periods of the XFB development are: 1) the formation of the XFB basement by the Xigaze ophiolite, and completed before the Aptian. The coeval deposits on the shelf are the Sangzugang Formation carbonate; 2) the earliest XFB deposits of the earliest Aptian age (ca. 125/ 120 Ma) represented by the Chongdoi Formation, related to the residual forearc; 3) composite forearc devel- oped, represented by the late Albian–late Cenomanian flysch megasequences (MS1–MS2) and the Turonian– Coniacian flysch megasequences (MS3–MS5), respectively; and 4) the final stage of the XFB development started the terminal forearc stage represented by the late Late Cretaceous–Early Paleogene shallow marine Cuojiangding Group and terminated during middle Ypresian, when the Gyalaze Formation was deposited. © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction The biostratigraphy of XFB sedimentary strata has been established during the 1970s–1980s (Wen, 1974; Cao, 1981; Wu, 1984; Liu et Extensive studies were carried out in the Yarlung Zangbo suture al., 1988), and lithofacies and provenance studies have been pub- zone (YZSZ) (e.g. Allègre et al., 1984; Gao and Tang, 1984; Coulon lished by Liu et al. (1993), Dürr (1993, 1996), and Einsele et al. et al., 1986; Gao, 1988; Dewey et al., 1990; Yu and Wang, 1990; Yin (1994). However, some recently published biostratigraphic data et al., 1994; Yin and Harrison, 2000; Liu et al., 2010; Dai et al., (e.g. Wan et al., 1998, 2001; Li et al., 2008a,b) and present works on 2011b; etc.). However, only few were concerned with the XFB, and structure and sedimentology demonstrate that previous hypotheses only few hypotheses on its evolution have been published (e.g. on the time, thickness and sequences of the XFB sediments must be Dürr, 1993, 1996; Einsele et al., 1994; Wang et al., 1999), despite revised. Consequently, new interpretations on facies architecture that the sedimentary strata appear to be well preserved and subaeri- and provenance will result in a reinterpretation of the XFB evolution. ally exposed. As part of the Yarlung–Zangbo subduction zone com- To improve our understanding of the XFB development, we have con- plex, the sediments of XFB document the late subduction processes ducted new geological field investigations of the Xigaze Group. Newly of the Neo-Tethys Ocean crust and the forearc basin development. described stratigraphic profiles and outcrops, combined with structural analyses, were used to systematically calibrate the stratigraphy, to rein- terpret sedimentary megasequence and provenance, and especially to ⁎ Corresponding author. reconstruct the sedimentary evolution of the XFB as well as its relation- E-mail address: [email protected] (C. Wang). ships to the YZSZ on the basis of the revised stratigraphic framework.

1342-937X/$ – see front matter © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2011.09.014 416 C. Wang et al. / Gondwana Research 22 (2012) 415–433

2. Geological background ophiolitic complex represented by highly deformed and serpenti- nized pyroxene peridotite with developed schistosity and by tectonic The YZSZ comprises four major tectonic units (from north to mélange (Gao and Tang, 1984; Gao, 1988). The accretionary wedge south): Gangdese magmatic arc of Cretaceous–Tertiary plutonic and crops out at Ngamring, Geding, Bailang, and Zedang (Fig. 1) where it volcanic rocks (GMA), Xigaze forearc basin (XFB), an ophiolite belt is characterized by olistoliths of various sizes ranging up to several kilo- and a Triassic–Eocene accretionary wedge (Fig. 1). meters, emplaced within the matrix of deformed and fragmented flysch The GMA in southern Tibet, together with the Ladakh magmatic sandstone and shale that comprises the Upper Triassic–Cretaceous arc in the western Himalayas, comprise a large Ladakh–Gangdese Xiukang Group. magmatic belt over 2000 km along the strike (i.e. the Trans- The strata of the XFB crop out along the Yarlung Zangbo in an Himalaya). The GMA consists of intrusive (including gabbro, diorite, east–west oriented belt, over 500 km long, which begins at Renbu tonalite, and granodiorite) and extrusive (including basalt, andesite, in the east and ends at Zhongba in the west. The maximum width of and rhyolite) rocks, in which the Cretaceous rocks with geochemical the belt is 22 km. Our study area was focused on the western part of compositions of I-type granitoids and adakites are thought to have the Xigaze region (Figs. 1 and 2A), where the flysch-dominated been generated by the northward subduction of the Neo-Tethyan Xigaze Group and shallow marine Cuojiangding Group are subaerially oceanic floor (cf. Wen, 1974; Zhu et al., 2006, 2009a, 2011; Zhang et well exposed. The Cuojiangding Group overlies the Xigaze Group in al., 2010). The intrusive rocks were emplaced over a long time period the Zhongba area (Liu et al., 1988; Wan et al., 2001). from the Late Triassic (ca. 205 Ma) to Miocene (ca. 10 Ma) (cf. Chung et al., 2009; Ji et al., 2009; Lee et al., 2009; Zhu et al., 2011). The extru- sive rocks, represented by the volcanic exposures in the Early Jurassic 3. Methods Yeba Formation (cf. Zhu et al., 2008b), Late Jurassic–Early Cretaceous Sangri Group (cf. Zhu et al., 2009a), and Early Paleogene Linzizong Three key methods were adopted in this study. The first method Group (cf. Zhou et al., 2004; Mo et al., 2008), extend about 1500 km was review of past works. Previous data was refined and reinter- in a W–E direction in southern Tibet. The Gangdese thrust fault preted based on newly described data on biostratigraphy, isotopic forms the contact between the GMA and the XFB in the eastern sec- chronology, petrology, sedimentary megasequence, paleocurrent di- tion (Yin and Harrison, 2000). rection, and provenance. The new results have been applied through- The ophiolite belt is composed of ultramafic rocks with gabbro cu- out most of the text for both the Xigaze Group and the Cuojiangding mulates, sheeted dyke complexes and pillow lava as well as Group. radiolarian-bearing chert. Radiolarians date the chert as of the late The second method was the field geological investigation. During Albian–early Cenomanian age (Marcoux et al., 1982). The bulk of radio- field work, observations and descriptions of profiles, measurement metric data from ultrabasic and basic rocks yielded an age of 120± of paleocurrents, and sampling for thin-sections and fossils (Figs. 2 10 Ma (Göpel et al., 1984). It had previously been thought that the and 3), were used to constrain the stratigraphy and to analyze the shortage of cumulates and sheeted dykes implies that the ophiolites provenance of the Ngamring Formation. Especially, five profiles previ- were product of an abnormal oceanic crust resulting from the slowly ously studied by Dürr (1993) and Einsele et al. (1994) were selected spreading mid-ocean ridge (Nicolas et al., 1981; Xiao, 1984; Girardeau for further re-examination: Zhaxigang, Geding, Kardoi–Gyangqingze, et al., 1985a,b). However, new evidence increasingly indicates that the Nadang–Deri, and Xishan–Beishan profiles (Fig. 2A). Several comple- ophiolites come from the supra-subduction zone (SSZ) (Bédard et al., mentary profiles to the south and west of Xigaze were also studied 2009; Bezard et al., 2011; Hébert et al., 2011; Dai et al., 2011b). The (Fig. 2A). Of these, the typical profile is the Kardoi–Gyangqingze pro- Renbu–Zedang thrust fault in Xigaze is the contact between the ophio- file which presents the structure and stratigraphy of the Ngamring lites and the southern accretionary wedge (Yin and Harrison, 2000). Formation (Fig. 2B). Traditional structures were analyzed for the pro- The accretionary wedge is a narrow elongate belt up to 1000 km file by the deformation and faulting approach, and the results of long and 10–50 km wide. It consists of rocks derived from the structural interpretation were completed in Section 4.

Fig. 1. Geological map of the Indus–Yarlung Zangbo suture zone showing location of the GMA, XFB, ophiolite belt, and accretionary wedge. The Sangri Group location is after Zhu et al. (2006), accretionary wedge after Wang et al. (1999). Inset: Simplified 1:2,500,000 geological map of Tibet (Pan et al., 2004). C. Wang et al. / Gondwana Research 22 (2012) 415–433 417

Fig. 2. A. Location of profiles from field studies shown on a geological map of the middle XFB (modified after Wang et al., 1999; Einsele et al., 1994). 1. GMA complex; 2. Qiuwu Formation; 3. Gyabulin Formation; 4. Sangzugang Formation; 5. Ngamring Formation; 6. granites; 7. ophiolites; 8. Liuqu Group.; 9. mélange; 10. Quaternary; 11. thrust fault; 12. Structural profiles investigated in this work; 13. village and town; and 14. localities studied in detail. ZXS: Zhaxigang profile; GDS: Geding profile; GKS: Kardoi–Gyangqingze profile (modified after Dürr, 1993: see panel B); DNS: Nadang–Deri profile; XGS: Xigaze Xishan–Nanshan profile. B. Interpreted structure of the Kardoi–Gyangqingze profile (modified after Dürr, 1993; Einsele et al., 1994). Note the asymmetry, deformation and dipping of the two flanks. Megasequences MS1–MS5: for description see text. S1–S65: locations of field studies. Short- ening ratio (e) is from Ratschbacher et al. (1992).

