Tectonic Evolution of Taimyr in the Late Paleozoic to Mesozoic from Provenance and Thermochronological Evidence

Xiaojing Zhang

To Bo and my parents 致万博和我的爸妈

Abstract

The Taimyr Peninsula is a key element in the circum-Arctic region and represents the northern margin of the Siberian Craton. The Taimyr Peninsula is a late Paleozoic fold and thrust belt and preserves late Paleozoic through Mesozoic siliciclastic sedimentary successions and providing an ideal location to investigate the Paleozoic to Mesozoic tectonic evolution associated with the Uralian orogeny, the Siberian Trap magmatism and opening of Amerasia Basin within a circum-Arctic framework. Multiple methods are adopted, including petrography, heavy mineral analysis and detrital zircon U-Pb geochronology for provenance investigation, apatite fission track dating for revealing thermal history and balanced cross section for understanding the deformation style of Taimyr. The results of this thesis indicate that the Late Carboniferous to Permian sediments of southern Taimyr were deposited in a pro-foreland basin of the Uralian orogen during the Uralian orogeny. In the Triassic, the siliciclastic deposits still show a strong Uralian signature but the initiation of Siberian Trap-related input begins to be significant. Erosion of the Uralian orogen has reached a deep metamorphic level. By Late Jurassic and Cretaceous time, the deposition setting of southern Taimyr is an intracratonic basin. Erosion and input from Uralian sources waned while greater input from Siberian Trap-related rocks of the Taimyr region dominated. The Taimyr Peninsula underwent at least three cooling and uplifting episodes: 280 Ma, 250 Ma and 220 Ma, corresponding to the Uralian orogeny, the Siberian Traps and the late Triassic transpression. Keywords: Taimyr, sandstones, provenance, heavy mineral analysis, detrital zircon, apatite fission track, tectonic setting, uplift

Sammanfattning

Tajmyrhalvön är en viktig del i polarcirkelområdet och representerar den norra gränsen av den Siberiska kratonen. Tajmyrhalvön är ett sen-paleozoiskt veckningsbälte som har bevarat sen-paleozoiska till mesozoiska klastiska sediment lagringar. Det är ett idealt område för att undersöka den tektoniska untvecklingen i samband med den uraliska orogenesen, den siberiska trapp magmatismen och öppningen av den amerasiska bassängen inom polarcirkel systemet. De metoder som har använts är petrografi, analyser av tungmetaller och U-Pb geokronologi av detritiska zirikoner för ursprungs undersökning, fissionsspårs datering av apatit för att visa den termala historien och balanserade tvärsnitt för att förstå deformations sättet i Tajmyr. Resultaten i den här avhandlingen visar att de sen karboniska till permiska sedimenten i södra Tajmyr avlagrades i en sedimentär bassäng i samband med den uraliska bergskedjebildningen. Klastiska sediment från trias visar fortfarande en stark uralisk signatur men även inslag av siberiska trapp relaterade sediment börjar bli markant. Erosionen av Uralbergen har nått den djupaste metamorfa nivån. Mellan yngre jura och krita perioden har avlagringarna i södra Tajmyr bildat en intrakratonisk bassäng. Erosionsmaterial med ursprung från Uralbergen minskar medan större mängder material från den siberiska trappen dominerar. Tajmyrhalvön påvisar minst tre upphöjnings perioder med termala variationer: 280 Ma, 250 Ma och 220 Ma, de representerar den uraliska bergskedjebildningen, den sibiriska trapp magmatism och yngre trias transpression.

摘要

泰米尔半岛位于西伯利亚克拉通的北缘，是环北极地区中一个重要的地点。泰米尔半岛是一个晚古生代的褶皱逆冲带，保存了晚古生代到中生代的硅质碎屑岩沉积序列，为我们提供了一个绝佳的在环北极背景下研究与乌拉尔造山带，西伯利亚大火成岩省岩浆作用和美亚海盆打开有关的古生代到中生代构造演化过程。本论文采用了多种研究方法，包括用于物源研究的岩石学，重矿物分析和碎屑锆石 U-Pb 年代学方法，揭示低温热历史的磷灰石裂变径迹定年，以及了解泰米尔半岛构造变形特征的平衡剖面方法。本论文的结果显示南泰米尔半岛的晚石碳纪到二叠纪砂岩是在乌拉尔造山过程中形成的前缘前陆盆地沉积物。在三叠纪时期，硅质碎屑沉积物依旧显示了强烈的乌拉尔造山带的物源特征，然而西伯利亚大火成岩省相关岩石的物源来源开始逐渐增强。乌拉尔造山带的剥蚀此时也达到了其变质岩层位。在晚侏罗纪到白垩纪时期，南泰米尔半岛的沉积构造背景已转化成为克拉通内陆盆地。乌拉尔造山带物源的剥蚀和贡献已经明显减弱，而西伯利亚大火成岩省相关岩石成为主要物源来源。泰米尔半岛经历了至少三阶段的冷却和隆升：280 Ma， 250 Ma 和 220 Ma，分别对应于乌拉尔造山作用，西伯利亚大火成岩省和晚三叠纪走滑挤压作用。

关键词：泰米尔，砂岩，物源，重矿物分析，碎屑锆石，磷灰石裂变径迹，构造背景，隆升

This doctoral thesis consists of a summary, three papers and one manuscript. The papers and manuscript are referred to as Papers I - IV in the text: Paper I: Zhang, X., Omma, J., Pease, V., and Scott, R., 2013, Provenance of Late Paleozoic-Mesozoic Sandstones, Taimyr Peninsula, the Arctic. Geosciences, v. 3, no. 3, p. 502-527.

Paper II: Zhang, X., Pease, V., Skogseid, J., and Wohlgemuth-Ueberwasser, C., 2015, Reconstruction of tectonic events on the northern Eurasia margin of the Arctic, from U-Pb detrital zircon provenance investigations of late Paleozoic to Mesozoic sandstones in southern Taimyr Peninsula. Geological Society of America Bulletin. In Press.

Paper III: Zhang, X., Pease, V., Omma, J., and Benedictus, A., Provenance of Late Carboniferous to Jurassic sandstones for southern Taimyr, Arctic Russia: A comparison of heavy mineral analysis by optical and QEMSCAN methods. Submitted to Sedimentary Geology

Paper IV: Zhang, X., Pease, V., and Carter, A., Thermotectonic history of the Taimyr fold and thrust belt, Russia: revealed by apatite fission track analysis and balanced cross section restoration. In Manuscript

X. Zhang was the main contributor in terms of analyses and writing for paper I, II and III, and modeling and writing for paper IV, together with the help of all co-authors. J. Skogseid produced the reconstruction map in Paper II. Dr. A. Benedictus produced the QEMSCAN heavy mineral data for paper III. Dr. A. Carter produced the apatite fission track data for paper IV. Reprints for all manuscripts are made with permissions from the publishers.

Papers published during my PhD but not included in this thesis: Pease, V., Drachev, S., Stephenson, R., and Zhang, X., 2014, Arctic lithosphere — A review. Tectonophysics, v. 628, no. 0, p. 1-25.

