Chronologie et évolution des reliefs dans la région Mongolie-Sibérie: Approches thermochronologique et morphotectonique

R.Vassallo (1), J-F.Ritz (1), M. Jolivet (1), R. Braucher (2), S. Carretier (3), C. Larroque (4), A. Chauvet (1), C. Sue (5), M.Todbileg (6), D. Bourlès (2), A. Arzhannikova and S. Arzhannikov (7)

(1) Laboratoire Dynamique de la Lithosphère, Université Montpellier II, France, (2) CEREGE, Aix-en-Provence, France, (3) LMTG,Toulouse, France, (4) Géosciences Azur, Sophia-Antipolis,Valbonne, France, (5) Département de Géologie, Université de Neuchatel, Suisse, (6) Mongolian University of Science and Technology, Ulaan Baatar, Mongolia, (7) Institut of Crust, Irkutsk, Russia.

Our study focuses on the mountain building process of a massif in an intracontinental transpressional domain.The Ih Bogd massif, situated along the Bogd , a major intracontinental active strike- slip fault in the eastern Gobi-Altay (Mongolia), presents a morphology exceptionally well preserved and shows clear relationships between topography and structures, allowing the analysis of its morphological and tectonic evolution. We coupled morphotectonics and 10Be ages to constrain the history of the incision in the main drainage basin of Ih Bogd massif. This study allows a better

understanding of the mechanisms and the timing of the interactions between climate and within Gobi-Altay massifs. 85˚ 90˚ 95˚ 100˚ 105˚ 110˚ 115˚

PROJECTION MERCATOR

Sa Using fission tracks on apatites, Vassallo et al. (in revision for EPSL) dated the peneplanation to the Jurassic. The preservation of this yan Est CRATON surface attests of the low rate in the region - except the occurrence of a few glacial deposits in the upper northern flank, the 55˚ 55˚ Ih Bogd massif does not present any large scale glacial morphology such as moraines, glacial stairways, rock bars - and shows that its ? ? SIBERIEN yanOuest Sa al recent uplift occurred without major tilting or folding. Fission tracks cooling path models suggest that this uplift started between 8 k and 2 Ma, which makes the long-term Cenozoic uplift rate of the massif lying between 0.25 and 1 mm/yr (Vassallo et al., in revision Baï for EPSL). In comparison, the vertical slip rates estimated along the bordering reverse faults during the Upper Pleistocene (0.1-0.2 <100 ka

50˚ Larroque et al., 1999 50˚ mm/yr) are slightly lower (Ritz et al., in press). Arzhannikova et al., 2004

De Vicente, M2 2006 1-5Ma Vassallo, thèse en cours A lta y Go bi-Altay

45˚ 2-8Ma 45˚ Vassallo et al., EPSL, en révision

Mesozoic uplifted peneplain

85˚ 90˚ 95˚ 100˚ 105˚ 110˚ 115˚ time (Ma) 200 150 100 50 82

Altitude : 1950 m IB03-1 Central age : 139 +/- 15 Ma 20 Mean track length : 11.1 +/- 0.2 μm Std dev : 2.0 μm

40

t e 60 m

PAZ p

e

ra t

80 u

e( re °

100 C ) 20 N=100 15 120 3D view of the Ih Bogd massif N Sketch map of the Gurvan Bogd system along the left-lateral Bogd strike-slip fault, Gobi-Altay mountain 10 on SPOT image 5 140 0 range, Mongolia (modified after Ritz et al. 2003) 0510 15 μ Track length ( m) 160 Modelling of the cooling path of the Ih Bogd massif using Fission Track data. These results show that the N S onset of the uplift is comprised between 8 and 2 Ma ago

1675 SW NE

T1 A 1650 T2 surface F3

1625

T3 main incision T4 1600

altitude (m) Δh=10.0± 0.5 m T4 1575 Bogd fault

1550

1525 frontal thrust F3 A'

