169 CHAPTER 1. THEORY OF FAULTING AND EVALUATION OF TIMING OF FAULT MOVEMENTS: METHODOLOGY CHAPTER 2. NEOTECTONICS OF ALTAI-SAYAN CHAPTER 3. GEOLOGICAL AND GEODYNAMICAL SETTINGS CHAPTER 4. THE TELETSK BASIN CHAPTER 5. NORTHEAST ALTA CHAPTER 6. bbb CHAPTER 7. bbbi CHAPTER 8. TUVA AND WEST-SAYAN 8.1 Introduction 8.2 Tannu-Ola range 8.3 Tuva basin 8.4 West-Sayan fault 8.5 Synthesis This chapter gives an overview of the deformation in the Tuva-Sayan region, affecting several Cenozoic sedimentary basins. Deformation inside the basins provides a relative age control. The deformation is compared to the deformation described for NE Altai, and the reasons for the differences are discussed. 8.1 Introduction Tuva is the name of an Autonomous Republic within the Russian Federation, forming its southern border with Mongolia (fig. 8.1). It is located East of the Shapshal range (fig. 1.2a; fig. 1.10, 1.11 and 1.12), and adjoins the Teletsk and Shapshal structures discussed in chapters 6 and 7. It forms the transition zone between the Altai-Sayan structures in the northwest and the Baikal structures in the East (fig. 8.1). In the northern part of Tuva, the West-Sayan mountain range stretches northeasterly, delimited at its northern edge by the West-Sayan fault (fig. 1.2a). The southern limit of Tuva is formed by the Ubsu-Nuur basin (number 7 on fig. 1.10 and fig. 8.1) and the Ureg-Nuur basin (number 6 on fig. 1.10 and number 9 on fig. 1.12). The occurrence of Neogene and Quaternary sediments in several basins allows for a stratigraphic age constraint of the Cenozoic deformation. The West-Sayan mountains are separated from the Ubsu-Nuur basin by the curvilinear, E-W trending Tannu-Ola mountain range (fig. 1.2a and fig. 8.1) and by the Khemchuk depression (number 11 on fig. 1.12 and fig. 8.1). A large amount of recent earthquakes are observed mainly along the southern and northern borders of the Tannu-Ola range and the Khemchuk basin (fig. 1.7). These zones are related to a high seismic activity (fig. 1.8). Both parameters show an ENE trend, suggesting that active deformation occurs along ENE trending structures. Investigation of the region allows for characterisation of the active deformations along the ENE trending zones, namely the West-Sayan fault, the Khemchuk basin and the Tannu-Ola range (fig. 8.1). Comparison of these structures with the studied zones to the West will provide information on the spatial evolution of the deformation in the area.
Chapter 8 Investigated regions: Tuva and West-Sayan 170 Fig. 8.1 Geographical settings of the studied zones in West-Sayan and Tuva. The DTM at the top shows the studied structures in more detail. Black arrows outline the West Sayan fault zone, and the inset shows the location of figure 8.5.
Chapter 8 Investigated regions: Tuva and West-Sayan 171 8.2 Tannu-Ola and the Ubsu-Nuur basin The Ubsu-Nuur basin forms a 300 km by 200 km broad depression (figs. 8.1 and 8.2). To the south it is bordered by the active Bolnai fault that forms a major left-lateral active strike-slip fault (cf. chapter 1). The Western border of the basin forms the seismically most active zone of the whole region (fig. 1.7 till fig. 1.9). The average altitude of the basin is about 1000 m (profile 3 on fig. 1.2b). The lowest, northern part is filled by the shallow and salty lake Ubsu Nuur. The Ubsu-Nuur basin has no outlet. The basin started to form in the Cretaceous [Dobretsov et al., 1996], and was possibly filled by sediments eroded from the uplifting ranges in the Altai. The uppermost sediments observed at the surface of the basin consist of red Neogene clay and silt of lacustrine origine, covered by Quaternary fluvial deposits. East from lake Ubsu-Nuur the basin is transected by a northeast trending narrow mountain range of about 100 km long and several km wide (Agardak range on fig. 8.2). Along this range, the sediments are faulted and folded, indicating a late Cenozoic (moderate) deformation (fig. 8.2A B and C). Clay-coated striae inside the red sediments indicate left-lateral strike-slip along northeast trending faults parallel to the range, and rightlateral strike-slip faults on similar faults, trending northwesterly (fig. 8.2B). The strike-slip along the Agardak range seems to induce the development of small pull-apart basins (fig. 8.2A and C). They are several decametres long and display many normal faults inside the clay, with small offset. Locally, gentle anticlinal folds within the sediments occur, with a N100E subhorizontal fold axis, suggesting local shortening. The extensional and contractional features develop along strike of the undulating