Toro, J., Miller, E.L., Prokopiev, A.V., Zhang, X., and Veselovskiy, R., Mesozoic Orogens of the Arctic. ISAC Special Volume, Under review X. Zhang/ Tectonic evolution of Taimyr in the Late Paleozoic to Mesozoic

1. Introduction 1

2. Geological setting 3

3. Stratigraphy and Sampling 4

4. Provenance investigation 8

4.1 Framework-grain Petrography (Papers I and III) 8

4.2 Heavy Mineral Analysis (Papers I and III) 10

4.3 Detrital Zircon U-Pb Geochronology (Papers I and II) 12

4.4 Discussion 15 4.4.1 Tectonic evolution of Taimyr 16 4.4.2 Regional implication 17

5. Low-temperature thermal history and shortening history (Paper IV) 19

5.1 Thermochronology by apatite fission track dating 19

5.2 Balanced cross section 20

5.3 Results and Discussion 20

6. Key Conclusions 23

7. Suggestions for Future work 23

Acknowledgments 24

References: 24

X. Zhang/ Tectonic evolution of Taimyr in the Late Paleozoic to Mesozoic

Tectonic Evolution of Taimyr in the Late Paleozoic to Mesozoic from Provenance and Thermochronological Evidence Xiaojing Zhang Department of Geological Sciences, Stockholm University, S-106 91 Stockholm, Sweden

1. Introduction The Taimyr Peninsula is located at the northern margin of the Eurasian landmass, facing the Kara Sea to the east and the Laptev Sea to the north and west (Fig. 1). Taimyr consists of three NW-SE trending terranes (Fig. 2) and has a complex accretion history. Central Taimyr was believed to accrete to southern Taimyr during the late Precambrian and then became the passive margin of Siberian craton (Pease and Vernikovsky, 2000; Vernikovsky et al., 1995). Northern Taimyr and Severnaya Zemlya, together with the crust underlying the nearby Arctic Ocean, are regarded as an independent crustal block in the Paleozoic, named the North Kara Block or North Kara Massif (e. g. Cocks and Torsvik, 2005; Metelkin et al., 2005). However, a growing body of evidence suggests that the North Kara Block was a part of Baltica from the late Neoproterozoic (Lorenz et al., 2012; Lorenz et al., 2008; Lorenz et al., 2007; Pease and Scott, 2009). Northern Taimyr was attached to Siberia (central-southern Taimyr) in the late Paleozoic, which is correlated with the Uralian orogeny (Inger et al., 1999; Pease, 2011; Vernikovsky, 1995; Zonenshain and Natapov, 1990). However, the timing and geometry of deformation in Taimyr are quite different (Pease, 2011). Furthermore, Taimyr experienced the Early Triassic Siberia Trap magmatism intrusion (Dobretsov et al., 2013; Kuzmichev and Pease, 2007) and a later dextral transpressional deformation (Inger et al., 1999), overprinting the earlier late Paleozoic deformation. Tectonic evolution of Taimyr in the late Paleozoic to Mesozoic is of great interest, but there is a lack of studies. The understanding of Taimyr tectonics will help us to understand the amalgamation history of Baltica and Siberia in the Russian Arctic, the relationship of the continental collision in Taimyr with the Uralian orogeny, the relationships between processes of the late Paleozoic to Mesozoic tectonic events, and then to constrain plate reconstructions of the paleo-Arctic. Furthermore, the nature of the tectonics of Taimyr plays an important role in basin initiation and development in this region because Taimyr is surrounded by some of the most important hydrocarbon basins in the Arctic (Barents Sea, West Siberian, south Kara Sea and Laptev Sea) (Drachev, 2011; Drachev et al., 2010). The Uralian orogen developed through collision of the eastern margin of Baltica with the western Siberian and Kazakhstan plates during the assembly of Pangea in the late Paleozoic (Brown et al., 2006). It extends from the Aral Sea in the south and is nearly 2500 km long, but the inferred continuation northward into the Arctic region is controversial. Some authors argue that the Uralian orogeny terminates at the Polar Urals (Gee et al., 2006; Puchkov, 1997) while others suggest its northward continuation reaches Taimyr (Drachev et al., 2010; Scott et al., 2010), or that it projects from Pai Khoi to Novaya Zemlya, then to Taimyr, Severnaya Zemlya and at last back to the Asian mainland (Otto and Bailey, 1995; Sengör et al., 1993). In the Mesozoic, Siberian Trap-related magmatism occurred at the Permian-Triassic boundary (Campbell et al., 1992; Dobretsov et al., 2013). It extends northwards to the Taimyr Peninsula and beyond to the Kara Sea (Dobretsov et al., 2013). The latest stage of Siberian Trap magmatism included bimodal volcanic rocks, syenite and A-type granites of 247 – 234 Ma (Dobretsov et al., 2013). The abundant basalt-related sills, dikes and tuffs in Taimyr (Dobretsov

A Dissertation for the Degree of Philosophy in Natural Science et al., 2013; Kuzmichev and Pease, 2007) and 249 – 241 Ma syenites in northern Taimyr (Vernikovsky et al., 2003) are believed to be related to trap magmatism. Taimyr experienced a significant phase of Middle to Late Triassic dextral transpression, which was broadly coaxial with earlier Uralian deformation and reworked the earlier thrust zones (Inger et al., 1999; Torsvik and Andersen, 2002).

Figure 1 Regional setting Taimyr and tectonic elements in the Arctic. The figure is modified after (Zhang et al., 2015). The inferred continuation of the Ural orogen into the Arctic is shown by the dashed line. Abbreviations: AACM – Arctic Alaska-Chukotka microcontinent; FJL – Franz Josef Land; LH – Lisburne Hills; NSI – New Siberian Islands; SAS – South Anyui suture; SZ – Severnaya Zemlya; WI – Wrangel Island. Bathymetry and topography are from the IBCAO Arctic Bathymetry database (Jakobsson et al., 2012).

Another important factor that may have influenced the tectonic evolution of Taimyr is the opening of the Amerasia Basin, which is thought to have taken place in the Late Jurassic or Earliest Cretaceous via a debated mechanism. A popular model suggests the Arctic Alaska– Chukotka Microcontinent (AACM) (Fig. 1) rotated counter-clockwise from Arctic Canada, opening the Amerasia Basin on one side and colliding on the other with northern Asia along the South Anyui Suture (SAS) in Chukota and with North America along the Angayucham zone in Alaska (Grantz et al., 1998; Grantz et al., 2011; Lawver et al., 2002; Rowley and Lottes,