0 500 1000 1500 2000 distance (m) Digital elevation model of the Bitut at the outlet of the valley, Topographic profile A-A' across the reverse fault scarp on the alluvial fan F3, Photograph of the strath terraces affected by the left-lateral strike- with the localization of the base level of terraces T1,T2 and T3 showing a vertical offset of 10.0 ± 0.5 m slip fault segment of the Bogd fault at the outlet of the Bitut valley

N S W E E W

F4 T2 strath terrace F3 A' R T2 T3 Bogd(-1) fault

bedrock A Bogd Fault T3 T4 Bogd(-1) fault h ~10 m T4 T1 T3 T4 T2 25 ± 2m

P0 ver

i

r

ut

t

i

B

Photograph of the Bitut valley showing the Photograph of the canyon carved into Vertical offset of ~10 m of the base level of the progressive downstream filling of the canyon by the bedrock under terrace T4 alluvial terrace T2 along the Bogd(-1) fault present river sediments. The canyon completely Surface Sample P0 (at/g/yr) 10Be (Mat/g) Uncertainty (Mat/g) T1(outlet) MO-03-01 20.5 1.35E+06 1.22E+05 disappears at the level of the Bogd(-1) fault T1(outlet) MO-03-02 20.5 1.30E+06 1.01E+05 T1(outlet) MO-03-03 20.5 1.51E+06 1.65E+05 Bogd(-1) Fault T1(outlet) MO-03-04 20.5 1.62E+06 1.34E+05 T1(outlet) MO-03-05 20.5 1.13E+06 8.41E+04 T1 weighted mean 20.5 1309304 50035 T2(outlet) MO-03-07 20.1 2.08E+06 1.17E+05 3957 m T2(outlet) MO-03-10* 20.1 1.23E+06 7.38E+04 N Mat/g T2(outlet) MO-03-11 20.1 1.75E+06 1.08E+05 T2(outlet) MO-03-12 20.1 1.87E+06 2.29E+05 <0,05 T2 (profile) MO-03-T2-TOP 20.1 1.69E+06 1.47E+05 T2 (profile) MO-03-T2-40 20.1 1.09E+06 1.39E+05 0,05-0,15 T2 (profile) MO-03-T2-60 20.1 8.22E+05 1.66E+05 2km T2 (profile) MO-03-T2-125 20.1 3.39E+05 5.21E+04 0,15-0,40 T2 (profile) MO-03-T2-160 20.1 4.29E+05 9.93E+04 0,40-0,80 T2 (profile) MO-03-T2-200 20.1 1.33E+05 3.08E+04 Cartography of the fault segments and of the alluvial surfaces along the Bitut valley T2 weighted mean 20.1 1899054 74947 0,80-1,40 N T3 (outlet) MO-03-14* 19.4 7.95E+05 8.01E+04 on aerial 1/35000 Russian photographs of 1958 T3 (outlet) MO-03-16 19.4 1.32E+06 1.29E+05 1,40-2,00 T3 (outlet) MO-03-17* 19.4 7.46E+05 2.65E+05 T3 (outlet) MO-03-18 19.4 1.44E+06 9.87E+04 2200 SN>2,00 B T3 (outlet) MO-03-19 19.4 1.21E+06 2.24E+05 T1 T3 (outlet) MO-03-21 19.4 1.27E+06 1.52E+05 T2 2150 T3 T3 (outlet) MO-03-22 19.4 1.49E+06 1.90E+05 T3 (valley) MO-03-34* 29.4 5.08E+05 4.50E+04 P0 T4 T3 (valley) MO-03-54* 27.3 4.53E+05 8.83E+04 2100 T3 (valley) MO-05-8* 22.5 4.59E+05 6.29E+04 * T3 (valley) MO-05-9* 22.5 8.45E+05 1.26E+05 2050 T3 (profile) MO-03-T3-0 19.4 1.04E+06 1.33E+05 = position of P0 on the 1/100,000 topographic map T3 (profile) MO-03-T3-30 19.4 7.96E+05 6.49E+04 = kinematic GPS data for T1 base level T3 (profile) MO-03-T3-50 19.4 4.63E+05 4.85E+04 2000 B:bedrock;T1,T2,T3,T4:strathterraces T3 (profile) MO-03-T3-75 19.4 2.65E+05 3.29E+04 Bitut river = kinematic GPS data for T2 base level T3 (profile) MO-03-T3-100 19.4 2.04E+05 4.14E+04 = kinematic GPS data for T3 base level Graphical representation of the distribution of the 10Be concentrations within the Bitut valley. 1950 T3 (profile) MO-03-T3-120 19.4 2.37E+05 1.06E+05 * T4 = kinematic GPS data for T4 base level Concentrations of terraces T1, T2 and T3 at the boundary of the massif, as well as the T3 weighted mean 19.4 1367828 48532 *= kinematic GPS data for the Bitut river T4(outlet) MO-03-29* 19.4 4.47E+06 3.18E+05 1900 concentration of the bedrock profile at the core of the massif, are weighted means

