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THE « TECTONICS, CLIMATE AND DENUDATION » DEBATE
On the basis of these discoveries, Raymo and Ruddiman (1992) and Molnar and England (1990) laid the foundations for the present debate upon the interactions between tectonics, climate and denudation (review e.g. in Champagnac et al., 2014). Raymo et al. (1988), Raymo (1991) and Raymo and Ruddiman (1992a, 1992b) advanced that uplift of mountain ranges could have triggered climate cooling through the consumption of CO2 by silicate chemical weathering, supposedly favoured by enhanced erosion due to relief increase. Their hypothesis was given even more weight with the findings that mountain uplift also favoured carbon burial (Derry and France-Lanord, 1996; France-Lanord and Derry, 1997; Galy et al., 2007), because of erosion and rapid burial in anoxic conditions.
In contrast, Molnar and England (1990) revive the hypothesis of Donnelly (1982) and dispute the premise of Raymo and Ruddiman (1992) by proposing that climate cooling was the primary driver of accelerated erosion. They advance as a cause that glacial processes should be more efficient to erode rocks than fluvial processes. Accelerated erosion would have led in turn to the uplift of mountain peaks and increase in relief. The two positions led to an explosion of studies about the potential links between climate, tectonics and erosion. The debate culminated for the late Cenozoic, with the revealed contradiction between evidences of the drop of greenhouse gases (Lüthi et al., 2008; Beerling and Royer, 2011; Foster et al., 2017), evidences for climate cooling and ice-sheet development (Zachos et al., 2001; Lisiecki and Raymo, 2005; Hansen et al., 2008), evidences of a global and substantial acceleration of erosion (Zhang et al., 2001; Herman et al., 2013), all of them apparently opposing to evidences of stable silicate chemical weathering rates (Willenbring and von Blanckenburg, 2010; Misra and Froelich, 2012).
Figure I-3. Deep sea average sediment accumulation rate since 150 Ma.
Accumulation rates are presented in function of the age of the underlying oceanic crust, with 1 Myr age-bin means and standard deviations. The data are compared with two models: a steady-state accumulation model considers an accumulation rate depending on ocean crust age and uniform in time; a time-dependent model considers an accumulation rate depending on time and uniform in space (from Olson et al., 2016; including data compiled by Hay et al., 1988).
To further investigate the debate about tectonics, climate and denudation, this thesis aims to obtain an independent temporal record of short-term erosion rates at the orogen-scale and at low-latitudes over the late Cenozoic time span. The Himalaya, at the convergence of the Indian and Eurasian plates, has epitomized this debate and will be the object of the present thesis, through geochemical and isotopic measurements applied on deep sea sedimentary sites drilled in the Bengal Bay (France-Lanord et al., 2016a; Clemens et al., 2016) and on a new continental sedimentary section in the Valmiki Wildlife Sanctuary, National Park & Tiger Reserve, Bihar, India.
The limits of the Ganga-Brahmaputra catchment are indicated in red. The Exp. 354 sites analyzed in this thesis and the new Siwalik section (Valmiki) along with a published Siwalik section (Surai) are indicated. Sediment thickness (compilation of Dasgupta et al., 2000 and Radhakrishna et al., 2010, including data of Curray et al., 1991) have poor resolution along the eastern coast of India and in the Nicobar Fan. Modern glaciers compiled in Armstrong et al., 2005; Raup et al., 2007. Asian South Lambert Conformal conic projection.
Physiographic and geological units
The Himalaya (Figure II-4-Figure II-5) is a fold-and-thrust belt that has developed since the collision at the early Cenozoic between the Indian and Eurasian plates (Patriat and Achache, 1984; Meng et al., 2012; DeCelles et al., 2014; Hu et al., 2015, 2016). The Himalaya forms an NW-SE trending 2,400 km-long and 300-400 km-wide arc, with two corners termed western and eastern syntaxes, and is classically divided in subparallel elongated physiographic and geologic units potentially homogeneous along strike (Gansser, 1964; Le Fort, 1986; syntheses of Hodges, 2000; Goscombe et al., 2006; DeCelles et al., 2016; review in Valdiya, 2015; Garzanti, 2019).