X. Zhang/ Tectonic evolution of Taimyr in the Late Paleozoic to Mesozoic

1988). In SE Siberia, the Kolyma-Omolon superterrane collided with the passive margin of the eastern Siberian plate in the Late Jurassic, forming the Verkhoyansk fold belt (Oxman, 2003). Provenance study of sediment is a powerful tool to constrain detritus erosion and deposition over time and evaluate hypotheses related to Arctic tectonics (e. g. Cawood et al., 2012; Fedo et al., 2003; Thomas, 2011). Late Paleozoic to Mesozoic siliciclastic deposition dominates southern Taimyr. The late Carboniferous to early Triassic siliciclastic succession in southern Taimyr is coeval with Uralian orogenesis and likely sourced from erosion of the developing Uralian collisional belt to the north and west. Uplift of the Siberian craton to the south due to the arrival of the Siberian plume head in the Permian-Triassic boundary, as well as plume-related magmatism, are also likely to have contributed to sediment input. The tectonic setting of, and sediment source(s) for, the late Carboniferous to early Triassic and Jurassic to Cretaceous sedimentary units are important for understanding the tectonic evolution of the Eurasian shelf from late Paleozoic through Mesozoic time and for constraining Arctic tectonic reconstructions. Although some geochemical and geochronological work has been conducted on igneous rocks of Taimyr (Pease et al., 2001; Pease and Persson, 2006; Pease and Vernikovsky, 2000; Vernikovsky et al., 2011; Vernikovsky et al., 2003; Vernikovsky and Vernikovskaya, 2001; Walderhaug et al., 2005), there are few investigations of the sedimentary rocks. In this thesis, multiple methods are used to determine sediment provenance and constrain sediment deposition through time, to develop an understanding of sediment erosional sources, as well as to evaluate hypotheses related to Arctic tectonics. This thesis aims to achieve the following objectives: 1. To establish the paleogeographical affinities of the Paleozoic and Mesozoic sedimentary successions in southern Taimyr using provenance studies (Papers I, II and III). 2. To compare the provenance investigation results from Taimyr sandstones with equivalent-aged Mesozoic stratigraphy from Eurasia and North America for a more comprehensive understanding of circum-Arctic tectonic evolution (Paper II). 3. To investigate the uplift and cooling history across Taimyr using thermochronological and balanced cross section methods in order to constrain the timing and style deformation, and establish links to regional tectonic events (Paper IV). 2. Geological setting The Taimyr fold and thrust belt is divided into three NE-SW trending structural domains, the southern, central, and northern domains (Bezzubtsev et al., 1983; Bezzubtsev et al., 1986; Vernikovsky et al., 1995) (Fig. 2). Southern Taimyr represents the passive margin of the Siberian platform in early Paleozoic time. The Ordovician to Middle Carboniferous shallow marine carbonate-dominated shelf succession is overlain by the Late Carboniferous to Early Triassic shallow-marine and continental siliciclastic succession. Permian-Triassic dikes and sills of the Taimyr igneous suite are interlayered with the siliciclastic strata and correlated with Siberian Trap magmatism (Walderhaug et al., 2005). High-level 249 – 241 Ma syenitic and granitic rocks (Vernikovsky et al., 2003) are widespread across western Taimyr and intrude the late Paleozoic siliciclastic succession (Vernikovsky et al., 2003). Subsequently, these rocks were locally folded and faulted during Late Triassic to Early Jurassic dextral transpression (Inger et al., 1999; Torsvik and Andersen, 2002). Central Taimyr is a structurally and lithologically diverse accretionary terrane, comprising a weakly to unmetamorphosed latest Neoproterozoic to late Paleozoic continental margin succession. It was deposited unconformably above a varied assemblage of Neoproterozoic volcano-sedimentary rocks, including fragmented ophiolites, island-arc volcanic rocks and

A Dissertation for the Degree of Philosophy in Natural Science continental crust of differing metamorphic grades (Uflyand et al., 1991; Vernikovsky et al., 2004; Zonenshain and Natapov, 1987; Zonenshain et al., 1990). These Neoproterozoic volcano-sedimentary successions form the basement to the Neoproterozoic to late Paleozoic carbonate rocks, and were in turn deposited unconformably above older Mesoproterozoic to early Neoproterozoic amphibolite-facies metasedimentary rocks that are intruded by c. 900 Ma granites (Pease et al., 2001). Later widespread magmatism and high-grade metamorphism occurred at c. 600 – 630 Ma in the Neoproterozoic volcano-sedimentary rocks (Pease and Vernikovsky, 2000; Vernikovsky et al., 1995), indicating latest Precambrian accretion of the allochthonous basement of central Taimyr to the Siberian margin. Central Taimyr is also

Figure 2 Simplified geological map of Taimyr Peninsula (after Bezzubtsev et al., 1983). Northern, central and southern Taimyr are divided by the Diabase thrust, the Main thrust and the Pyasino-Faddey thrust. The three squares indicate the three regions where siliciclastic sandstone samples were collected for provenance study. The figure is adapted from (Zhang et al., 2015). intruded by dikes and sills associated to Siberian Trap magmatism. Northern Taimyr, dominated by greenschist-facies Neoproterozoic to early Paleozoic turbidites formed on a continental slope, is an allochthonous terrane compared to central and southern Taimyr. Granitic plutons of Carboniferous to Permian age intruded into northern Taimyr, including syn-collisional deformed c. 305 Ma granites (Pease et al., 2015; Vernikovsky et al., 1995), and post-collisional undeformed c. 265 Ma granites which have well-defined contact metamorphic aureoles and Late Permian cooling ages (Vernikovsky et al., 1998) (Fig. 2). The post-collisional granites intrude across both northern and central Taimyr (Fig. 2). Thus, they are interpreted as “stitching” plutons that signal the collision between northern and southern–central Taimyr (Inger et al., 1999; Zonenshain and Natapov, 1987; Zonenshain et al., 1990). 3. Stratigraphy and Sampling Samples used in this thesis for provenance investigation were mainly collected from the Late Paleozoic to early Mesozoic siliciclastic successions in southern Taimyr. The late Carboniferous and Permian strata, comprising seven formations up to 7km thick, represent a sequence of clastic sedimentary rocks that include coal-bearing continental and shelf facies, as well as marine facies. The Triassic strata are mainly volcanic sequences comprising

X. Zhang/ Tectonic evolution of Taimyr in the Late Paleozoic to Mesozoic basaltic lavas, tuffs and tuffaceous sediments. Jurassic and Cretaceous strata are only exposed in the eastern part of Taimyr, comprising siltstone, sandstones, mudstone with layers of conglomerates and limestones. Samples are mostly medium grain size sandstones from three different regions：the Buotankaga River region，the vicinity of Taimyr Lake and the Chernokhrebetnaya River region in southern Taimyr and one spot close to the northern coast at Chelyuskin Cape in northern Taimyr (Fig. 2). Samples used for apatite fission track dating were collected from several locations in northern, central and southern Taimyr. Lithology of samples includes granite, metamorphosed arenite, and sandstones, and the age of samples varies from Neoproterozoic to Early Jurassic. The sampling details are listed in Table 1 and 2.

A Dissertation for the Degree of Philosophy in Natural Science

Table 1: Sample data for Taimyr sandstones used for provenance investigation Framework Heavy mineral Detrital zircon Region Sample Age Unit petrography analysis dating XZ12-08 Late Carboniferous Makarovskaya Fm X X X XZ12-01 Late Carboniferous to Early Permian Turuzovskaya Fm X X X XZ12-05 Late Carboniferous to Early Permian Turuzovskaya Fm X XZ12-06 Late Carboniferous to Early Permian Turuzovskaya Fm X X X XZ12-17 Late Carboniferous to Early Permian Turuzovskaya Fm X X X Buotankaga XZ12-10 Early Permian Byrrangskaya Fm X X X River XZ12-15 Early Permian Byrrangskaya Fm X X X

XZ12-11 Early Permian Sokolinskaya Fm X X X

XZ12-13 Late Permian Baykurskaya Fm X X X XZ12-16 Late Permian Baykurskaya Fm X X X XZ12-02 Middle to Late Triassic Mamonova Fm X X XZ12-03 Middle to Late Triassic Mamonova Fm X X X XZ12-09 Middle to Late Triassic Mamonova Fm X X X RAS98-8 Early Permian Byrrangskaya Fm X X X Taimyr Lake RAS98-9 Early Permian Byrrangskaya Fm X X 6

RAS98-32 Early Permian Byrrangskaya Fm X X RAS98-23 Late Carboniferous to Early Permian Turuzovskaya Fm X X

VP10-25 Late Carboniferous to Early Permian Turuzovskaya Fm X X X VP10-33 Late Carboniferous to Early Permian Turuzovskaya Fm X X X VP10-44 Late Carboniferous to Early Permian Turuzovskaya Fm X X X VP10-19 Early Permian Byrrangskaya Fm X X X VP10-41 Early Permian Byrrangskaya Fm X X X VP10-42 Early Permian Byrrangskaya Fm X X X VP10-14 Early Permian Sokolinskaya Fm X X X VP10-16 Early Permian Sokolinskaya Fm X X X Chernokhrebetnaya VP10-36 Early Permian Sokolinskaya Fm X X X River VP10-43 Early Permian Sokolinskaya Fm X X X