T4(outlet) MO-03-30 (T) 19.4 1.68E+05 3.39E+04 altitude (m) * T3 T4(outlet) MO-03-31 (B) 19.4 1.93E+05 3.10E+04 1850 Bogd(-1) fault T4(outlet) MO-03-32 19.4 3.05E+05 3.16E+04 10Be Concentration (at/g) T2 T4(outlet) MO-03-35* (T) 19.4 7.58E+05 2.69E+05 * 0.0E+00 5.0E+05 1.0E+06 1.5E+06 2.0E+06 0 T4(outlet) MO-03-36 (B) 19.4 1.52E+05 2.34E+04 1800 20 T4 (valley) MO-03-46 25.0 1.06E+05 6.12E+04 * 40 T4 (valley) MO-03-47 25.0 2.85E+05 2.96E+04 1750 60 T4 (valley) MO-03-48 25.0 4.02E+05 5.17E+04 * T1 T4 (valley) MO-03-49 25.0 2.52E+05 8.94E+04 80 1700 * 100 T4 (valley) MO-03-50 25.0 1.77E+05 3.99E+04 * T4 (valley) MO-03-51 (T) 25.0 1.16E+05 2.00E+04 * 120

* Depth (cm) T4 (valley) MO-03-55 (B) 25.0 1.78E+05 1.87E+04 Bogd fault * frontal thrust 140 1650 * T4 weighted mean 25.0 188686 8882 * 160 bedrock T4 MO05-1 25.9 21169 5330 * 180 1600 bedrock T4 MO05-2 25.9 19832 5334 200 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 T2 bedrock T4 MO05-3 25.9 11240 4262 220 bedrock T4 MO05-4 25.9 6852 3432 distance (m) bedrock T4 weighted mean 25.9 12566 2181 Longitudinal profiles of the base levels of the strath terraces along the Bitut river, Boulder embedded in an alluvial surface with erosion of the sandy Results of the 10Be analysis obtained from kinematic GPS data and 1/100,000 Russian topographic map matrix by wind deflation, and depth profile of the 10Be distribution within a 2 m pit-soil in terrace T2 t0 ~600 ka S N

Gurvan Bulag thrust fault Summit plateau Bitut river Bogd fault Bogd(-1) fault 4000 m P0 position of 3000 m the theoretical riverbed P0 2000 m STUDY AREA

1km 1000 m

0 t1 ~330 ka 0 5km 10 km Metabasite Quartz-diorite Granite Metasediment (micaschist and marble) Orthogneiss Basalt T1 Gneiss Migmatite Conglomerate P0 Alluvial fans Major fault with pre- Major fault with cenozoic and cenozoic only pre-cenozoic activity activity

SNAB t2 ~230 ka 5 ± 3Ma 1 N

Bogd fault

T1 T2 A P0 20 km

2

t3T3 ~110 ka T1

T2 P0 3

t4 0-5 ka Present T1 4 Bogd(-1) fault Sketch of the evolution of the morphology of the Bitut valley over the Upper Pleistocene - Holocene. (A) T4 10 km