The third method was the use of aerial photos at 1:75,000 scale, lighter-colored stripe. All the results of the lithological markers are which in addition to tectonic features studied in the field allowed to lo- shown in Fig. 3. cate and map regional distribution of lithological markers (Fig. 3)and megasequences in the Ngamring Formation. This process focused on 4. Structures of Ngamring Formation two lithological markers: a sandstone–conglomerate assemblage mark- er that appears as steeply sloping, high, sharp ridges that are darkish in The Ngamring Formation forms a synclinorium striking W–E color, indicating the channel-fills of the submarine fan system; and a pe- (Fig. 2). It is asymmetric, with the northern flank 10–15 km wide lagic marlstone marker, which is identifiable on the images as a narrow, and southern flank 6–8 km wide (Fig. 2B). This asymmetry may

Fig. 3. Locations of samples on geological map (simplified after Einsele et al., 1994 and Wan et al., 1998). 1. Numbers and localities of the foraminiferal fossil samples (after Wan et al., 1998). 2. Numbers and localities of ammonite fossils: (a) and (d) after Wiedmann and Dürr (1995); (b) after Xiao (1984); (c) after Wen (1974). 3. Numbers and localities of detrital zircon samples (after Wu et al., 2010). 4. Limestone layers (modified after Dürr, 1993 and Einsele et al., 1994); solid line: observed, dashed line: inferred; 5. village and town. 418 C. Wang et al. / Gondwana Research 22 (2012) 415–433 have been result of difference in the thicknesses of sediments depos- This age was also confirmed by radiolarian association found in the ited in the distal and proximal part of the basin and/or by a northward Chongdoi (Wu and Li, 1985; Wu, 1986), which together with the de- overriding thrust on the southern flank, and/or by the lack of the trital zircon U–Pb isotope age of 116 Ma (Wu et al., 2010) from the Aptian–Albian sediments on the southern flank. siltstone in the upper of the Chongdoi Formation indicates that the The structural deformation increases northward as shown by the strata are late Aptian to the early Cenomanian in age. profile (Fig. 2B). To the south, the folds are open, associated with Previously, the Chongdoi Formation has been thought to be a part of smaller sliding folds striking west–east, commonly having axial cleav- an oceanic arc system (e.g. Cao, 1981; Xizang BGMR, 1997; Aitchison age evident in the weak layers. In the center of the synclinorium the et al., 2000; Ziabrev et al., 2004). However, in this paper we have includ- folds become small-scale subordinate folds with axial cleavages, and ed it into the Xigaze Group instead of the oceanic crust association for strata are horizontal; in the north, the strata are strongly deformed three reasons: (1) The Chongdoi is conformable with the overlying with tightly-closed, imbricated folds. flyschoid Ngamring Formation (Aitchison et al., 2000; Ziabrev et al., All strata strike W–E as result of contraction in the N–S direction. 2004), which we have also observed during our field investigation; The folds are asymmetric, with the structural style observed along the (2) Limestone fragments of the Sangzugang Formation are found in strike at both, the southern and northern flanks of the basin. The fold the Chongdoi, as well as in the Ngamring Formation (Einsele et al., axial surfaces dip to the south at the northern flank, and northward at 1994; Dürr, 1996; Wang et al., 1999); and (3) This assemblage is similar the southern flank (Fig. 2B). Pale strips on images which mark pelagic to the sequence of radiolarian chert and clastic rocks in the forearc basin marlstones outcrop near the center of the synclinorium. Two of these of the Great Valley, western USA, which is an example of the typical identified on the southern flank and one at the northern flank belong forearc basin sequence. to the same layer (Figs. 2B and 3), as documented by the foraminiferal biostratigraphy (Wan et al., 1998). Therefore, they are not two differ- 5.1.2. Sangzugang Formation ent horizon as previously indicated by Einsele et al. (1994). The name of this formation is after the village of Sangzugang, Sajia A set of imbricated thrusts are located at the northern boundary of County, east of Xigaze, where it crops out as tectonic fragments. It the synclinorium (Fig. 2B), where the mid-Cretaceous calcareous consists of dark gray, thick-bedded bioclastic limestones with abun- Sangzugang Formation and the Tertiary terrestrial Gyabulin Forma- dant foraminifera and rudists, indicating shallow carbonate platform tion are present in the thrust (Yin et al., 1994). At the southern facies. The unit is 60–230 m thick (Wu et al., 1977), and decreasing boundary area, the contact of the Ngamring Formation is less clear to a few meters thickness about 10 km east of Xigaze, and disappears than in the other areas, as most of the outcrop is covered by the farther east at Dazhuka. It has fault contacts with either the Ngamring Quaternary sediments. However, sedimentary contact of the Ngamr- Formation or the Gyabulin Formation. The Sangzugang is of Aptian– ing flysch with the underlying pillow lava of the ophiolitic series Albian age (Cherchi and Schroeder, 1980; Bassoullet et al., 1984). can be seen in the Dazhu area. Also at Qunrang and Naxia villages, the flysch overlies the Chongdoi Formation radiolarian cherts. An al- 5.1.3. Ngamring Formation most vertical fault separates the Tertiary terrestrial Liuqu Group and The name was used by Wu et al. (1977) to describe Cretaceous deep- Ngamring (Yin et al., 1988a) in the Kardoi–Gyangqingze profile. sea flysch in Ngamring County. Later it was redefined as the Cretaceous deep-sea flysch occurring above the carbonate Sangzugang Formation 5. Revision of stratigraphy (Yin et al., 1988b; Xizang BGMR, 1993, 1997). The Ngamring Formation is composed by sandstone and shale, and is characterized by a series of Verification of previously established litho-, bio-, and chronostra- large channelized conglomerates in the lower part. It is conformable tigraphy for the XFB is critical for basin analysis. Below we present with the underlying Chongdoi Formation (Fig. 5) as well as the overly- new data that will revise previous studies re-interpret the ages and ing Padana Formation, west of Ngamring County (Fig. 6). But it has fault relationships of the stratigraphic units (Fig. 4). Two main lithostrati- contact with the Xigaze ophiolite in the south and with the Gyabulin graphic associations represent the XFB fill. They are the Xigaze Formation or Sangzugang Formation in the north (Fig. 2A). However, Group dominated by flysch of the Ngamring Formation, and the shal- the stratigraphic sequence, thickness and dating of the Ngamring has low marine Cuojiangding Group. been misinterpreted, mainly due to the strongly deformed strata and the scarcity of fossils. Below they are calibrated on the combination of 5.1. Xigaze Group the recently published biota and age maximum of detrital zircon U–Pb isotope plus the present observation and structural interpretation in The Xigaze Group was originally defined by Wen (1974) as sedi- Section 4. mentary flysch about 6000 m thick, unconformably overlying the It is generally accepted that the structural analyses shows that Yanshanian granites with basal conglomerates of Cenomanian–Turonian rocks of the Ngamring Formation become progressively older from age at its base. Later, the shallow marine carbonates of the Sangzugang the synclinorium axis towards the southern and northern flanks. Formation were included into this group (Wu et al., 1977). In this However, there are problems associated with the interpretation of paper, besides the Ngamring Formation and Sangzugang Formation, stratigraphic sequence and thicknesses. From the axial center of the the Chongdoi Formation is included as part of the Xigaze Group (Fig. 4). basin towards the north, the Ngamring Formation was dated as of the late Turonian age (samples 46-1, 46-2), and the middle-early 5.1.1. Chongdoi Formation Turonian (samples 47-3, XB-4, XB-5) (Fig. 3) using foraminiferal This formation was named by Cao (1981) after Chongdoi village, microfossils, and at eastward locations by the occurrence of ammo- about 20 km SE of Xigaze, where it is composed by two members. nite Mammites sp. (Wen, 1974). Farther to the north, the rocks The lower member is ophiolite breccia interbedded with purplish- were dated as Cenomanian by the ammonite Turrilites (T.) acutus red radiolarian chert. The upper member is tuffaceous siltstone inter- (Wiedmann and Dürr, 1995). The Aptian–Albian age is indicated by bedded with shale that ranges in thickness from 73 to 197 m (Xizang the occurrences of larger benthic foraminifera Orbitolina (Wan et al., BGMR, 1993, 1997). Its basal contact is depositional upon the ophio- 1998) in the sample 14-1 (Fig. 3) and of an ammonite Neophlycticeras lite (Fig. 5) and pillow lava and its upper contact is conformable (T.) brottianum (Wiedmann and Dürr, 1995). On the northern bound- with the overlying flysch of the Ngamring Formation (Fig. 5). In ary, the Sangzugang Formation was dated by foraminifera as of Naxia locality, the planktonic foraminifer Rotalipora spp. was found Aptian–Albian age (Cherchi and Schroeder, 1980; Bassoullet et al., in the limestone at the top of the Chongdoi Formation (Wu, 1984) 1984), indirectly indicating that the deposition of the Ngamring which indicates that it was deposited no later than Cenomanian. could not have started prior to the Aptian–Albian. C. Wang et al. / Gondwana Research 22 (2012) 415–433 419

Fig. 4. Diagram showing the stratigraphic framework of the XFB with data of biostratigraphy, maximum age of detrital zircon U–Pb isotopes, and lithofacies. The chronostratigraphic system is cited from Gradstein et al., 2008. The column of lithostratigraphy and lithology is composite from the review (details see in text). In the column of the biostratigraphy, the fauna of the Cuojiangding Group are quoted from Wan et al. (2001), and the biozones of the Ngamring Formation are cited from Wan et al. (1998). The column of the age maximum is cited from Wu et al., 2010.