VP10-12 Late Permian Baykurskaya Fm X X X

VP10-46 Late Permian Baykurskaya Fm X X X VP10-63 Late Permian Baykurskaya Fm X X X VP10-64 Late Permian Baykurskaya Fm X X X VP10-69 Late Permian Baykurskaya Fm X X X VP10-65a,b Early Triassic Unnamed Unit X X X VP10-66 Early Triassic Unnamed Unit X X

X. Zhang/ Tectonic evolution of Taimyr in the Late Paleozoic to Mesozoic

Framework Heavy mineral Detrital zircon Region Sample Age Unit petrography analysis dating VP10-11b Middle Triassic Unnamed Unit X X VP10-56 Middle Triassic Unnamed Unit X X X VP10-57 Middle Triassic Unnamed Unit X X X VP10-58 Middle Triassic Unnamed Unit X X X VP10-68 Middle Triassic Unnamed Unit X X Chernokhrebetnaya VP10-53 Early Jurassic Unnamed Unit X X River VP10-59 Early Jurassic Unnamed Unit X X X

VP10-51 Middle Jurassic Unnamed Unit X X VP10-55 Middle Jurassic Unnamed Unit X X X VP10-47 Late Jurassic Unnamed Unit X X X VP10-48a Late Jurassic Unnamed Unit X X X VP10-48b Late Jurassic Unnamed Unit X X X Northern Taimyr T99-26 EarlyCretaceous Unnamed Unit X X X T99-32 EarlyCretaceous Unnamed Unit X X

A Dissertation for the Degree of Philosophy in Natural Science

Table 2. Sample details for Taimyr apatite fission track analysis Samples Lithology Location Elevation(m) Rock Age VP98-002a Granite Northern Taimyr 279 304 Ma VP02-059a Meta-arenite Northern Taimyr 38 Early Paleozoic VP98-025 Granite Central Taimyr 197 940 Ma VP98-060 Granite Central Taimyr 211 800 -900 Ma VP98-071 Gabbro Central Taimyr 188 c. 600 Ma VP98-085 Volcanoclastic Central Taimyr 137 Neoproterozic VP98-089 Granite Central Taimyr 93 Camb

XZ12-08 Sandstone Southern Taimyr 361 C2mk XZ12-01 Sandstone Southern Taimyr 365 C2-P1tr VP10-44 Sandstone Southern Taimyr 459 C -P tr 2 1 XZ12-10 Sandstone Southern Taimyr 417 P1br VP10-36 Sandstone Southern Taimyr 333 P1sk VP10-64 Sandstone Southern Taimyr 37 P bk 2 VP10-65a Sandstone Southern Taimyr 33 T1 VP10-57 Sandstone Southern Taimyr 40 T2 VP10-53 Sandstone Southern Taimyr 59 J1

4. Provenance investigation The sedimentary successions in basins are direct records of mountain uplift and erosion. Provenance investigations of sediments and sedimentary rocks aim to identify the source area(s) and assemblages of parent rock(s) (Weltje and von Eynatten, 2004). Our paper I, II and III utilize framework-grain petrography, heavy mineral analysis and detrital zircon U-Pb dating to investigate provenance of the Paleozoic to Mesozoic sedimentary rocks of Taimyr. Two early Cretaceous samples are from northern Taimyr and the remaining 41 samples are from Late Carboniferous to Late Jurassic strata in southern Taimyr (Fig. 2). 4.1 Framework-grain Petrography (Papers I and III) The framework-grain petrographic method is used to determine tectonic settings and provenance by identifying the detrital modes of sandstone obtained from point counting of grain mounts by thin section (Dickinson, 1985; Dickinson and Suczek, 1979). The most widely used method is that of Dickinson and Suczek (1979), who made quantitative statistical assessments of the composition of sandstone from type areas, and then proposed a suite of discrimination diagrams (Fig. 3). Sandstone detrital framework minerals larger than 0.0625 mm are counted and include the following: 1. stable quartzose grains (Qt), including monocrystalline quartz (Qm), and polycrystalline quartzose lithic fragments (Qp); 2. monocrystalline feldspar grains (F), including plagioclase (P), and K-feldspar (K); and 3. unstable polycrystalline lithic fragments (L), including volcanic and metavolcanic lithic fragments (Lv), and sedimentary and metasedimentary lithic fragments (Ls). There are two complementary triangular plots in common use, and each of them is based on different detrital grain suites (Fig. 3) (Dickinson and Suczek, 1979). In QFL (Q = Qp + Qm) and QmFLt (Lt = Lv + Ls) plots, all the grain types are used. Both polycrystalline quartz and monocrystalline quartz are calculated for the QFL plot, reflecting the stability of grains, and

X. Zhang/ Tectonic evolution of Taimyr in the Late Paleozoic to Mesozoic thus the influence of weathering, provenance relief and transport mechanism(s), along with source rock composition. All the lithic fragments and polycrystalline quartz are combined in the QmFLt plot, emphasizing the grain size of the parent rock. Three main provenance types are identified in these plots: continental block, magmatic arc and recycled orogen (Dickinson, 1985; Dickinson and Suczek, 1979). This method is easy, quick, widely used (e.g. Miller et al., 2008) and has subsequently been developed further by many other researchers (Garzanti et al., 2008; Garzanti et al., 2002; Ingersoll et al., 1984; Ingersoll et al., 1993; Weltje, 2002).

Figure 3 Framework grain petrographic discrimination diagrams of Taimyr samples showing tectonic settings related to source areas (after Dickinson, 1985). The figure was produced for this thesis. Squares with cross represent samples from the vicinity of Taimyr Lake, normal squares and circles represent samples from the Buotankaga River region, and solid squares and circles represent samples from the Chernokhrebetnaya River region.

In this thesis (Papers I and III), we assess sandstone composition and maturity via point counting of thin sections made from 43 sandstone samples (Fig. 3). The late Carboniferous to Permian sandstones from the three different regions in southern Taimyr are immature, mostly poorly to moderately sorted and contain subangular to subrounded grains. The seven Triassic samples are highly immature and poorly sorted, mainly containing angular to subrounded detrital grains. The framework-grain point counting results suggest that late Paleozoic to Mesozoic samples from southern Taimyr are derived from a variety of source rocks, including felsic and mafic volcanics, microcline-bearing plutonic, sedimentary and meta-sedimentary rocks. The generally immature Late Carboniferous to Permian and Triassic samples imply proximal source areas and abundant first-cycle detritus in the late Paleozoic to early Mesozoic depositional basin. They are plotted in the “transitional arc”, “dissected arc”, “recycled orogenic” and “mixed” fields (Fig. 3). The seven Jurassic and two Cretaceous sandstone samples are more mature than the previous samples and are are moderately to well sorted, containing subangular to subrounded grains. They define a trend from “transitional recycled orogenic” to “cratonic interior” in the provenance QtFL and QmFLt plots (Fig. 3). The higher maturity of Jurassic and Cretaceous suggests longer transport and/or input from recycling of previously

A Dissertation for the Degree of Philosophy in Natural Science deposited sediments. We reveal that sandstone deposition evolved from recycled orogen and dissected arc terrane sources to cratonic interior sources through time from Late Carboniferous to Cretaceous (Papers I and III). 4.2 Heavy Mineral Analysis (Papers I and III)

Minerals with a density greater than 2.86 g/cm3 in siliciclastic sediments are called heavy minerals. They usually occur as accessory minerals in rocks. Heavy mineral analysis is one of the most powerful and widely used methods in determining sediment provenance because of the sensitivity of heavy minerals to source rock lithology (Morton, 1992; Morton and Hallsworth, 1994). It is especially useful in investigations of sedimentation related to tectonic uplift, since the evolution and unroofing of orogens are reflected in their foreland deposits (Mange and Maurer, 1992).