T2 Projections of the longitudinal river profile and of the alluvial markers. (B) Corresponding block diagrams of the T3 P0 valley at the core of the massif. (C) Corresponding block diagrams of the outlet of the drainage basin. 1) At the Bogd fault transition between the penultimate glacial and interglacial periods (~125-110 ka): enhanced stream power caused by the wetter climate triggers important erosion removing the colluviums that had accumulated within the drainage basin during the previous glacial period. Huge debris-flows, gathering the available sediment load, Sketch of the chronology of the evolution of (A) Block diagrams representing the growth of the Ih Bogd massif, from the transport the material within the drainage network abrading the bedrock in the drainage basin, and resulting in the Bitut valley morphology during the last onset of the uplift to the present stage. This growth is characterized by the the formation of the T3 alluvial terrace inside the massif, and of the F3 fan at the outlet of the valley, which is shift of the active deformation from the successive accretion of crustal blocs, according to the outward migration of rapidly abandoned. On figure (A), dotted line represents the following incision and abandonment of the terrace. 2) Bogd(-1) to the present Bogd fault the brittle deformation. (B) Corresponding stages in plan view. Note the bi- During the subsequent glacial period (~100-20 ka), the massif is uplifted and is affected by very little river incision, directional (transversal and longitudinal) growth of the massif almost localized at the outlet of the valley. At the core of the massif, detrital material accumulates on the slopes and in the drainage network by thermoclastic processes (Note that the main incision occurred during the previous interglacial). 3) At the transition between the last glacial and the present interglacial period (20-15 ka), CONCLUSION the new climate pulse leads to the formation of the terrace T4 and of the fan F4, which is rapidly abandoned. Note that, at the outlet of the valley, the alluvial fan F4 is set in the alluvial fan F3 even downstream the reverse fault. 4) Our study shows that the process of incision/abandonment of the alluvial terraces along the Bitut valley integrates the During the interglacial period, between ~5 ka ago and the present, a strong river incision creates a new canyon response to the uplift of the Ih Bogd massif as well as the response to the growth of the hydrographic network. From and causes the abandonment of the terrace T4. 5) After this incision phase, the canyon is progressively filled up our morphotectonic and 10Be data, it is not possible to estimate the amount of each of these two components along downstream by the present river load the river. In any cases, the evolution of the river profile through time, documented by the geometry of the abandoned strath terraces, is mainly the result of the growth of the hydrographic network associated with the retreat of the summit FINANCEMENTS Reliefs de la Terre : 8000 euro (2004-05) + 25000 euro (2005-06) = 33000 euro plateau. This is particularly obvious at the core of the massif, where the river incision rate is 0.6-0.9 mm/yr during the UMR 5573 Montpellier : 2800 euro (2004-05) + 1500 euro (2005-06) + 2000 euro (2006-07) = 6300 euro Upper Pleistocene and ~6 mm/yr during the Holocene. Total : 39300 euro The morphology of the Bitut valley is strongly controlled by the cyclicity of the climate, which is characterized by the PUBLICATIONS alternation of long and dry glacial periods with short and wet interglacial periods.The formation of the alluvial surfaces Jolivet, M., Vassallo, R., J-F. Ritz, R. Braucher, A. Chauvet, C. Larroque, C. Sue, M. Todbileg, N. Arzhannikova and S. Arzhannikov. Long-term preservation of an uplifted mesozoic occurs at the transition between glacial and interglacial periods, when the capacity of the rivers due to important peneplain , in prep. waterfalls allows removing and transporting the sediments accumulated within the basin. This fluvial process, Vassallo, R., Chronologie et évolution des reliefs dans la région Mongolie-Sibérie: Approches morphotectonique et thermochronologique, thèse, 2006. apparently controlled by the global climate changes, is probably related to the aridity of the region. A similar pattern De Vicente, R. , Surrection du massif de Baatar Khairhanii (Altay, Mongolie): analyse morphotectonique et thermochronologique traces de fission sur apatites, master 2, 2006. were described in the hot deserts of the American southwest and the Middle East (Bull, 1991). Vassallo, R., J-F. Ritz, R. Braucher, M. Jolivet, A. Chauvet, C. Larroque, S. Carretier, D. Bourlès, C. Sue, M. Todbileg, N. Arzhannikova and S. Arzhannikov. Timing and evolution of the Ih While the abandonment of the alluvial fans is almost instantaneous, the incision and abandonment of the alluvial Bogd restraining bend (Gobi-Altay, Mongolia) combining structural , morphotectonics and 10Be dating,Tectonics, soumis. terraces occurs several thousand years later. This lag illustrates the time for the river to pass from an aggradational Vassallo, R., M. Jolivet, J-F.Ritz, R. Braucher, C. Larroque, C. Sue, M.Todbileg and D. Javkhlanbold. Uplift age and rates of the Gurvan Bogd system (Gobi-Altay) by apatite fission track analysis, Earth Planet. Sci. Lett., en revision. regime to a downcutting regime. This process is probably due to the progressive decrease of the sediment load transported by the river. Ritz, J-F., R. Vassallo, R. Braucher, E. Brown, S. Carretier and D.Bourlès, 2006. Using in situ-produced 10Be to quantify active tectonics in the Gurvan Bogd mountain range (Gobi- Altay, Mongolia), GSA Bulletin, sous presse. The staircase morphology of the massif and the flatness of the summit plateau attest that the uplift of the massif is CONGRES mainly controlled by reverse faults, and that the active faulting progressively migrates outward, on the edges of the