Most of the Ngamring Formation in the southern Kardoi– of Sagoi indicates the Albian–Cenomanian age of the strata (Xiao, Gyangqingze profile (Fig. 2) are of Cenomanian age based on the fora- 1984). Foraminifera and radiolarian fossils in samples 43-16 and minifera Rotalipora cushmani (sample 43-11) (Caron, 1985). The pres- 43-17 (Fig. 3)indicatetheearlyLateCretaceousage(Yin et al., ence of an ammonite from Brancoceratidae Family found to the west 1988b). Abundant foraminiferal assemblage (e.g. Whiteinella 420 C. Wang et al. / Gondwana Research 22 (2012) 415–433

Fig. 5. Field photograph showing the sequence from the ophiolite belt to the XFB at Qiongrang village (29°09′34″E, 89°01′57″N), SE Xigaze. A. Reversed stratigraphic sequence; folded succession of the Chongdoi Formation (rhythmic turbidite sandstone and shale Bouma-cde, shale with ash intercalation, red thin radiolarian cherts) up to the ophiolites. B. Enlargement of contact relationship between radiolarian cherts and ophiolites. The figures in the photograph are shown at the contact. archaeocretacea, Marginotruncana renzi, M. pilediformis)insamples 5.2. Cuojiangding Group 44-4 and 44-5 (Fig. 3) indicate deposition during the Cenomanian– Turonian (Wan et al., 1998). Similarly, foraminifera microfossils in The Cuojiangding Group which overlies the Ngamring flysh is in- samples ND-15 and ND-16 (Fig. 3), south of the Nadang–Deri profile cluded here as part of XFB sedimentary fill to provides auxiliary infor- (Fig. 2), indicate the Turonian age (Wan et al., 1998). Unfortunately, the mation about forearc basin development. The name of this Group is Sangzugang Formation has not yet been observed on the southern derived after a small lake (ca. 5400 m high) approximately 40 km flank of the basin, as only the Cenomanian–Turonian component of the north of the new Zhongba town. The group consists of the Padana Ngamring Formation crops out on the southern flank of the synclinorium. Formation, Qubeiya Formation, Quxia Formation, and the Gyalaze The youngest strata of Ngamring Formation are of the Coniacian Formation (Fig. 4), which crop out to the west of Ngamring county. age and are present at the axial part of the synclinorium. The forami- It has a total thickness of 1472 m and is comprised of predominantly niferal assemblage M. sinuosa in sample 46-2, between samples 44-5 by shallow marine sediments (Liu et al., 1988) with their age ranging and 47-3 (Fig. 3) indicates an age younger than the late Turonian from the Late Cretaceous to the Early Paleogene (Qian et al., 1982; Liu (Wan et al., 1998). To the east, the foraminifera Dicarinella primitiva et al., 1988; Wan et al., 2001; Li et al., 2008a,b). in sample XB-1 at the Xiaburi profile (Fig. 2) is an index species of In previous studies the Padana Formation and Qubeiya Formation the early Coniacian (Caron, 1985). D. concavata is very abundant in have been included in the Xigaze Group. However, both of them were de- sample XB-3, indicating the late Coniacian age (Wan et al., 1998). posited in a shallow marine environment (details in Section 9), therefore The thickness of the Ngamring Formation on the northern flank has the depositional setting is very different from the deep-sea flysch of the been revised to approximately 4100 m down from 6000 to 8000 m pre- Xigaze Group. In addition, they are similar to the Quxia Formation and viously estimated by Einsele et al. (1994), and it changes to 1000– Gyalaze Formation of the Cuojiangding Group. Therefore based on the 4100 m thick in the region. The biostratigraphic data indicates lithofacies similarity we have changed the stratigraphic classification of Aptian–Coniacian age for the Ngamring Formation (Wan et al., 1998). the Padana and Qubeiya and included them into the Cuojiangding Group. Considering the age maxima of detrital zircon U–Pb isotopes: 107 Ma, 101 Ma, and 88 Ma from the lower, middle, and upper part of this for- 5.2.1. Padana Formation mation (Wu et al., 2010), the age of the Ngamring Formation can be re- The Padana Formation was named after the Padana valley south- stricted to late Albian–Coniacian in age (ca. 106 to about 86 Ma). east of Sangsang village (Liu et al., 1988). The lower part of this C. Wang et al. / Gondwana Research 22 (2012) 415–433 421 formation is comprised by gray sandstone interbedded with shale and found in the strata indicates their late Paleocene to early Eocene in the upper part by red shale interbedded with gray sandstone and (Selandian–early Ypresian, ca. 61–51 Ma) age. Because the top of shale. The thickness of this formation ranges from about 733 to the stratum is not exposed, its age may extend to the latest Ypresian, 2000 m (Xizang BGMR, 1997). It has a conformable contact with i.e. 50–49 Ma, even younger. both the underlying Ngamring Formation (Fig. 6) and the overlying Qubeiya Formation. No age diagnostic fossils were found in the 6. Megasequences of Ngamring Formation Padana Formation, therefore the Santonian age suggested is an inter- pretation from the age of underlying Ngamring Formation and over- Before discussing the evolution of the XFB, we provide revision of lying the Qubeiya Formation as well as a maximum age of 88 Ma megasequence of Ngamring Formation as it has impact on the basin fill- derived from detrital zircon U–Pb isotopes (Wu et al., 2010). ing process interpretation. Previous studies documented that the Ngamring Formation sediments were deposited in a submarine fan sys- 5.2.2. Qubeiya Formation tem (e.g. Dürr, 1993; Liu et al., 1993; Einsele et al., 1994). To better de- The Qubeiya Formation, named at the same place as the Padana scribe the submarine fan system, seven basic lithofacies terms (A, B, C, Formation, is composed of yellowish-gray blocky sandy wackestone D, E, F, G) of Shanmugam and Moiola (1985) were used here. Allaby intercalated with sandstone, shale, mudstone and packstone. It is and Allaby (1999) defined a megasequence as comprised by units 430 m thick with the thickness increasing eastward up to ca. deposited during a distinct phase of basin evolution, separated by the 1000 m in Saga County (Xizang BGMR, 1997). In the Zhongba area, major unconformities that mark changes in the basin-controlling sedi- it has gradational contact with underlying Padana Formation and a mentary deposition processes. We have distinguished the megase- hiatus separates it from the overlying Gyalaze Formation. The quences of the Ngamring chiefly from lithological associations because benthic assemblages (Fig. 4) represented by foraminifera, bivalves, of the difficulty in recognizing unconformities in a deep-sea basin envi- ammonites, gastropods, and crinoids (Qian et al., 1982; Liu et al., ronmental setting. 1988; Wan et al., 2001), plus the maximum 78 Ma age of detrital Combining structural analysis and revised stratigraphy of the zircon U–Pb isotope from an indeterminate stratigraphic interval in Ngamring Formation, we distinguish five megasequences, termed the Qubeiya (Wu et al., 2010), indicate a Campanian–Maastrichtian MS1 to MS5 (Figs. 2B, 7, 8), in difference to the three megasequences age of the strata. suggested indirectly by Einsele et al. (1994). The five flyschoid mega- sequences we have recognized have different distributions in space, 5.2.3. Quxia Formation according to their proximal/distal positions in the basin (Fig. 8). For The Quxia Formation, named at Quxia valley in the Cuojiangding example, MS2 and MS3 are confined to the western area of the prox- area, is dominated by conglomerates with shale and sandstone inter- imal northern flank of XFB, where lateral lithofacies lack channelized calations. At the typical section the formation is 107 m thick and has sandstone–conglomerate beds. In addition, MS3 is not easily separa- paraconformable contact with the underlying Qubeiya Formation and ble from MS4 because both were deposited in a distal basin environ- conformable contact with the overlying Gyalaze Formation (Liu et al., ment where sediments were much thinner than in the proximal part 1988; Wan et al., 2001). Similar to the Padana Formation, no age- of the basin. diagnostic fossils have been found in the formation since it was estab- lished. Therefore, its age has been assumed to be Danian (Wan et al., 6.1. MS1, early submarine fan megasequence 2001). MS1 occurs on both flanks of the synclinorium (Fig. 2B) and appears 5.2.4. Gyalaze Formation to be truncated by faults in the southeastern part of the XFB. On the The Gyalaze Formation has been also named at Cuojiangding, southern flank, MS1 crops out between Kardoi and Geding where it is where it is composed of gray and dark gray foraminiferal wackestone about 2–3 km wide and 300 m thick and is in fault contact with the and packstone interbedded with sandstone and conglomerate. The ophiolites (Fig. 8) and has a sharp contact with the MS2 channel con- thickness is over 145 m at the typical section, and the top of the for- glomerate (Fig. 7). On the northern flank the megasequence is well pre- mation is at the center of a syncline so its contact with the overlying served in an outcrop zone about 5 km wide, and it is in fault contact unit is unclear. The age is relatively better constrained than the ages with the Gyabulin Formation and/or Sangzugang Formation. It can be of other units of the Cuojiangding Group because foraminifera are subdivided into two sub-megasequences MS11 and MS12. abundant in the formation. Assemblages of the large foraminiferal Miscellanea–Daviesi and Nummulites–Discocyclina (Wan et al., 2001) 6.1.1. MS11, slope apron facies sub-megasequence This sub-megasequence is comprised by slope facies C and D (mudstone–sandstone), intercalated with facies F (debris-flow brec- cias and gravelly shale). Facies C and D are comprised by alternating graded, fine-grained sandstones, siltstones and shales that drape the slope. Thin black siliceous limestone also occurs as intercalations, probably formed during interruption in terrigenous supply into the basin. A characteristic feature of this sub-megasequence is the pres- ence of facies F (debris-flow sediments). On the northern flank, facies F was found in the profiles at Gyangqingze (sample S14) and Deri (S59). There are two debris- flow layers in the northern Gyangqingze–Kardoi profile at Gyalie vil- lage (Fig. 2A): the first layer is debris-flow and slump sediments about 46 m thick, enclosing giant olistoliths up to 30 m×15 m in size; the second layer, about 20 m thick, cropping out north of the thrust slice, contains some limestone gravels from the Sangzugang Formation. Also some Orbitolina fragments were found in sample Fig. 6. Photograph showing the conformable contact between the Padana Formation of S14, located about 3 km east of Gyangqingze village. The up-section the Cuojiangding Group and Ngamring Formation of the Xigaze Group, in the Padana fi groove (29°21′32.8″N, 86°43′52.8″E), Ngamring area. For scale, see figures circled at pro le is composed of back shale (5 m thick), deformed sandstone lower right-hand corner. (7 m) and paraconglomerate (10 m). The paraconglomerate gravels 422 C. Wang et al. / Gondwana Research 22 (2012) 415–433