Table 3 Heavy mineral order of stability in deeply buried sandstone modified after Morton and Hallsworth (1994). Most stable Rutile, Anatase, Brookite, Zircon, Apatite Tourmaline, Monazite, Spinel Garnet, Chloritoid Allanite Staurolite Sodic amphibole Kyanite Titanite Epidote Calcic amphibole, Andalusite, Sillimanite Sodic pyroxene Orthopyroxene, Clinopyroxene Olivine Least stable

Many heavy minerals have restricted parageneses that indicate the involvement of particular source rocks (Morton and Hallsworth, 1994), and thus the number and the species of possible minerals present in sediments can reflect the parent rock. Determining the relative abundance of heavy minerals in sedimentary rocks is one of HMA methods, which is achieved by counting minerals on grain mounts. To obtain precise data, at least 200 non-opaque minerals should be counted (Mange and Maurer, 1992). However, source rock is not the only control on the heavy mineral assemblage; other parameters have the ability to change the relative abundance of heavy minerals, including weathering at the source area, mechanical destruction during transport from the source area, hydraulic factors referring to mineral shape and size, as well as diagenetic processes (intrastrata dissolution). The most important factors are hydraulics and burial diagenesis (Mange and Maurer, 1992; Morton and Hallsworth, 1999). Hydraulic factors influence the variety of heavy minerals deposited under certain hydraulic

X. Zhang/ Tectonic evolution of Taimyr in the Late Paleozoic to Mesozoic conditions and burial diagenesis can dissolve unstable minerals resulting in a decrease of heavy mineral diversity. To constrain the nature of the source rocks using the heavy mineral abundance information, it is necessary to evaluate these factors first. The relative stability of heavy minerals in deeply buried sandstone is shown in Table 3. Another HMA method is to study the ratios of specific mineral pairs. These mineral pairs should have similar hydraulic and diagenetic behavior and be relatively stable (Morton and Hallsworth, 1994). The mineral ratios typically used are outlined in Table 4. Such mineral pairs are less likely to reflect hydraulic or diagenetic factors, so they generally reflect the characteristics of their source rock(s). For example, a high monazite to zircon (MZi) ratio (or index) indicates the involvement of granite or pegmatite, because monazites are common in granitic rocks and pegmatites; high chrome spinel to zircon (CZi) index suggests a significant contribution from ultramafic rocks because chrome spinels are common in ultramafic rocks (Mange and Maurer, 1992; Morton and Berge, 1995; Morton and Hallsworth, 1994; Morton et al., 2011).

Table 4 Provenance sensitive mineral pairs (Morton and Hallsworth,1994). The minerals are hydraulically similar, stable heavy minerals.

Index Mineral pair Definition ATi apatite-tourmline 100×apatite/(apatite+tourmaline) GZi garnet-zircon 100×garnet/(garnet+zircon) RZi rutile-zircon 100×rutile/(rutile+zircon) CZi chrome spinel-zircon 100×chrome spinel/(chrome spinel+zircon) MZi monazite-zircon 100×monazite/(monazite+zircon)

We present comprehensive heavy mineral analysis data of more than forty sandstone samples from Late Carboniferous to Early Cretaceous in Papers I and III. The heavy mineral assemblages determined by optical ribbon method (Galehouse, 1971) are very similar across southern Taimyr and contain six main species: apatite, tourmaline, zircon, rutile, garnet, and epidote. A further seven species are present in lesser amounts, including clinopyroxene, Cr-spinel, monazite, chloritoid, staurolite, titanite and cassiterite (Fig. 4). The heavy mineral assemblages of the Late Carboniferous to Permian samples are stable and dominated by mixed populations of apatite, zircon and tourmaline. Abundant euhedral zircons and apatites suggest a volcanic arc source. Cr-Spinel indicates an ophiolitic source (Mange and Maurer, 1992). The Early Triassic samples are similar to the Late Carboniferous to Permian samples and indicate common sources. They also contain 4% garnet, which can indicate a metamorphic rock source. The Middle to Late Triassic samples show a dramatic decrease in apatite and significant increase in garnet and clinopyroxene. The increasing garnet and clinopyroxene is consistent with detrital input from metamorphic rock and basic volcanic rock sources, respectively. Heavy mineral assemblages from the Jurassic and Cretaceous samples contain substantial metamorphic mineral like garnet and staurolite and minor amounts of chloritoid (Fig.4). The average ATi index is relatively high, whereas the Late Jurassic and Cretaceous samples have lower ATi value. The Late Carboniferous to Early Triassic samples display lower average RuZi, MZi and GZi ratios, while the Middle Triassic to Late Jurassic samples have higher average RuZi, MZi and GZi. The average CZi varies from 1 to 11 in Late Carboniferous to Middle Triassic samples and becomes zero in the Jurassic samples. We also

A Dissertation for the Degree of Philosophy in Natural Science compare heavy mineral assemblage data obtained by both optical counting and automated QEMSCAN methods and reveal compatible results, suggesting great potential for automatic heavy mineral analysis of sandstones.

Figure 4 Relative abundance of heavy mineral species for Taimyr samples. The figure is adapted from Paper III.

4.3 Detrital Zircon U-Pb Geochronology (Papers I and II)

Zircon is a common accessory mineral in most rock types. It is mechanically and chemically stable and therefore can endure successive cycles of transport, burial and erosion (Carter and Bristow, 2000). The development and application of secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) techniques can generate ages efficiently and with high accuracy, which make zircon U-Pb geochronology by far the most powerful and popular approach for sedimentary provenance study today (Fedo et al., 2003, and references therein; Gehrels, 2011; Gehrels et al., 1995). Detrital zircon ages are compared with the ages of potential source terranes to determine the ultimate source(s) of the sediment in provenance investigations (Gehrels, 2011; Thomas, 2011). The aim of zircon analysis is to produce an age distribution that reflects the population in the sediment and hence is an accurate signature of its source rock(s). The strategy to achieve this is to select zircons randomly, irrespective of grain size, color, shape and so on (Gehrels, 2011). However, due to analytical requirements of the technique, very small grains are excluded. Morton et al. (1996) suggest using the 63 - 125 μm (very fine sand) fraction in detrital zircon studies. At least 117 analyses for each sample are needed to achieve statistical representation (Vermeesch, 2004).