At the south of the Himalaya (Figure II-5) lies the foreland sedimentary basin, consisting in the Ganga and Brahmaputra floodplains (avg. < 120 m.a.s.l.). The floodplains extend to the Bengal delta (avg. < 20 m.a.s.l.), with the sedimentary system continuing in the Indian Ocean, through the submarine delta on the Bengal shelf to the turbiditic Bengal Fan (e.g. Curray et al., 2003). The Neogene cover of the foreland basin has been exhumed and folded at the front of the Himalaya, in the Siwalik hills (avg. Asian South Lambert Conformal conic projection (map drawn from compilations Tables SII-1 and SII-2). Published complementary structural sections are provided in Figure II-9.
500 m.a.s.l.), bound to the south by the Main Frontal Thrust (e.g. Mugnier et al., 1999; Lavé and Avouac, 2000). The Cenozoic cover of the southeastern flank of the basin has also been exhumed in the Indo-Burman fold-belt (avg. 1,300 m.a.s.l., with high relief; e.g. Maurin and Rangin, 2009), along with Mesozoic marine sediments and ophiolitic assemblages (Mitchell, 1993; Allen et al., 2008), in the continuity of the Sunda subduction Trench. Remnants of the Cretaceous traps and Indian Precambrian craton (< 1,000 m.a.s.l.) rise at the southern limits of the Ganga floodplain and along the Brahmaputra (the Shillong or Meghalaya plateau, avg. 1,300 m.a.s.l., and the Mikir hills).
Separated from the Siwaliks by the Main Boundary Thrust (Gansser, 1964; Meigs et al., 1995; DeCelles et al., 1998), rises the Lesser Himalaya (avg. 2,000 m). The Lesser Himalaya consist of low- to medium-grade metasediments, mainly of Precambrian age and with an inverted metamorphic gradient, with occasional crystalline or sedimentary nappes of lithologically similar to the High Himalaya and the Tethyan unit respectively (e.g. Célérier et al., 2009; Yu et al., 2015).
Separated from the Lesser Himalaya by the Main Central Thrust (e.g., Gansser, 1964; Le Fort, 1975) rises from 3,000 m to more than 8,000 m.a.s.l. the High Himalaya, also coined Greater Himalaya, with avg. elevation > 6,000 m.a.s.l. and relief occasionally > 5,000 m. The High Himalaya consists in medium to high-grade metasedimentary and meta-igneous rocks, with an inverted metamorphic gradient, intruded by synorogenic leucogranites (Le Fort et al., 1987).
Separated from the High Himalaya by the South Tibetan Detachment (Burg et al., 1984; Burchfiel et al., 1992; Searle et al., 1997) the Tethyan formations partly cover the north of the high range, with a relief similar to the High Himalaya, and the south of the Tibetan plateau, with avg. elevation northwards progressively decreasing to 4,500 m. The Tethyan unit consists in the Precambrian to Eocene low-grade metasedimentary succession deposited at the north of the Indian margin, and occasionally contains granitoid intrusions or older crystalline rocks.
Separated from the Tethyan realm by the ophiolitic suture, an assemblage of sediments and ophiolitic complexes, the Transhimalayan formations extend northwards on the Tibetan plateau, and are derived from the magmatic arcs linked to the Cretaceous to Paleocene subduction of the Neotethys.
Precipitations and hydrography
Asia is subject to two seasonal monsoons, the South Asian monsoon and the East Asian monsoon (review in Wang, 2006; Molnar et al., 2010), which produce a contrast between dry winter and wet late spring/summer seasons. The South Asian monsoon, which includes the regional Indian monsoon, results from the cross equatorial heat transfer between the Southern Hemisphere, dominated in the region by the Indian Ocean and the Northern Hemisphere partly covered by the Asian continent. This heat transfer is partly controlled by the orographic effect of the Himalayan range (modelling of Boos and Kuang, 2010, 2013; discussion in Molnar et al., 2010).