strike-slip zone. Together with the paleostress reconstructions from the observed strike-slip faults (fig. 8.2B), they indicate a general northeast directed S Hmax. The fault kinematics inside the Cenozoic sediments provide an upper age constraint for the deformation in the region, that should post-date the late Neogene depositional age of the sediments. In the North, the Ubsu-Nuur basin is bordered by the Tannu-Ola mountain range, that emerges abruptly from the basin (profile 2 on fig. 1.2b) with a foreberg system of foreland hills (fig. 8.3), in which the Neogene sediments are folded and locally faulted. The forebergs extend a few kilometres from the mountain range front and vanish gradually into the flat basin. In some places inside the forebergs, the sediments are tilted along a horizontal E-W trending axis, with a dip increase approaching the mountain front (fig. 8.4). In other places, highly variable bedding dips and trends (on a 500 m² area) indicate small fault blocks, separated by thrust, back-thrust and reverse faults. Sometimes the forebergs have an internal anticlinal structure (fig. 8.4). Outside the foreberg system, outcropping Neogene sediments along river valleys indicate regular and sub-horizontal red clay and fluvial deposits. The foreberg hills are probably driven by an underlying thrust fault accommodating the shortening by propagating into the foreland. Inside the forebergs, no signs for strike-slip movements were observed. Inside the Paleozoic bedrock, outcropping along the mountain range front, steep reverse faults with thick unconsolidated breccia gauge were observed (fig. 8.2E). They indicate a top-to-thesouth reverse movement responsible for the mountain range uplift (fig. 8.4). Several thrusts and NE trending dextral strike-slip faults also occur. More inside the mountain range, leftlateral strike-slip and oblique faults with an easterly to northeasterly trend occur (fig. 8.2F). It
Chapter 8 Investigated regions: Tuva and West-Sayan 172 indicates a left lateral component in the shallow deformation inside the mountain range. Inversion of the fault slip data yields a compressional local stress field with a NE trending S Hmax (fig. 8.2E and table 8.1). Fig. 8.2 DTM of the Ubsu-Nuur basin and surrounding ranges. Observed fault kinematics for Cenozoic deformation are indicated on the stereoplots (A to F). Extrapolation of the regional deformation is indicated on the DTM for the Agardak and Tannu-Ola regions. The other regions show fault kinematics derived from earthquake focal mechanisms (fig. 1.9). The question mark indicates the unstudied western Tannu-Ola.
Chapter 8 Investigated regions: Tuva and West-Sayan 173 Fig. 8.3a Example of the foreberg system in front of the Tannu-Ola range. They appear as folded Neogene rising up from the surrounding plane. Fig. 8.3b View from the Tannu-Ola range towards the Ubsu-Nuur basin. Lake Ubsu Nuur is indicated by the open triangles. Forebergs inside the Ubsu-Nuur basin (some of them indicated by the black triangles) are the narrow ridges of hills rising up over the surrounding erosional surface and alluvial fans, and situated within a few kilometres of the neighbouring mountain.
Chapter 8 Investigated regions: Tuva and West-Sayan 174 Agardak- range: Pure extension and strike-slip in pull-apart basins Site Lat. (dec.) Long. (dec.) Description n nt ó1 ó2 ó3 R á Q R' Tuv08 50 39 94 84 Agardak range, Neogene basin 10 11 90/000 00/030 00/120 0.01 01.0 A 0.01 Tuv09 50 24 94 43 Agardak range, folded neogene 08 10 25/039 65/215 02/308 0.84 09.5 A 0.84 Tuv11 50 23 94 56 Neogene basin 14 16 83/295 06/155 05/065 0.15 10.0 A 0.15 Weighed mean: 3 tensors 38 86/335 03/000 03/090 0.33 0.33 Tannu-Ola range: Pure compression Site Lat. Long. Description n nt ó1 ó2 ó3 R á Q R' Tuv23 50 73 94 18 South Tannu-Ola: iron-coated striae 8 12 01/023 05/292 85/124 0.54 13.7 B 2.54 Tuv25 51 03 94 62 North Tannu-Ola: iron-coated striae 18 25 04/353 22/262 68/094 0.40 16.5 B 2.40 Weighed mean: 2 tensors 26 03/008 23/277 66/105 0.47 2.47 West-Sayan Fault Zone, Sayan range, transpression Site Lat. Long. Description n nt ó1 ó2 ó3 R á Q R' Tuv46 52 10 89 76 faults with non-consolidated breccia and clay str. 15 19 22/351 68/172 22/262 0.21 09.0 A 1.79 Table 8.1 Paleostress determination for active deformation along the Agardak, Tannu-Ola and West-Sayan ranges. Explanation of the symbols on table 6.2. Fig. 8.4 Schematic cross section of the northern border of the Ubsu-Nuur basin and it junction with the Tannu-Ola range. S o indicates the bedding in Neogene sediments. S 0 ' bedding in Cambrian conglomerates and volcanic rock.