Vassallo, R., J-F.Ritz , R. Braucher, S. Carretier, M. Jolivet, D. Bourlès, A. Chauvet, C. Larroque , C. Sue , M.Todbileg, 2006.Timing and evolution of the deformation in a restraining bend: relief. On the northern flank of Ih Bogd, the present frontal fault began to uplift the massif ~600 ka ago, while the example of the Ih Bogd massif, Gobi-Altay, Mongolia. European Geosciences Union,Vienna, SRef-ID: 1607-7962/gra/EGU06-A-09044. previous one ceased its activity between 100 and 200 ka ago.Therefore, during the shifting process, the two faults have Vassallo, R., M. Jolivet , J-F.Ritz , R. Braucher, N. Arzhannikova, S.Arzhannikov , A. Chauvet , C. Larroque , C. Sue and M.Todbileg , 2006. Chronology and uplift rates of the relief in the been both active for a period of ~400-500 kyrs. From the vertical offset of the alluvial fan that was abandoned during Altay and the Gobi-Altay mountain ranges (Mongolia). European Geosciences Union,Vienna, SRef-ID: 1607-7962/gra/EGU06-A-00573. the penultimate interglacial period, when the vertical movement was already limited to the most external fault, we Vassallo, R., J. F.Ritz, M. Jolivet, R. Braucher, C. Larroque, C. Sue, M.Todbileg, D. Javkhlanbold, S. Carretier and D. L. Bourlès, 2005. Age and rates of the late Tertiary-Quaternary uplift of estimate a ~0.1 mm/yr Upper Pleistocene - Holocene uplift rate of the massif. The lateral growth of the outermost the Gurvan Bogd system (Gobi-Altay) coupling fission tracks and 10Be analysis, Bulnay Field Conference, Mongolia, 22-29 July. reverse faults (Dalan Turuu foreberg to the North and Gurvan Bulag foreberg to the South) would correspond to the Vassallo, R., J-F. Ritz, R. Braucher, M. Jolivet, C. Larroque, C. Sue, M. Todbileg, D. Javhaa and D. L. Bourlès, 2004. Timing and uplift rates in Cenozoic transpressional mountain ranges within the Mongolia-Siberia region, RST Strasbourg, September 2004, Abstract n°RSTGV-A-00210. early stage of a new shift of the deformation in the mountain building process of the Ih Bogd massif.