Fig. 7. Vertical stratigraphic diagram of sedimentary sequence of flysch, Ngamring Formation of Xigaze Gp. Sample numbers of fossils as in Fig. 2. Note the thickness discrepancy between southern and northern flanks. 1. Shale; 2. mudrock; 3. sandstone and mudrock; 4. sandstone; 5. conglomerate; 6. marlstone; and 7. limestone. are predominantly from basic volcanics, granite and sedimentary On the southern flank, the counterpart of MS11 is the lower- rock. Pillow structures and chilled borders can be observed in some middle Chongdoi Formation in Naxia. Because it is located farther of the gravels. from the source, the facies are lenticularly distributed gravity-flow

Fig. 8. Lateral extent of Xigaze Group submarine fan megasequences. Geological background map as in Fig. 2. Note the axis of the synclinorium. 1. Megasequences and numbers; 2. megasequence MS0; 3. megasequence MS1; 4. megasequence MS2; 5. megasequence MS3; 6. megasequences MS2 and MS3; 7. megasequence MS4; 8. megasequences MS3 and MS4; 9. megasequences MS2, MS3 and MS4; 10. megasequence MS5; 11. axis of synclinorium; 12. village and town; and 13. localities of supply channels (after Einsele et al., 1994); other symbols as in Fig. 2A. C. Wang et al. / Gondwana Research 22 (2012) 415–433 423 sandstone, forming lenses less than 20 m long and representing chan- channel is about 16 km×8 km west of the town, and about 8 km to nelized turbidities of facies B. This facies was deposited on the north- the east. The total thickness of the channel system is about 1250 m, ern slope of the accretionary wedge in the southern XFB. Orbitolinia with a single channel fill up to 100 m thick, making it one of the larg- fragments found on both the northern and southern flanks of the syn- est submarine fans reported in geological history (Shanmugam and clinorium and associated late Albian ammonite Neophlycticears spp. Moiola, 1985). 7 km SSW of Gyangqingze village (Wiedmann and Dürr, 1995) indi- Five channel systems represented by sandstone–conglomerate cate that sub-megasequence MS11 was deposited during the late bodies were identified in the Xigaze region: Quarry Ridge, Minor Albian, probably up-to the Cenomanian. Ridge, Castle Ridge, Monastery Ridge, and West Ridge (Fig. 9A). Litho- facies in Quarry Ridge change upwards from abundant channel sand- 6.1.2. MS12, overbank facies sub-megasequence stone–conglomerate to interchannel fine-grained sediment deposited MS12 is dominated by fine-grained, non-channelized sediments on the lower slope: channel sandstone–conglomerate, transitional with many small-scale coarse trough fills (facies B), and lacks the fine-grained intrachannel sediment, sandstone and shale between large-scale channels of MS2. This implies that there was no point- mid-fan and lobe, to middle-outer fan sediment. The Minor Ridge sys- source supply from the continent during formation of MS12, which tem comprises channel sandstone–conglomerate and interchannel was related by Dürr (1993) to an infant submarine fan stage. sediment was deposited on the slope by episodic supply from the A characteristic feature of this sub-megasequence is that no coarse shelf. The Castle Ridge system appears to be a channelized sandstone– clasts are observed on either the southern or northern flanks. Both fa- conglomerate composition overlain with an uneven surface by inter- cies D and G occurring between Naxia and Kardoi villages on the channel slope sediments (Figs. 9B, 10); the Monastery Ridge system is southern flank of the synclinorium are comprised by fine-grained tur- composed by superimposed channel fills and slope facies, and the biditic sandstones and pelagic sediments, indicating a relatively sta- West Ridge system consists of two channel-filled sequences. ble tectonic period. A slump layer approximately 50 m thick appears in the upper part of facies D. Slumps, sandy pillows and convolutions 6.2.2. Channel system of Kardoi submarine fan associated with this layer suggest deposition on the lower-slope. The A 3 km-wide channel sequence of the Kardoi Channel system ex- slumped layer is overlain by a calcareous layer 34 m thick, which sug- posed near Kardoi, south of the synclinorium, is the only evidence gests a lower terrigenous supply due to rising sea level. The MS12 is of this system deposited in a distal basin environment. They are cov- truncated by the channel conglomerate of the MS2. Facies F is also ered by Quaternary sediments. Despite that, two sets of channel sand- found in the upper sub-megasequence MS12 on the southern flank stone and conglomerate sequences can be distinguished as (Fig. 7), differing from the MS11 in that it is not associated with chan- constructing the North and South Kardoi Ridges. neled facies. The gravel within this facies is mostly angular and of var- The South Kardoi Ridge is composed of channel facies approxi- iable composition, indicating a proximal source from the slope apron mately 82 m thick, displaying fining-upward and thinning-upward during earlier basin development. Facies F outcrops are locally found strata. Six superimposed channel sandstone–conglomerate bodies in MS12 throughout the XFB. On the northern flank, MS12 is charac- represent six incisions, one of which is 2 m deep. A lateral progression terized by the presence of overbank and interchannel facies, that is, was observed in the first channel body present in the lower part of by facies C and D comprising graded beddings of centimeter-thick the channel sequence, indicating eastward migration of the channel fine-grained sandstone/siltstone and shale (Tc-e or Te-f). These two axis. A gravel layer partly composed of boulders was found at the facies extend laterally for up to 100 m. Locally, medium-thick base of the channel system. The largest boulder is about 1.3 m in di- (10–100 cm) coarse greywackes with suspended muddy gravels ameter, with flute casts up to 0.4 m wide and 0.15 m deep. Flute occur below facies C, which could be misinterpreted as either wide casts and the imbrication orientation of the gravel indicate deposition channel or channelized high-density turbidity. However, we have by a SW-flowing paleocurrent. not yet found in the field any main channel or supply channel for sed- It has been puzzling that the sandstone and conglomerate of the iments in the MS12. We thus interpret this facies association as over- South Kardoi Ridge are located within the distal basin lithofacies. bank and interchannel facies, with the channels being small, similar Two possible explanations can be suggested: 1) the submarine fans to those of the Eocene Hecho Group in the Pyrenees (Mutti, 1977; were built directly onto the basin facies when the sea level dramati- in Geding by Dürr, 1996). cally fell below the shelf break, resulting in exposure of continental MS12 is dominated by fine-grained, non-channelized sediments shelf; and 2) the proximal channel coarse sediments directly over- with many small-scale coarse trough fills (facies B) but lacking the lapped on the distal basin facies due to basinward migration of its large-scale channels of MS2. This implies that there was no point- depocenter. We prefer the first explanation, which implies that the source supply from the continent during formation of MS12, which basin center was located farther to the south and close to the South corresponds to an infant submarine fan (Dürr, 1993). Kardoi Ridge. Presence of the planktonic foraminifera R. cushmani at the top of MS12 (Wan et al., 1998) indicates a Cenomanian age for MS1 and MS2. 6.3. MS3, mature submarine fan megasequence