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U-Pb analyses are usually plotted on a conventional or inverse (Tera and Wasserburg, 1972) concordia diagram. This is done using a spreadsheet with macros such as Isoplot (Lugwig, 2010). Due to the better precision associated with 206Pb/238U ages for “young” zircons, while for “older” zircons the 206Pb/207Pb ages have better uncertainties (Gehrels, 2011), a threshold age between 1.2 Ga and 900 Ma is selected to determine the usage of 206Pb/238U or 206Pb/207Pb. If the 206Pb/238U, 207Pb/235U and 206Pb/207Pb ages are similar (within error), an analysis is considered to be concordant (Wetherill, 1956). However, there are always some discordant analyses in a dataset (Gehrels, 2011; Nemchin and Cawood, 2005). Usually, analytical results with more than 10% discordance or with large errors (>10%) are excluded from the final data synthesis. The acceptable results are plotted in a probability plot with histograms in order to represent both the age and associated uncertainty (DeGraaff-Surpless et al., 2003; Gehrels, 2011; Lugwig, 2010). The probability plots are used to make comparisons between samples (Fig.5). We present detrital zircon U-Pb analyses of 43 sandstone samples from Taimyr, covering the complete depositional sequence from the Late Carboniferous through Permian and Triassic, to the Late Jurassic and Early Cretaceous in Paper I and II. The U-Pb relative probability curves of detrital zircons from the Carboniferous-Permian formations from the three regions in southern Taimyr are similar and consistent within a regional scale depositional system at least 500 km across (Fig. 2 and 5). Late Carboniferous to Permian sandstones contain detrital zircon populations of 370 – 260 Ma, which were derived from the late Paleozoic Uralian orogen in northern Taimyr and/or the Polar Urals. Late Neoproterozoic to Silurian ages (688 – 420 Ma) are consistent with derivation from Timanian and Caledonian age sources and indicate an ultimate Baltica source (Fig. 5A and B). The presence of minor Mesoproterozoic zircons in Late Permian samples is significant because these ages are not present in Siberia and represent a unique Baltica signature. The Triassic, Jurassic and Cretaceous samples all contain zircons with ages of Late Devonian to Permian (370 – 260 Ma), Middle Ordovician to Early Devonian (470 – 380 Ma) and Middle Neoproterozoic to Early Ordovician (700 – 480 Ma), sourced from the Urals and the recycled Caledonides and Timanides, respectively (Fig. 5B and C). The Neoproterozoic (1000 – 700 Ma) and Archean to Paleoproterozoic ages in Mesozoic samples were probably sourced from central and northern Taimyr and the Siberian craton, respectively.

Figure 5 Relative probability curves (with histograms) of U-Pb detrital zircon ages of A) all samples from the Buotankaga River region; B) late Paleozoic samples from the Chernokhrebetnaya River region; and C) Mesozoic samples of the Chernokhrebetnaya River region. Note the maximum depositional age is determined by the three youngest grains and N = number of accepted analyses. Top inset shows the potential sources with age ranges. The figure is after Zhang et al. (2015).

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4.4 Discussion

Sediment composition in basins is controlled by many parameters, such as the parent rock type, tectonic setting, transport pathway, deposition and digenesis, of which the parent rock type and tectonic setting exert the primary effects (Dickinson, 1985; Dickinson et al., 1983; Dickinson and Suczek, 1979; McLennan et al., 1993; Zuffa, 1987). A sedimentary basin may accumulate detritus from several sources. For example, multiple magmatic events can occur in a single locality, or rocks of different lithology and age may come from different localities mixing during transport or during recycling of older sediments with younger primary sources (Thomas, 2011). The combination of heavy mineral analysis and detrital zircon age dating has made high resolution characterization and differentiation of sediment provenance viable (Hallsworth et al., 2000). While heavy mineral data are used to identify the source rock types, the detrital zircon age data are used to constrain the ages of source terrains (Fig. 6). In this way, both the nature and the age of the possible source(s) can be determined.

Figure 6 Schematic illustration showing the application of detrital zircon ages, heavy mineral analysis and petrography to constrain tectonic setting of sediment sources (after Cawood et al., 2012). Red arrows indicate the depositional age of sediment. It is well recognized that tectonic evolution is reflected in sedimentary rocks (McLennan et al., 1993). Cawood et al. (2012) classified the tectonic setting of a sedimentary package as extensional, convergent and collisional, which correspond to the continental block, magmatic arc and recycled orogen of Dickinson et al., (1985), respectively. They proposed that detrital zircon age spectra can also reflect the tectonic setting of basins (Cawood et al., 2012) (Fig. 6). Detrital zircons from basins in an extensional setting, including rift basins and post rift passive margins, usually lack grains with syn-sedimentary ages and are dominated by older input from the continent. Detrital zircon spectra of sediments in arc flanking basins along convergent

A Dissertation for the Degree of Philosophy in Natural Science margins are characterized by young ages of syn-sedimentary igneous activity, and minor older continental input is involved in their deposition. Foreland basin sediments in continental collisional settings contain zircons with ages close to the deposition age along with significant older zircons from the continent (Cawood et al., 2012; Cawood et al., 2007). The heavy mineral assemblage and morphology also preserve features reflecting the tectonic setting of basins: sediment in extensional settings have a higher proportion of stable heavy minerals (zircon, tourmaline, rutile and apatite) and a higher proportion of rounded grains; sediment from continental collisional settings have a moderate proportion of stable heavy minerals and both rounded and euhedral grains are common; sediment in magmatic arc settings have a lower proportion of stable heavy minerals and are more euhedral (Nie et al., 2012).

4.4.1 Tectonic evolution of Taimyr

The framework-grain petrography, heavy mineral and detrital zircon data are consistent with each other. We reveal the provenance and tectonic evolution of the Late Paleozoic to Mesozoic siliciclastic succession in Paper I, II and III. The immature Late Carboniferous to Permian sandstones indicate derivation from composite sources representing magmatic arc and recycled orogenic settings, consistent with mixing of volcanic, plutonic, ophiolitic and recycled sedimentary rock contributions. All Carboniferous to Permian samples represent a dissected arc and recycled orogenic source (Fig.3), suggesting these rocks were produced through uplift and erosion of a rising collisional belt (Zhang et al., 2013). Heavy mineral assemblage results suggest a proximal igneous source by angular and subangular euhedral detrital zircon and apatite grains. Cr-spinel indicates probable influx from exposed ophiolitic basement in the Late Carboniferous to Permian (Paper III). The detrital zircon populations of the Late Carboniferous to Permian samples are dominated by 350 to 262 Ma zircons, which are equivalent with the samples’ depositional age, and correlate well with the syn- to post-tectonic timing of the Uralian orogen in the north (Zhang et al., 2015). The occurrence of syn-depositional ages and older ages indicates that the Carboniferous to Permian sediments were deposited in a foreland basin setting (Zhang et al., 2013). The diachronous and bivergent Uralian orogen involved double sided thrusting (Puchkov, 2009; Scarrow et al., 2002). Thrusting in Taimyr is predominantly south-vergent (see discussion in Pease, 2011) and recent geophysical interpretations suggest that north-vergent thrusting is located off-shore north of Taimyr (Malyshev et al., 2012). Southern Taimyr is interpreted to represent the pro-foreland basin of the Uralian orogen in Late Carboniferous to Permian time receiving detritus from the Polar Urals, northern and central Taimyr and/or Severnaya Zemlya (Zhang et al., 2015). We reveal the similar framework-grain, heavy mineral and detrital zircon results of the Early Triassic samples to the older Carboniferous-Permian samples, indicating that erosion from the Uralian orogen was still supplying significant input to Early Triassic siliciclastic sediments (Papers II and III). Erosion of the Uralian orogen reached a deeper metamorphic level by the Middle Triassic (Paper III). The addition of c. 250 Ma ages in the Early Triassic samples reflects the influence of Siberian Trap magmatism in the region (Dobretsov et al., 2013; Kuzmichev and Pease, 2007; Vernikovsky et al., 2003). The Middle Triassic samples contain an increasing Siberian Trap-age component (Fig. 5) and abundant clinopyroxene (Fig. 4), suggesting erosion and great contributions from Siberian Trap-related rocks of Taimyr. The eastern part of Taimyr became an intracratonic basin of the Eurasian continent in the Late Jurassic and Cretaceous suggested by the lack of syn-deposition detrital zircon ages (Zhang et al., 2013; Zhang et al., 2015). The Siberian Trap magmatism was supplying the most

X. Zhang/ Tectonic evolution of Taimyr in the Late Paleozoic to Mesozoic detritus to Taimyr in the Jurassic and Cretaceous (Zhang et al., 2013; Zhang et al., 2015), while staurolite-bearing medium-grade metamorphic rocks of Uralian orogen were also important detritus sources (Paper III).