Chapter 8 Investigated regions: Tuva 175 The observed reverse structures in the Cenozoic sediments indicate that the Tannu Ola range continued its uplift along E-W trending faults after the Neogene, and the deformation front since then moved towards the basin, causing the shortening features in the deformed foreberg Neogene sediments. There is a component of left-lateral strike-slip in the uplift deformation (observed in the mountain range proper, but much less in the foreberg zone), and the Tannu- Ola range can therefore be described as a transpressional flower structure. Unfortunately, a detailed field study of the characteristics of the contact between the northern border of the Tannu-Ola mountain range and the Khemchuk depression could not be carried out. The single outcrop zone studied in this zone indicated strong oblique (contractional) rightlateral movements along various faults, most of them trending northwesterly (fig. 8.2D). The data were not obtained from deformed Cenozoic sediments, but from iron-coated brittle faults in the Paleozoic northern mountain front. Age control therefore is less clear. Morphological indications for the existence of E-W trending reverse faults, however, are widely spread. Inversions of earthquake focal mechanisms indicate the active movements at depth (average 20 km in the area), and show (except for many reverse movements) some left-lateral movements on E-W trending fault systems [Delvaux et al., in preparation -b]. Paleostress reconstructions from the fault-slip data suggest a strike-slip related extensional regime in the Agardak range (so inside the Ubsu-Nuur basin), and a compressional regime in the Tannu-Ola region (table 8.1). Recent paleostress reconstructions from inverted focal mechanisms [Delvaux et al. in preparation -b] show a northeasterly trending S Hmax. The characteristics of the studied zones suggest that the whole Tannu-Ola region forms as a pop-up transpressional flower structure with a sinistral component in a N-S trending shortening zone between the Ubsu-Nuur block and the Khemchuk- Tuva basin. 8.3 The Khemchuk depression The Khemchuk depression is in fact a general term for a complex assemblage of tilted blocks, squeezed between the West-Sayan- and Tannu-Ola ranges. Inside the Khemchuk depressions, several small Cenozoic basins filled by lacustrine Neogene sediments developed (fig. 1.10). The sediments seem to be deposited in gentle extending basins, as is suggested by the irregular shape of the basins. The sediments proper are hardly deformed. The structural outline of the Khemchuk depression is complex, but reverse movements seem to dominate the deformation. An example of recent reverse movements along the northern border of the Khemchuk depression is given on fig. 8.5 (outline on fig. 8.1). The observed reverse faults are associated to fault-breccia zones and deformed slope-deposits. Remnants of Neogene silty sediments are found uplifted for 200 metres. Layering in Ordovician sediments trend N85E, whereas the reverse faults trend almost northeasterly. No indications for strike-slip faulting have been observed. This deformation indicates the shortening related uplift of the West-Sayan range relative to the Khemchuk depression.