6.2. MS2, channeled submarine fan megasequence The MS3 megasequence is present on both flanks of the synclinor- ium. At the southern flank it merges with the MS4 between Kardoi MS2 is easily recognized in the field and distinguishable from the village and Sino-Nepal highway, where it is very difficult to separate underlying MS1 in the XFB (Fig. 7), being very thick and dominated them (Fig. 8) apart; and at the northern flank, MS3 becomes narrow, by coarse-grained terrigenous clastics. In the field it is recognizable incorporating MS4 to the west of Geding village. by the strata forming high, steep relief. Five sets of channel systems This megasequence is dominated by middle-outer fan facies B and at 2 to 10 km spacing were identified by mapping and remote- G, with a few occurrences of basin facies C and D. It represents an as- sensing image interpretation within a west–east oriented zone semblage of interchannel, overbank and channel sediments similar to 120 km long. The MS2 in the Kardoi and Xigaze submarine fan sys- MS11-but, despite these similarities, the style of channel superimpo- tems is described in detail below. sition is different, indicating northward source of the sediments. The lithofacies differ between the southern and northern flanks 6.2.1. Channel system of Xigaze submarine fan because of the difference in the distance from the source. In the Outcrops of the channel system of the Xigaze submarine fan are north, MS3 facies change from retrogradation to progradation: facies mainly present 10–20 km west and east of Xigaze town. The widest B (branched channel) and facies C (lobe) grade into thinly bedded, 424 C. Wang et al. / Gondwana Research 22 (2012) 415–433

Fig. 9. A. Channel systems in MS2 of lower Ngamring Formation west of Xigaze town. 1. Rhythmic alternation of sandstone and shale; 2. channel conglomerate; 3. town district; 4. highway; and 5. Quaternary. B. Sketch of the lithology at Castle Ridge at eastern terminal, NW corner of Xigaze town. Note the slumped sandstone block. The largest cobble is >80 cm in diameter.

fine-grained turbidite-shale and medium-bedded lobe sandstone, MS4 is characterized by pelagic sediments, which can be subdivided with thinly bedded pelagic limestone at the top. In the south, MS3 into three sequences. The lower sequence is dominated by facies B consists mainly of facies C and D (outer fan-basin). Towards the top branched channel sandstone and facies C lobe sandstone (Fig. 11). To- of the megasequence, sediments have a pelagic character and thinly ward the south the sediments become progressively finer, as noted in bedded limestone occurs in the Kardoi profile. southern Xigaze (Nanshan). In the middle of MS4, the strata are grading abruptly to pelagic carbonate, showing as a pale stripe on remote- sensing images and indicating a scarcity of outer fan-basin facies C 6.4. MS4, highstand submarine fan megasequence and D between the lower and middle sequence, perhaps due to eustatic sea level change. In the upper sequence, the strata are typical pelagic This megasequence is well exposed on the northern flank, but basin facies, that is, rhythmically interbedded black/siliceous shale, sili- merges with the MS3 on the southern flank, partly as result of ceous marlstone and siltstone, alternating on a centimeter scale, and faulting. enclosing few siliceous concretions. The lack of carbonates suggests C. Wang et al. / Gondwana Research 22 (2012) 415–433 425

Fig. 10. Lithostratigraphic column of the sedimentary sequence at the Castle Ridge in the MS2. deepening of the basin, with deposition perhaps occurring below calcite 7. Basement of XFB compensation depth (CCD). Previous geologic studies suggested that the Xigaze ophiolite 6.5. MS5, outer fan and basin megasequence forms the basement of the XFB (e.g. Einsele et al., 1994; Wang et al., 1999). Results of new studies concur with this interpretation, but in- Megasequence MS5 occurs in the core of the synclinorium (Fig. 8), dicate that their age, nature, relation to the Neo-Tethys ocean crust, is where it comprises the following three sequences: more complex and less certain. For the convenience of discussion, the The lowest sequence, about 240 m thick, consists of facies B and C central segment of the YZSZ will here be called the Xigaze ophiolitic massif, to include the Dazhuka, Bailang, Geding, Lhaze, Ngamring, sandstones. It can be further subdivided into three sub-sequences, Sangsang, and Saga submassifs to the west. the lowest of which contains many coarsening-upwards and In the 1980s, the Xigaze ophiolitic massif was thought to be a rem- thickening-upwards cycles, with poorly sorted mostly matrix- nant of oceanic lithosphere formed at the spreading center (e.g. Nicolas supported sandstones, containing muddy intraclasts. The middle et al., 1981; Allègre et al., 1984; Girardeau et al., 1985a,b; Girardeau and sub-sequence contains beds that are fining-upwards and thinning- Mercier, 1988; Pearce and Deng, 1988) but later studies have indicated upwards, resulting from retrogradation and abandoned of the chan- that the Xigaze ophiolites were formed in the supra-subduction zone nels. The uppermost of the sub-sequences is composed of dark shale (SSZ) (e.g. Aitchison et al., 2000; Wang et al., 2000; Huot et al., 2002; interbedded with facies E (thin sandstone), representing overbank Hébert et al., 2003; Xia et al., 2003; Ziabrev et al., 2003; Dubois-Côté or basin-outer fan sediments. et al., 2005; Dupuis et al., 2005; Guilmette et al., 2008, 2009; Bédard The middle sequence is about 55 m thick and is comprised by facies et al., 2009). Recent studies note that the Xigaze ophiolitic massifs B sandstones. Sandstones in the channels have an erosional base, have different ages and tectonic settings. In the Dazhuka submassif, a zircon U–Pb age of 126±1.5 Ma with the source being proximal to the north. reported from quartz diorite (Malpas et al., 2003) is consistent with In the uppermost sequence, facies C and D sandstones are interca- the late Barremian–late Aptian radiolarian age (Ziabrev et al., 2003). lated with shale. The occurrence of subfacies Tc-e indicates devel- Geochemical studies of the same submassif indicate that it was opment of an outer fan facies. Foraminifera microfossils (Wan formed in the SSZ environment (Xia et al., 2003). Subsequent studies et al., 1998) suggest Coniacian age for MS5, which thus represents of mafic rocks support this conclusion, and it was proposed that the the youngest deep-sea flysch deposit of the Ngamring Formation mafics were formed in the back-arc basin spreading center (Dubois- of the Xigaze Group. Côté et al., 2005). 426 C. Wang et al. / Gondwana Research 22 (2012) 415–433

Fig. 11. Photograph showing typical sequence of lobe facies in MS4 and MS5, 8 km SW of Xigaze town. a, shale intercalated with thin beds of fine-grained sandstone and siltstone; b, poorly sorted turbidite or rhythmic alternation of sandstone, siltstone, and shale. The circled Tibetan prayer flag on the top of the mountain is about 3 m high.