4.4.2 Regional implication

We compare our results to the published data surrounding the Arctic Ocean to reveal the regional tectonic correlations (Zhang et al., 2015). We conclude that in the Permian detritus from the Uralian orogen was deposited across Taimyr, Novaya Zemlya and the New Siberian Islands, but not on Wrangel Island or in the Lisburne Hills (Fig. 7A). In the Triassic, Taimyr, Chukotka, Wrangel Island, the Kular Dome, the Lisburne Hills, Franz Josef Land and Svalbard shared common sources from Taimyr, the Siberian Traps and the Urals. The widespread distribution of material eroded from Taimyr and the Urals was likely resulted by uplift associated with the Siberian plume head in the Triassic. Detritus from Taimyr and the Urals was probably shed northward/northeastward into a retro-foreland basin and ultimately transported to Chukotka, Wrangel Island, the Kular Dome, the Lisburne Hills Franz Josef Land and Svalbard. Consequently, although these areas share similar sediment provenances in the Triassic, restoration of the Chukotka part of AACM to the Barents Shelf as suggested by Miller et al. (2006; 2010) is not needed (Fig.7B). At least from the Late Jurassic, Taimyr was in an intracratonic setting. The provenance results from Jurassic sedimentary rocks reveal that a reduction of erosional detritus from the Urals in Taimyr, together with docking of the Kolyma-Omolon superterrane and the formation of the SAS, which restricted the regional distribution of detritus from Taimyr and the Urals (Fig. 7C).

Figure 7 Schematic paleotopography at 300 Ma, 220 Ma and 150 Ma, after plate tectonic models slightly modified from Grantz et al.(1998) and Lawver et al. (2002). Abbreviations: EI – Ellesmere Island; FJZ – Franz Josef Lands; KD – Kular Dome; K-O – Kolyma-Omolon Superterrane; LH – Lisburne Hills; NZ – Novaya Zemlya; S – Svalbard; SZ – Severnaya Zemlya; WI – Wrangel Island. Stars indicate the sample locations in this study; black arrows indicate the transportation direction of detritus to Taimyr and white arrows indicate transportation direction of detritus to other locations. The transparent ellipses in A and B indicate the e retro-foreland basin of the Uralian orogen. The figure is adapted from Zhang et al., (2015).

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5. Low-temperature thermal history and shortening history (Paper IV) 5.1 Thermochronology by apatite fission track dating

To determine the low temperature thermal history of Taimyr, apatite fission track dating is employed. Thermochronology data record cooling history of rocks in response to tectonic thickening, rock uplift and erosion (Erdős et al., 2014). Apatite fission track (AFT) analysis is one of the most widely used thermochronological methods to constrain the low-temperature thermal histories of various rocks in a wide range of geological settings (Donelick et al., 2005). AFT analysis utilizes the damage formed by fission decay of 238U. New tracks accumulate at a constant rate, allowing the calculation of fission track ages; tracks have approximately the same initial length of ~ 16.3 µm and are partially or entirely shortened at elevated temperatures, resulting in easily measured reductions in both track lengths and fission track densities (Dumitru et al., 2001; Gallagher, 1995; Gleadow et al., 1986; Stockli, 2005). The partial annealing zone (PAZ) represents the temperature range between ~60 °C and ~110 °C, where fission tracks are partially annealed; above ~110 °C all fission tracks are totally annealed and below ~60°C, fission tracks are effectively stable, annealing at a very slow rate (Gallagher, 1995; Green et al., 1986).

Figure 8 Predicted fission track age and length distributions of three different hypothetical cooling histories (within the same temperature interval between 120° and 20°C). The resulting track length distributions are shown on the right. The three numbers represent the predicted fission track age (top), the mean track length (middle), and the standard deviation of the length distribution (bottom) (after Gleadow et al., 2002).

The distribution of confined track lengths together with the AFT ages can provide unique thermal history information in the temperature range below the closure temperature (~110 °C)

A Dissertation for the Degree of Philosophy in Natural Science over times of the order of 106 to 109 years (Gleadow et al., 1986). Different length distributions result from different types of thermal history (Fig. 8) (Gleadow et al., 2002). In rocks which have cooled very rapidly and not reheated above ~60°C, the track length distribution is very narrow and nearly symmetric with a long mean track length of 14 – 15 µm and a standard deviation of ~1.0 µm; the AFT age closely approximates the time of rapid cooling (case a in Fig. 8). In rocks which have undergone a steady cooling path and not reheated significantly, the track length distribution is broader and skewed with a mean track length of 12 – 13 µm and a standard deviation of 1.2 – 2 µm (case b in Fig. 8); the AFT age is younger than the time of cooling. In rocks which have undergone a two-stage cooling history, the track length distribution is even broader; when the peaks are clearly resolved, i.e.- the older track length is shortened to a mean of 9 – 10 µm by the later thermal event, the distribution becomes bimodal (case c in Fig. 8); otherwise, the length distribution remains unimodal but with an intermediate mean length and a greater standard deviation (Gleadow et al., 2002; Gleadow et al., 1986). 5.2 Balanced cross section

The balanced cross section technique is a fundamental tool for providing a geometric model of the subsurface and estimating the amount of shortening that occurred in a specific region of a orogen or fold and thrust belt (Judge and Allmendinger, 2011). It is based on the assumption that the area of the cross section is preserved during deformation and that an original sequence of flat-lying beds has been folded and faulted to form the present configuration of units (Hossack, 1979). Strata volume is only altered by erosion and compaction. A geologically reasonable cross section must be consistent with the structural style of the region and restore to a pre-deformational state with no large gaps or overlaps in strata; such a cross section is considered viable and may be non-unique (Elliott, 1983). Cross section balancing removes the displacements of faults, folding related to faulting and flexural slip, and volume changes caused by compaction and erosion. The balanced cross section is drawn in the direction of tectonic transport and perpendicular to the main structural trends. Two methods are mainly applied for rendering balanced cross sections, i.e.- forward modeling and inverse modeling (Dahlstrom, 1969). The forward method models the procedure from pre-deformational state to deformed state. The inverse method restores a currently deformed section to its pre-deformational state based on the sequence of faults and folds in the section, and maintaining unit length.

5.3 Results and Discussion

The Taimyr fold and thrust belt underwent a series of geological events in the late Paleozoic to Mesozoic, including the Uralian collision, Siberian Trap magmatism and Mesozoic transpression. The thermal history of Taimyr will enable us to better understand these events. We integrate these results in Paper IV when constructing a balanced cross section for central-southern Taimyr. This permits us to better understand the shortening pattern and thermal history of the Taimyr fold and thrust belt (e. g. Erdős et al., 2014; Lock and Willett, 2008). Based on the AFT central age and mean track length (MTL), the AFT data are grouped into three types. Type I contains 13 samples with AFT central ages much younger than the emplacement or deposition age of the host rock, indicating that a distinct reduction in single-grain age has occurred in respond to thermal annealing after the rock was deposited or