Chapter 8 Investigated regions: Tuva 176 Fig. 8.5 Tectonic contact between the West-Sayan range (northwest) and the Khemchuk depression (southeast). Yellow colour on the satellite image (Landsat MSS) indicate Neogene silt and sandy deposits. Brownish red colours are Ordovician Quartzite and granites, grey colour reflects Quaternary slope deposits and weathered crust. Cross section shows the reverse faults affecting Neogene deposits (Ng). S 0 indicates layering in Ordovician sediments. Figure location on fig. 8.1. 8.4 The West-Sayan fault To conclude the description of the deformation observed in the West-Sayan and Tuva region, we discuss the West-Sayan fault (fig. 1.2a). This ENE trending reactivated Paleozoic shear zone marks the northern limit of the West-Sayan Range, and runs from the Teletsk basin to the East-Sayan mountains in the Baikal area. Deep cataclastic planes and mylonitic foliation trend northeasterly and show top-to-thenorthwest thrust movements. In several zones, reactivation of these deep structures, with the occurrence of unconsolidated fault breccia and clay gauge suggest a shallow and more recent reactivation. Also, joint sets of variable trends showed iron-coated slickensides. In places where a sense of movement could be related to this reactivation, reverse and thrust movements dominated the strike-slip motions. The latter showed generally left-lateral movements on northeasterly trending segments. Only in the northern extremity of the West-Sayan range, just at the border with the foreland depression, a good set of conjugated strike-slip faults
Chapter 8 Investigated regions: Tuva 177 developed. In the examined segments (red dots on the right side on fig. 8.1), the West-Sayan fault is a transpressional left-lateral strike-slip fault with a large reverse component in its movement. This corresponds to a local compressional strike-slip stress regime with a NNW trending S Hmax (table 8.1). 8.5 Conclusions As observed for northeastern Altai, the West-Sayan and Tuva region are characterised by a post-neogene phase of transpressional deformation. Except for local variations, the general convergence direction and S Hmax direction are the same over both regions. The main difference in tectonism is the trend of the major faults. In Tuva and West-Sayan, they are overall oriented E-W and northeast. The northwestern and meridional active structural grain of northeast Altai is, except again locally, not observed, but replaced by a E-W to NE trend (fig. 1.11). The similarity between the deformation observed in the field and that derived from earthquake focal mechanisms (in terms of fault orientation and slip sense) indicates that the post-neogene deformation with a N-S to NE trending S Hmax is continuing at present. The exact onset of the deformation, however, could not be determined due to a lack of stratigraphic control. The West-Sayan fault, forming the northern border of the deformation zone, shows sinistral oblique-slip movements. The Bolnai fault, forming the southern border of the Ubsu-Nuur basin undergoes active pure sinistral strike-slip [e.g. Baljinnyam et al., 1993]. The zone in between these two bounding structures is characterised by a strong component of shortening in the movements, resulting in reverse pop-up, with a strike-slip component. The Khemchuk basin is separated from the West-Sayan range by a system of reverse faults, and the E-W trending Tannu-Ola range emerges from the Ubsu-Nuur basin as a transpressional flower structure. The intensity of the deformation, qualitatively estimated from the morphological expression of the faults, clearly diminishes towards the north, as does the seismic activity (fig. 1.7) and the topographic differences (fig. 1.2b). The observations indicate a change in the deformation between the strike-slip zones in the East (Altai) and south (Bolnai) and the shortening related movements in Tuva and West-Sayan. Apparently, the main reason for the strain partitioning seems to be the orientation of the basement structural grain, that is oriented at a higher angle to the overall shortening direction in Tuva and West-Sayan than in the West (Altai). This results in more contractional deformation in the East than in the West. The presence of the Quaternary sedimentary basins in such settings can be best explained as inheritance basins, where they form regions in the crust that are more resistant to deformation (Ubsu-Nuur, Khemchuk), and thus are not uplifted yet, in contrast to the ranges separating them (Tannu-Ola and West-Sayan). The bigger size (than in northeast Altai), the high altitude and the relatively small amount of Quaternary sediments in the basins support this idea. The reason for the tectonic quiescence of the Ubsu-Nuur basin could be related to the existence of a negative seismic velocity anomaly in asthenospheric mantle under the basin (fig.
Chapter 8 Investigated regions: Tuva 178 1.5), keeping its lithosphere more plastic and susceptible for bending (and subsiding) under horizontal shortening. The West-Sayan fault, displays mylonitic bedrock, ophiolites and strongly sheared schists along its strike, on which the neotectonic reactivation has been superimposed. The Agardak range, undergoing active sinistral strike-slip and related uplift is composed of ultra-mafic melange rocks, also associated to a Precambrian suture. However, the other main active fault zones, along the Tannu-Ola range and north of the Khemchuk depression mainly reactivate nonmetamorphic bedding planes of Paleozoic clastic sedimentary rocks and volcanics. No polyphase shear zones were observed in both areas.