In the Bailang submassif, 40Ar/39Ar dating of hornblendes from Zircon SHRIMP U–Pb dating of diabase in the Sangsang submassif three amphiobolite samples yielded ages of 123.6±2.9 Ma, 127.7± shows that it was formed around 125.2±3.4 Ma (Xia et al., 2008). 2.2 Ma and 127.4±2.3 Ma (Guilmette et al., 2008, 2009), in good Geochemical studies of mafic rocks from the Saga and Sangsang sub- agreement with the zircon SHRIMP U–Pb age of the gabbro (125.6± massifs indicate that they are similar to back-arc basin basalts, with 0.8 Ma) (Li et al., 2009). Guilmette et al. (2008, 2009) interpreted small negative Nb–Ta anomalies (Bédard et al., 2009). In combination these ages as indicating the initiation of intra-oceanic subduction. This with the composition of the peridotites, Bédard et al. (2009) pro- conclusion is supported by geochemical studies of the ultramaficand posed that the mafics were formed in an intra-oceanic SSZ. mafic rocks that indicate the presence of a north direction Early Creta- In summary, the above data show that the intra-oceanic subduction ceous intra-oceanic subduction system within the Neo-Tethyan Ocean zone was located within the central segment of YZSZ during the late (Huot et al., 2002). Early Cretaceous, which was temporally formed as the basement of In the Geding submassif, a gabbro yielded zircon U–Pb age of 128± the XFB. However, the western and eastern segments of YZSZ are 2Ma(Wang et al., 2006). This result was interpreted as the age of sea- more complex. In the eastern segment, the ages of rocks from the Luo- floor spreading in the Neo-Tethys. However, the mineral chemical data busa and Zedang massifs range from 177 to 152 Ma (McDermid et al., in contrast indicates that the Geding ophiolite was generated in an SSZ 2002; Zhou et al., 2002; Zhong et al., 2006), making them older than setting (Hébert et al., 2003). Geochemical studies of the Geding basaltic the Xigaze ophiolites. This may explain why there was not a similar rocks suggest that they represent a fragment of mature mid-ocean- forearc basin developed over there. In the western segment, the ages ridge basalt (MORB) lithosphere that has been modified in the SSZ en- of the ophiolitic rocks from Zhongba (Dai et al., 2011b), Xiugugabu vironment (Chen and Xia, 2008). (Wei et al., 2006), and Yungbwa (Li et al., 2008a,b,c; Bezard et al., In the Ngamring submassif, the elemental and Sr–Nb–Pb isotopic 2011), show that most of them were formed around 120–130 Ma. The geochemical data of the mafic rocks show an affinity of N-MORB occurrence of the Devonian maficrocks(Zhou, 2002; Dai et al., 2011a) without Nb–Ta anomalies, indicating a mid-ocean-ridge or mature that were inferred as the remnant of Paleo-Tethys indicate a much back-arc tectonic setting (Niu et al., 2006). more complex geologic evolution of the ophiolites in YZSZ.

Fig. 12. Map of paleocurrent flow directions in the Ngamring Formation flysch. 1. Flute cast; 2. groove cast; 3. current ripple; 4. over-banking direction; 5. pebble orientation (uni-/ non-polar); 6. numbered data points: 1–48 for data based on the present study, and 49–94 for data from Dürr (1993); and 7. village and town. C. Wang et al. / Gondwana Research 22 (2012) 415–433 427

8. Provenances of XFB sediments 100000

Volcanic pebbles Sandstones 8.1. Paleocurrent pattern

Paleocurrent analysis provides important information about prov- enance, geometry of the depositional units, and depositional process- Gangdese Volcanic rocks es. During field work, paleocurrent directions were obtained at 48 Within-plate lavas localities (Table S1). Another set of 46 paleocurrent data was pub- MORB

Ti 10000 lished by Dürr (1993). Both of these sets of data demonstrate a pre- dominantly southward paleocurrent flow and subordinately westward and eastward directed flows (Fig. 12). This has previously been interpreted as indicating that the XFB was chiefly supplied by Volcanic arc lavas sediments from the GMA located to the north, and partly from the ophiolite belt to the south (e.g. Dürr, 1993, 1996). Our data supports this conclusion. For example, much of the ophiolitic detritus-basic volcanic rocks, chert, and calcite- and epidote-replaced pyroxenes 1000 from the MS11 in the south were founded in the sandstones in the 10 100 1000 Zr Naxia profile where the paleocurrent flow directions are NNE 10°. However, another group of paleocurrents in MS1 indicated an east- Fig. 13. Ti–Zr covariation diagram for the volcanic pebbles and sandstones from the fl ward ow direction (Fig. 12), and that at both the southern and Xigaze Group superimposed on fields of Mid-Ocean Ridge Basalt (MORB), Volcanic- northern flanks. This suggests a change in flow direction after current arc lavas and Within-plate lavas (Pearce et al., 1981). All the Xigaze volcanic pebbles reached a southerly point, where it might have been obstructed by ei- and sandstones show compositions of volcanic arc rocks, resembling those of Jurassic ther morphological elevation on the seafloor, or by the presence of an to Cretaceous Gangdese volcanic rocks. Data sources of andesite volcanic pebbles and fl sandstones are from Dürr (1996), Gangdese volcanic rocks (Sangri Group and Yeba accretionary wedge that constrained the current to ow in the axial Formation) from Dong et al. (2006), Kang et al. (2009), andZhu et al. (2008a,b, direction of the basin. The observed eastward paleoflow supports 2009a,b). basin asymmetry and a W–E axial direction of the XFB. Paleocurrent flow direction was unified during MS2, when the flow was southward, with minor SW flows, indicated by flute cast and gravel the Ngamring Formation (Wu et al., 2010). The Yeba Formation is imbrications in the Kardoi and Naxia area (Table S1, Fig. 12). This can be characterized by the rare occurrence of intermediate rocks (Zhu et attributed to the complete infilling of the basin, causing sediment to by- al., 2008b) while the Sangri Group is dominated by the presence of pass it into the trench, and therefore indirectly implying that the basin andesites and andesitic breccias with dacites (Dong et al., 2006; subsidence center had migrated farther to the south. Kang et al., 2009; Zhu et al., 2009a). This means that the sedimenta- After MS2 deposition, both eastward and westward paleocurrent ry–volcanic sequence of the Late Jurassic–Early Cretaceous Sangri flow are found in MS3–MS5, as well as the southward and northward Group in the southern GMA was the primary source of andesitic ma- flow. The highly variable paleocurrent flow directions can be terial from the Ngamring Formation flysch. This conclusion is consis- explained by source supplies from the south and the north super- tent with the recent recognition of Wu et al. (2010), who identified a posed on westerly and easterly sources, in a similar fashion to the principal magmatic period between 130 Ma and 80 Ma (peaking at Great Valley forearc basin, in western USA (e.g. Ingersoll, 1979; around 110 Ma) and a subordinate period between 190 Ma and Moxon, 1988, 1990; Williams, 1997). 150 Ma (peaking at 160 Ma) from detrital zircons from sedimentary rocks in the XFB. 8.2. Sources of andesitic component The explanation could be the presence of older volcanic strata south of the GMA, as represented by the Yeba Formation and Sangri Petrological texture, mineralogical composition and geochemistry Formation. Macrofossils (Yin et al., 1998) and isotopic chronology in- (Fig. 13) have shown that volcanic detritus in the Ngamring Forma- dicate that the Yeba Formation is early to middle Jurassic (e.g. Geng et tion was mainly derived from andesites (Dürr, 1996). The shortage al., 2006; Zhu et al., 2008a,b, 2009a,b) and that the Sangri Formation of rhyolitic and dacitic detritus indicates that the source of the detri- is late Jurassic to early Cretaceous (Dong et al., 2006). These ages tus could have been remelting of originally basaltic mantle (e.g. lower match the two principal magmatic periods obtained from detrital zir- crust or subducted oceanic lithosphere), but not from remelting of the con studies of the Ngamring Formation (Wu et al., 2010). The Sangri middle to upper crust (Gill, 1981), which is dominated by SiO2-rich Group is dominated by andesitic lava and andesitic breccia with magma, as seen in northern Chile, northern Peru, and western Amer- dacites, indicating that their origin is related to arc-type volcanic ica (Schmincke, 1986). Previous studies indicate that a thinned or rocks formed in the lower crust during the Neo-Tethys oceanic crust transitional continental crust could have existed in the southern subduction (Dong et al., 2006; Kang et al., 2009). The above ages of GMA during the Early Jurassic through the Early Cretaceous (e.g. the Yeba and Sangri formations strongly suggest that the two Meso- Dong et al., 2006; Geng et al., 2006; Zhu et al., 2008a,b, 2009a,b, zoic sedimentary–volcanic sequences in the southern GMA were the 2011), where andesitic volcanic rocks could have been formed (e.g. source of andesitic material for the Ngamring flysch. the Sangri Group andesites, Zhu et al., 2006, 2009a) and thereafter The composition and geochemistry of the sediments also show eroded. that the andesitic and other volcanic lithiclastics are common in the What could be the sources of the andesitic components in the lowest sequence of the Ngamring Formation, but their occurrence de- Ngamring Formation? A probable derivation is that they were creases upwards. A possible explanation is that as increasing amounts sourced from older volcanic strata in the GMA, as represented by of terrigenous materials were transported into the XFB, older rocks the Yeba Formation and Sangri Formation. Macrofossils (Yin et al., were eroded as the GMA was denuded. As a result, the younger volca- 1998) and isotopic chronology indicate that the Yeba Formation is nic rocks were the source for the lower part of the sequence, while early to middle Jurassic (e.g. Geng et al., 2006; Zhu et al., 2008a,b, the upper part of the XFB strata was supplied from progressively 2009a,b) and that the Sangri Formation is late Jurassic to early Creta- older rocks during unroofing of GMA. This reversal is supported by re- ceous (Dong et al., 2006; Zhu et al., 2009a). These ages match the two cent results of detrital zircon U–Pb and Hf isotopic analyses. Most zir- principal magmatic periods obtained from detrital zircon studies of con ages from the lower flysch of the Ngamring Formation are 428 C. Wang et al. / Gondwana Research 22 (2012) 415–433

characterized by positive εHf(t) values similar to those of magmatic sediments from both the carbonate platform and GMA in the north zircons from the Gangdese batholith and from the upper Ngamring and from the Xigaze ophiolite and the accretionary wedge in the and Padana–Qubeiya formations. The younger sequences record ei- south. ther pre-Mesozoic U–Pb zircon ages or negative εHf(t) values (Wu Frequent debris of orbitolinid-bearing limestone found in the ba- et al., 2010). sinal Chongdoi Formation indicates that erosional clastics from the northern carbonate platform reached as far as the southern XFB.