X. Zhang/ Tectonic evolution of Taimyr in the Late Paleozoic to Mesozoic crystallized. The fairly long MTL of type I (13.34 – 15.06 µm) with narrow distributions (standard deviation = 0.86 – 1.26 µm) is indicative of a rapid cooling history. The weighted mean AFT central age at c. 215 Ma of these samples approximates time of cooling. Type II includes the Early and Middle Triassic samples with AFT central ages equivalent to or slightly younger than their stratigraphic ages. The long MTL (13.45 - 13.9 µm) with broad track-length distributions (standard deviation = 1.72 - 1.82) is evidence of partial annealing at 60 – 80 °C (Green et al., 1989b) in the early stage before most tracks formed and slow cooling below 60 – 80 °C. The Early Jurassic sample VP10-53 is the type III, with the AFT central age older than its stratigraphic age (c. 216 Ma), demonstrating no significant thermal annealing after the host rock was deposited. The short MTL of c. 12.79 µm with a wide distribution and less than one fourth tracks longer than 14 µm suggest exposure to burial temperatures of 60 – 80 °C in the middle to late stage of this sample’s history (Green et al., 1989a; Green et al., 1989b) followed by slow cooling to ambient temperatures. We select good quality data (more than 50 confined tracks) to model the thermal history of these samples using the HeFTy software (Ketcham, 2005) (Paper IV). Three samples of type I VP98-002a, VP02-059 and VP98-025 show similar temperature-time (t-T) paths in the modeling results (Fig. 9), suggesting a regionally significant cooling event occurred in the early Mesozoic c. 220 Ma across Taimyr. The modeling result for the type II early Triassic sample VP10-65a suggests that a rapid cooling below 110 °C occurred at c. 280 Ma in the sediment source areas and confirms thermal overprinting (reheating) after deposition due to later burial (Fig. 9). The thermal modeling of the Early Jurassic sample VP10-53 of type III documents a rapid cooling episode at ca. 250 Ma of the source rocks followed by shallow burial in the Early and Middle Jurassic (Fig. 9). The modeling implies that the Taimyr fold and thrust belt underwent three cooling episodes at 280 Ma, 250 Ma and 220 Ma, corresponding to the shortening and uplifting of Uralian orogeny, cooling related to Siberian Trap magmatism, and later uplifting during Mesozoic transpression. The interpretation of the balanced cross section reveals a stack of thin-skinned thrust sheets related to Uralian orogenesis. Folding and thrusting occurred in the Early Permian, resulting in shortening and uplifting at c. 280 Ma. The total shortening estimated from the cross section is about 27%.

Figure 9 Results of inverse thermal history modeling together with track length distributions. Time (Ma) is plotted against temperature (°C) for each sample. Goodness of fit (GOF) gives an indication about fit between observed and predicted data (values close to 1 are the best). The figure is adapted from Paper IV.

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6. Key Conclusions

 In latest Carboniferous and Permian time, the late Paleozoic Uralian orogen extended into Taimyr. Detrital zircon and heavy mineral investigations of the late Paleozoic siliciclastic succession reveal that southern Taimyr was the pro-foreland basin to the Uralian orogen and received detritus from the proximal, evolving Uralian orogen to the north and northwest.  In the Triassic, the siliciclastic deposits still show a strong Uralian signature. Heavy mineral assemblages suggest erosion of the Uralian orogen reached a deeper metamorphic level. The initiation of Siberian Trap-related input begins to be significant.  In the Jurassic and Cretaceous, southern and northern Taimyr received detritus as an intracratonic basin of the Eurasian platform. Erosion and input from Uralian sources decreased while greater input from Siberian Trap-related rocks of the Taimyr region dominated. The abrupt appearance of staurolite in the Jurassic samples signals rapid uplift and erosion of a staurolite-bearing medium-grade metamorphic rock source. It may reflect the erosion through a thrust belt, where older and deeper rocks were thrusted over younger and shallower rocks.  Comparison of our results to the published literature demonstrates that in the Permian detritus from the Uralian orogen was deposited across Taimyr, Novaya Zemlya and the New Siberian Islands, but not on Wrangel Island or in the Lisburne Hills. In the Triassic, Taimyr, Chukotka, Wrangel Island, the Kular Dome, the Lisburne Hills, Franz Josef Land and Svalbard shared common sources from Taimyr, the Siberian Traps and the Urals. In the Late Jurassic there was a reduction in erosional detritus from the Urals in Taimyr which, together with docking of the Kolyma-Omolon superterrane and the formation of the SAS, restricted the regional distribution of detritus from Taimyr and the Urals.  The AFT data and thermal history modeling of samples with varied lithology, age and locations across the Taimyr fold and thrust belt define Middle Permian, earliest Triassic and Middle Triassic cooling events, reflecting uplift and/or cooling associated with the Uralian orogeny, Siberian Trap magmatism and Mesozoic transpression. The balanced cross section reveals thin-skinned thrusting during Uralian orogeny in the Early Permian and an estimated c. 30% shortening.

7. Suggestions for Future work

Significant progress has been made understanding Taimyr geology and tectonic evolution in recent years, yet many fundamental questions remain, including the age, kinematics, causes, and spatial variations of Paleozoic and Mesozoic deformational events. For example, high resolution zircon age dating for the widely intruded syn- and post- tectonic granitoids in northern and central Taimyr should be performed to better constrain the timing of late Paleozoic tectonic events since the existing age data are few (Vernikovsky, 1995). More low temperature thermochronology work, such as apatite (U-Th)/He dating with a partial annealing zone of 80 – 40 °C can provide information on the lowest-temperature thermal history in the region (Stockli, 2005). The only onshore geophysical work in the public domain related to Taimyr is from the western Yenisei–Khatanga regional trough (Kontorovich, 2011). To better understand the deformation style and structural features, more geophysical surveys and structural studies across Taimyr are needed. This thesis presents a balanced cross section across a short segment of Taimyr; two longer balanced cross sections across both eastern and western Taimyr are under construction and will produce nearly complete transects across the orogen. This will provide a more comprehensive quantification of shortening associated with

A Dissertation for the Degree of Philosophy in Natural Science the Arctic Uralides.

Acknowledgments

How time flies! Now I am writing the last part of my thesis to finish the four-year’s PhD journey. But I still remember exactly how excited and nervous I was on the very first day when I came to Sweden alone. I remember Vicky picked me up at the airport with smile on her face and let me feel so comfortable. There are so many thanks and special appreciation to my main supervisor Professor Victoria Pease. I would like to thank you for accepting me for this project, encouraging my research and helping me to grow as a research scientist. Your advice and support on my research and on my career is most invaluable. Many thanks go to my co-supervisor Dr. Eve Arnold for improving papers and great support throughout my PhD studies. I would also like to thank other people who have helped me during the work of this thesis: Alexey Soloviev and Alina Proskurnina for help in the field; Per-Olof Persson and Kerstin Lindén at the Natural History Museum, Dan Zetterberg, Cora Wohlgemuth-Überwasser and Curt Broman at the Department in Stockholm for assistance with the laboratory work; Swedish Polar Research Secretariat for logistical support. Thank Elin for helping me translate the abstract of my thesis into Swedish. Thank all the administration people for all your work to make everything running efficiently in the department. I would like also thank Hildred Crill for helping me improve my English. I would like to say thank you to my friends in the department, especially the “hard rock people” Wen, Barbara, Clifford, Alex, Hagen and Frank, as well as other PhD students on the second and third floor for all the fun times we have had together. Thanks also go to all my other friends in Stockholm; without you guys, this journey could have been more difficult. Special thanks I would like to express to my husband Bo Wan. I would never have achieved what I have today without your unconditional support and love. Even though we have been apart for most of the time over these four years, I never felt lonely because I know you are always there for me. I also thank my family for always believing in me and supporting me. This work has benefited from financial support from YMER-80 to X. Zhang and Swedish Research Council Funding to V. Pease. Last but not least I would like to express my deepest appreciation of Dr. Robert Scott, who unexpectedly passed away in 2012. He was a co-supervisor and I will never forget his kindness and humor.

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A Dissertation for the Degree of Philosophy in Natural Science

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