At the time of the debate between Raymo and Molnar, Asian monsoons were supposed to have appeared or strengthened in the late Cenozoic (Quade et al., 1989) but they revealed to be as old as ca. 24 Ma (Clift et al., 2008, 2014; Clift and Webb, 2018), potentially dating back to ca. 34 Ma (Licht et al., 2014; Gupta et al., 2015) or earlier (Caves et al., 2015; Caves Rugenstein and Chamberlain, 2018).
The map is interpolated from gauge station annual data of the Aphrodite Network. The swath profile presents for central Nepal the annual precipitations interpolated from different datasets, along with topography. Minimum-maximum are represented by shaded curves. Gauged data are plotted with an error-bar corresponding to the 30 yrs maxima and minima (modified from Andermann et al., 2011).
reported stations being in valleys and receiving 1,000-2,000 mm/yr (Anderman et al., 2011). A strong contrast exists between the wet southern flank and the drier areas in the rain shadow of the higher summits, such as the Mustang area, and the Tibetan plateau ( 450 mm/yr in Lhasa). Similar observations can be made for the eastern Himalaya which is probably subject to precipitations more intense than the Central Himalaya (Bookhagen et al., 2006a, 2006b).
The Himalayan hydrographic network (Figure II-4) organizes around three main rivers, from west to east, the Indus, the Ganga, and the Brahmaputra, the two latter joining into the Lower Meghna in the Bengal delta plain. Several significant transversal Himalayan tributaries join the network, among them the Karnali-Ghaghara, the Narayani-Gandak and the Arun-Kosi, as well as cratonic tributaries, the Chambal and the Son. The majority of the river discharge occurs during the monsoonal season. Marked differences exist between the Ganga and the Brahmaputra. The rivers present distinct morphologies (meandering Ganga vs braided Brahmaputra), which characterizes a different capacity to transport sediments, indicated by the Brahmaputra discharge double of the Ganga one (GRDC, 1996, quotation of Lupker, 2011) and a floodplain 5 times smaller than the Ganga one. Suspended sediment flux present variability, but are potentially of comparable order, 500 – 600 Mt/yr (1966 to 1970 measurements, RSP, 1996, quotation of Lupker, 2011).
The Himalayan glaciers (Figure II-4) presently cover 2% of the Lower Meghna drainage basin and 5% of the Himalayan part of the catchment (including the eastern syntaxis, Figure II-4). The Himalayan glaciers might have increased their extent to 20% of the mountain range during the Last Glacial Maximum (Shi, 2002). The hypothesis of an extensive ice-sheet during the last 500 ka was proposed by Kuhle (e.g. Kuhle, 1995; Kuhle, 2011) but contradicted by geochronologic constraints (e.g. Lehmkuhl et al., 1998; review in Owen and Dortch, 2014). According to Owen and Dortch, 2014’s review, in the central and eastern range, including the SE Tibetan plateau, maritime glaciers are dominantly fed by monsoons and have a warm-based sole, whereas in the western range, continental glaciers are fed by monsoons and rainfalls caused by westerlies and have a mix-based sole. Accumulation is favoured by frequent snow avalanches during the summer and a protecting debris cover originated from tectonics, which varies according to locations.
In their review, Owen and Dortch, 2014 exposed that the Himalaya were subject to alternate phases of increases and decreases of ice extent during the Quaternary, although there is no direct evidence before 300-400 ka. Evidences consist in the identification and dating of moraines, using 14C, OSL and cosmogenic nuclides. Moraine identification is not straightforward, because of a possible confusion with landslides or rock avalanches (Hewitt, 1999). Additionally, in the central and eastern Himalaya, intense rainfall combined with tectonics rapidly erases the older moraines.
There is some debate whether glacial advances were synchronous or not across the orogen during the last glacial cycle, i.e. since 120 ka, particularly between the drier western part and the wetter central and eastern part (e.g. Owen and Dortch, 2014).
TECTONICS VIEWED BY THERMOCHRONOMETRY
The Himalayan tectonic structures were described by the early works of Gansser (1964) and Le Fort (1975).