9. Discussions Such interpretation is supported by positive initial εHf(t) values of zir- cons derived from the GMA (Wu et al., 2010). The east–west paleo- 9.1. Sedimentary evolution of XFB current direction (Fig. 12) indicates that the accretionary wedge formed a morphologic high on the sea floor and that the subsidence The forearc basins develop as part of the trench–arc system above center was located south of the XFB. the subduction zone. Dickinson and Seely (1979) classified the fore- During the middle-late Cenomanian, the XFB changed from a re- arc basins into five types: intramassif basin, residual basin, accretion- sidual basin to a composite basin, and the accretionary wedge also be- ary basin, constructed basin, and composite basin. Combination of the came a part of the basement of the basin. At this time, large revised stratigraphic framework and of the basin basement, lithofa- submarine fan channelized systems developed in the deep-sea basin cies architecture (megasequences), and provenance study have (Figs. 7, 8, 9), while the carbonate platform was exposed to subaerial resulted in our recognition of four distinct stages in the XFB develop- conditions and eroded when sea level dropped sharply. Subsequently, ment: basement formation, residual forearc, composite forearc, and large amounts of sediment were brought up to the accretionary terminal forearc. The basement formation period is represented by wedge, and in places channels approached the trench. This is evident the Sangzugang Formation which was deposited on a narrow shelf by the occurrence of only two sets of channel fill sequences deposited at an active continental margin. Residual and composite forearc in the distal basin setting where they directly downlapped onto pe- basin periods are represented by the Ngamring Formation, compris- lagic limestone in the Kardoi region. During the stage, the direction ing five successive megasequences of the submarine fan. The terminal of sediment supply to the basin changed from lateral to longitudinal, forearc period is represented by the Cuojiangding Group. The change attributable to the tectonic barrier caused by emplacement of the ac- of the forearc basin type could indicate a linkage to the termination of cretionary wedge and trapping sediments in the XFB. This tectonic ac- the subduction of the New-Tethys oceanic lithosphere. tivity resulting from ocean crust subduction extended farther to the As discussed in Section 8.2, andesitic clastics are common within north, as evidenced by folding of the Takena Formation in the Lhasa sediments of the XFB, indicating that partial melting of lower crust retroarc basin (Coulon et al., 1986). or subducted oceanic lithosphere should have been processed in its The composite forearc basin matured during the early-middle Tur- north before the XFB formation (SU1, Fig. 14A). Such melting event onian (Fig. 7) when MS3 was built by middle and outer fan facies. could be attributed to the subduction of the Neo-Tethyan oceanic lith- During this time period sediment supply decreased. On the north of osphere that resulted in extensive volcanism occurred in the southern the basin, the megasequence is formed by branched channel sand- margin of the Lhasa terrane (Zhu et al., 2006, 2009a,b, 2011), as stone of facies B and lobe sandstone of the lower part of facies C, grad- represented by the Sangri Group that could have provided andesitic ing upward into fine-grained turbiditic and lobe sandstones; the top materials for the XFB. By the standing subduction, an intra-oceanic strata contain thin pelagic limestone layers, indicating a pelagic envi- system (including the northward-subducted zone, oceanic island- ronmental setting. In the south, MS3 is dominated by facies C lobe arc and intra-arc basin) was formed in south of the open Neo- sandstone and facies D outer fan-basin fine-grained sediments. Tethys ocean (SU2, Fig. 14A, for details refer to Hébert et al., 2011), During the middle-late Turonian, the composite forearc basin was during which the SSZ-type Xigaze ophiolites were generated. Accord- constructed by MS4 (Fig. 7) consisting of branching channel lobe sand- ing to the ages of the Xigaze ophiolite (see in the Section 7) and XFB stone and abundant basin facies. At the top of this megasequence, light- (see in Section 5.1), it is reasonable to conclude that the subduction gray pelagic limestone provides significant late-Turonian marker for may have terminated about 120 Ma ago. correlation between both flanks of the synclinorium. The composite In comparison to the older normal oceanic crust, the SSZ-type forearc basin development stage weakened during the Coniacian. It is Xigaze ophiolite has a light density, for which it would have never represented by MS5, which is comprised by the outer-fan and basin been intermixed with the mantle lithosphere. Therefore, it was facies (Fig. 7). obducted toward the south and became the basement of the XFB in As revised in Section 5, the change from the deep-sea facies to the late Early Cretaceous (SU2, Fig. 14B and C). At the same time, shallow-sea facies lies between the Ngamring Formation of the the Sangzugang Formation carbonates were deposited on a narrow Xigaze Group and the Padana Formation of the Cuojiangding Group. continental shelf. During the same time, the residual forearc basin That means either the age at the base of the Padana or the age at was formed by the initiation of the typical arc–trench system of the the top of the Ngamring represent the termination of the deep-sea YZSZ. deposition. This time would mark either the termination of the com- The residual forearc basin represents the earliest stage of the XFB posite forearc basin or the initiation of the terminal forearc basin. The development, represented by the late Albian–mid Cenomanian revision of stratigraphy (details refer to Section 5) shows that the Ngamring Formation flysch (MS1) and by the Chongdoi Formation. Ngamring Formation is of late Albian–Coniacian age and the Padana During this period, submarine fan and slope facies spread out into Formation is of Santonian age, indicating that deposition in the XFB the basin. As described in Section 6.1, the evolution of the megase- changed from deep-sea facies to shallow-sea facies at ca. 86 Ma. quence MS1 can be subdivided into two stages (Fig. 7): in the first The terminal sedimentary succession of the XFB is represented by stage, MS11 is characterized by slope facies mudrock and siltstone in- the Cuojiangding Group comprised by Late Cretaceous–Early Paleo- tercalated with debris-flow breccia and gravelly mudrock, dominated gene shallow marine carbonates and terrigenous rocks that crop out by non-channelized debris-flow sediment in apron facies in the north west of Ngamring. This shallowing-up sequence could be attributed and by rare lenticular and sheet turbidite sandbodies of facies B. Dur- to the decreasing accommodation space for basin filling material ing the second stage, MS12 was formed by overbank sediments, and it (Fig. 14C). The hypothesis is supported by the conformable contact features non-channelized fine-grained sediment facies, during which between the Xigaze Group and the Cuojiangding Group (Fig. 6). supply channels were not incised for transportation of detritus from Relatively, it is simple to date the termination of the XFB. The Gya- either the GMA or the carbonate platform on the narrow margin, in- laze Formation at the top of the Cuojiangding Group is generally dicating no point sources, despite the fact that the XFB contains thought to be the marine sediment record of the XFB. We then C. Wang et al. / Gondwana Research 22 (2012) 415–433 429

Fig. 14. Model showing the evolution of the XFB and the middle YSZS. A. First (early) subduction (SU1) in the earliest Barremian (~130 Ma), indicating possible bi-directional sub- duction of the Neo-Tethys oceanic crust and an intra-oceanic obduction (OB) before the formation of XFB, resulting in spreading of southern back-arc and the forming of the obducted Xigaze ophiolites. Note the potential overriding thrust in northern back-arc and ophiolitic mélange in forearc basin. B. Second subduction (SU2) and XFB formation during the early Albian. New typical trench, arc, and forearc basin were formed by persistent subduction, during which the Xigaze ophiolite was obducted onto the forearc area of the Gangdese arc and became the basement of the XFB. Note three contemporaneous geological processions: the retro-arc basin had already started in the northern Gangdese arc (Leier et al., 2007), the shallow carbonate platform (Sangzugang Formation) had been built up on the narrow shelf in the southern Gangdese arc, and the prism had been added to the southern edge of the XFB blocking the southward transportation of clastics into the trench. C. Complete filling of the XFB in the Late Cretaceous–Early Paleogene (86–50 Ma) by shallow sea Cuojiangding Group sediments. The accretionary wedge may be itself raised above sea level after the transition of the deep-sea flysch to shallow-sea facies, being a potential source area of the XFB, although not the main source. D. Present structure of the YZSZ, showing the locations of the YZSZ and the accretionary wedge after the collision (50–45 Ma) of India with Eurasia. Note width of accretionary wedge, the inferred localities of MORB ophiolites, and the change in source of volcanic rocks to both volcanic and batholithic rocks.

suggest that the termination of the Gyalaze Formation deposition for publication), and is regarded as the site of the collision between represents the termination of XFB. As revised in Section 5.2, the the Indian and Eurasian continents (e.g. Gansser, 1977, 1980; Gyalaze Formation is of Selandian–Ypresian age (top age, ca. Shackleton, 1981; Tapponnier et al., 1981). Similar to the XFB, the 50–49 Ma), so we propose that the XFB terminated at about 50–49 Ma. Indus Forearc Basin developed in the Indus Suture Zone. Starting on As a whole, the sedimentary succession of the XFB represents a the Ladakh magmatic arc in the mid-Cretaceous, development of the typical composite basin (Dickinson, 1995). The XFB developed in Indus Forearc Basin terminated in the early Eocene, as marked by an the forearc area with sediments extensively supplied from the conti- intermontane molasse (Van Haver, 1984; Garzanti and Van Haver, nental magmatic arc. The sedimentary succession of pelagic radiolar- 1988). It is obviously instructive to compare the two. ian chert, shale and microfossil-bearing tuff in the lowest layer, flysch As is evident from their present structure, both are asymmetrical, submarine fan facies in the middle, and costal-shallow facies in the with the one exception of near-symmetry appearing in the eastern upper strata documents the evolution of the basin subsidence center. XFB. This could have resulted from thrusting in collision and/or from different thicknesses in the original deposition. 9.2. Comparison between Indus Forearc Basin and XFB A complete terrigenous to carbonate succession 1500 m thick was deposited in the southern Indus Forearc Basin, similar to the XFB. Fa- In Ladakh–Indus, the suture is named the Indus Suture Zone. Con- cies change through time show similar sequences of shallowing- sidered together with the YZSZ, it was named the Indus–Yarlung upwards megasequences (turbidite to nummulitic limestone), and al- Zangbo Suture Zone (Searle et al., 1997; Henderson et al., accepted luvial to deltaic sediments occur in both forearc basins. Platform 430 C. Wang et al. / Gondwana Research 22 (2012) 415–433 deposition began with orbitolinid and rudist carbonates at almost the characterized by shallow-marine mixed terrigenous and carbon- same time, shown by the Khalsi Formation in Indus and the Sangzu- ate rocks. The gradual upward transition from the deep-sea flysch gang Formation in Yarlung Zangbo. facies of the Ngamring Formation into the shallow marine facies of Further, the terrestrial coarse successions above the forearc basin the Padana Formation indicates progressive basin filling rather that started to develop following the early Eocene show a great deal than tectonic activity, which is tentatively dated at the boundary of similarity in both regions. A set of conglomerates and sandstones (ca. 86 Ma) of the Santonian and Coniacian . 250–1600 m thick, nonconformable on the marine Gyalaze Formation 3) The deep-sea fan represented by the Ngamring Formation consists limestones and sandstones of the top XFB in Cuojiangding Lake, of sandy turbidites, channeled sandstones and conglomerates northern Zhongba, correlate with those of the Hemis conglomerate with a few thin pelagic limestone and calcareous shale. Sedimen- in the Indus basin. It is interesting that the terrestrial succession tary beds are arranged into five megasequences (MS1–MS5), com- was transported westward in the Indus basin, mostly fed from the prised by the early submarine fan, channeled submarine fan, northeast (Frank et al., 1977; Baud et al., 1982) and became more dis- mature submarine fan, highstand submarine fan, and outer fan tal toward the west and south. It is inferred that this change repre- and pelagic sediments. MS1 has been further subdivided into the sents the initiation of collision between two continents and implies slope apron facies sub-megasequence (MS11) and the overbank an oblique collision from east to west; however, much work needs facies sub-megasequence (MS12). Megasequence MS2 can be sub- to be done in the future to test this hypothesis. divided into the channel system of the Xigaze submarine fan and There are two distinctions between the two basins. One is that the the channel system of the Kardoi submarine fan. position of shallow water carbonate deposition is different: the Khalsi 4) Former data plus ours show that the paleocurrent flows were east- Formation is located between the Indian Ophiolite Complex and Indi- wards and westwards during the early and late stages of basin de- an Forearc Basin, whereas the Sangzugang Formation is positioned on velopment and northwards and southwards during its middle the northern margin of the XFB. The other difference is the timing of stage, indicating typical forearc basin sedimentation with longitu- the beginning of the deep-sea flysch; the deposition of the Ngamring dinal supply and lateral filling. This change in directional sediment Formation started earlier than the late Albian in XFB, and the Tar For- supply to the basin can be attributed to the tectonic barrier caused mation (deep-sea turbidite) began in the Maastrichtian or even in the by emplacement of the accretionary wedge and trapping sedi- Paleocene. This distinction is profound, and implies either diachrone- ments in the XFB. ity or obliquity of the Neo-Tethyan oceanic crust subduction. This is 5) The provenance analysis supports a new interpretation that the also evident in the Indus Basin. Shallow-sea nummulitic limestone andesitic detritus of the XFB may have been derived from the was deposited in the west followed by deltaic succession, with Jurassic–Early Cretaceous arc-volcanic Yeba Formation and Sangri deep-sea turbidite occurring in the south-east (Garzanti and Van Group south of the GMA, but not from the batholith or/and Paleo- Haver, 1988). gene volcanic rocks in GMA, implying that obduction of the SSZ Xigaze ophiolite may have been directed toward to the south. 10. Summaries and conclusions 6) Four distinct phases of the XFB development we recognize are the basement formation, residual forearc, composite forearc, and ter- The XFB resembles a typical forearc basin in its filling pattern and minal forearc. The main basement of the XFB is formed by the basement lithology (Dickinson, 1995; Dickinson and Seely, 1979). The Xigaze ophiolite, formed prior the Aptian (ca. 120 Ma), with the basin has a shallowing-upwards sequence of pelagic and turbiditic coeval counterpart on the continent being the carbonate Sangzu- deep-sea facies, from near-shore shelf through to continental facies, gang Formation. The residual forearc may have initiated in the dated from the late Aptian through the Ypresian (ca. 120–49 Ma) earliest Aptian (ca. 125/120 Ma), as represented by sedimentary with a duration of about 71 Myr. The basin is narrow (20–22 km deposits of the Chongdoi Formation. Then the residual forearc wide in the north–south direction) and 550 km long in the east– and composite forearc developed, represented by the late Albian– west direction. The sedimentary fill is asymmetric. The N–S shorten- Coniacian flysch megasequences of the Ngamring Formation. The ing ratio has been estimated as ≥31% for the Gyabulin Formation terminal forearc continued through the late Late Cretaceous–Early and ≥65% for the Ngamring Formation (Ratschbacher et al., 1992), Paleogene (ca. 86–49 Ma), represented by the shallow marine which makes the dimensions of the original XFB were ≥71 km wide Cuojiangding Group, and would have not terminated until the and 50–100 km long, thus comparable in size to a large forearc latest Ypresian (ca. 50–49 Ma) by the Gyalaze Formation. basin (Dickinson, 1995; Dickinson and Seely, 1979). 7) The studies of stratigraphy, lithofacies and provenance as well as The combination of the revised stratigraphic framework, reinter- comparison to other typical forearc basins not only can contribute pretation of lithofacies architecture, provenance, and basin basement, to the understanding of the sedimentary evolution of the XFB, but leads to the following conclusions: also are significant to the evolution of YZSZ. From the records of XFB, we suggest that the Neo-Tethys oceanic crust may have 1) The XFB consists of two main lithostratigraphic associations: the been subducted at two different times and obducted once. Xigaze Group and the Cuojiangding Group. The former group con- sists of the Sangzugang Formation, Chongdoi Formation, and Supplementary material related to this article can be found online Ngamring Formation, with their ages revised as the late Aptian– at doi:10.1016/j.gr.2011.09.014. early Albian, middle Aptian–Albian, and middle Albian–Coniacian, respectively. The thickness of the Ngamring Formation was revised to less than 4000 m on the basis of the structural analyses of studied Acknowledgements profiles. The Cuojiangding Group is composed of the Padana Forma- tion, Qubeiya Formation, Quxia Formation, and Gyalaze Formation, We acknowledge the help of many colleagues in carrying out the ages of which were revised as Santonian, Campanian–Maastritchian, field work: Zhou Xiang, Lu Yan, Wan Xiaoqiao, Chen Jianping, He Danian, and Selandian–Ypresian, respectively. Most of the forma- Zhengwei, M. Dobs, Li Anren, and I. Westlake. G. Einsele and S. Dürr tions have conformable contacts with each other except of the Sang- provided some materials during this study. Comments from R. Hébert zugang Formation, which is disconformable with others. were useful in shaping the manuscript. We also thank Zhu Dicheng 2) The Xigaze Group is dominated by deep-sea fan flysch lithofacies, for constructive suggestions and comments. This project is funded except for the Sangzugang Formation, which is comprised by shal- by National Natural Science Foundation of China (41072075), Nation- low carbonate platform lithofacies. The Cuojiangding Group is al Basic Research Program of China (2012CB822000), 111 Project of C. Wang et al. / Gondwana Research 22 (2012) 415–433 431

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