A definition of tectonics could cover all processes of deformation and transfer of energy that affect the crustal rocks (e.g. Burbank and Anderson, 2011). At the Earth’s surface, the most common type of deformation is brittle deformation, when rocks fracture and eventually slip along faults, resulting in earthquakes, the majority of them being small, and a minority being of large magnitude (e.g. 2008s Wenchuan earthquake, Sichuan, China, moment magnitude Mw 7.9, Parker et al., 2011; 2015s Gorkha earthquake, Nepal, Mw 7.8, Elliott et al., 2016). At greater depths, because of pressure and temperature, rocks deform by sliding in a ductile way, without producing significant fractures.
Tectonic drivers and elevation change
Tectonics find energy from the deep Earth’s mechanisms, through the coupling of mantle convection and plate motion. Plate subduction and collision form mountain ranges by elevating topography and increasing relief. In turn, topography and relief are secondary drivers for tectonics, as they redistribute crustal masses over a region and alter the stress field with gravity (e.g. Molnar and Lyon-Caen, 1988; Beaumont et al., 1992; Avouac and Burov, 1996; Willett and Pope, 2004).
Table of contents :
1. STATE OF ART
1.1 Genetic Model of Deposits Associated with Veins-Stockworks
1.2 W-Sn- (Cu) Panasqueira Deposit
1.3 Common Types of Fluids Characterizing Transport and Deposition of W: Fluid Inclusions Data
2. SET OF ISSUES AND SUBJECT OF THE THESIS
3. OBJECTIVES AND EXPECTED RESULTS
4. MATERIALS AND METHODS
4.1 Geological Part- Data Collection
4.2 Geochemical Collection Data
Major elements conditions
Trace, Rare Earth elements
4.3 Experimental Part- Dataset
5. MANUSCRIPT ORGANIZATION
First Depositional Stage
Paragenetic succession at Panasqueira
General characteristics of the paragenetic succession
At the pillar scale
At the sample scale
Revised Panasqueira paragenetic chart
CHAPTER ONE INCIPIENT DEPOSITION STAGE OF WOLFRAMITE: W-RUTILE AND TOURMALINE CHEMISTRY AS PROXIES FOR EARLY FLUIDS AT PANASQUEIRA (PORTUGAL)
Geological setting and previous work
Samples and methods
Petrography of the tourmalinized wallrocks
Tourmaline chemical composition
Rutile chemical composition:
Age of the early hydrothermal stage
Tourmaline evidence for the involvement of two fluids
The boron problem and the feasibility of a magmatic source of fluids
Metasedimentary source(s) of the fluids: the contribution of REE data
Contrasted compositions of F1 and F2 end-members
Rutile evidence for short term fluid dynamics and mixing
Proposed conceptual model
CHAPTER TWO RUTILE FROM PANASQUEIRA (CENTRAL PORTUGAL): AN EXCELLENT PATHFINDER FOR WOLFRAMITE DEPOSITION
2. Geological Setting
3. Material and Methods
4.2. Rutile Chemical Composition
5.1. Crystal Chemistry
5.2. Compositional Zoning: Sector Zoning
5.3. Compositional Zoning: Oscillatory Zoning
6.1. Oscillatory Zoning: External or Internal Control?
6.2. Open or Closed System Evolution?
6.3. Oscillatory Zoning as a Consequence of Seismic Activity?
6.4. Nature of the Fluid(s)
Panasqueira Chemical Fluid Evolution
CHAPTER THREE FLUID EVOLUTION IN THE PANASQUEIRA DEPOSIT: THE ROLE OF EXHUMATION AND MULITPLE FLUID PULSES IN THE TRANSPORT AND DEPOSITION OF W-Sn-Cu
3 Paragenetic succession at Panasqueira
General characteristics of the paragenetic succession
Revised Panasqueira paragenetic chart
4 Fluid inclusions results
5 Bulk chemical evolution
6 P-T estimation
Thermal gradients and proximity to magma intrusions: the convective fluid system
Long-lived hydrothermal systems likely related to multiple intrusions
Experimental Part On W-Beahviour
CHAPTER FOUR TUNGSTEN (VI) SPECIATION IN HYDROTHERMAL SOLUTION UP TO 400°C AS REVEALED BY INSITU RAMAN SPECTROSCOPY
2.2 Spectroscopic cell and Raman data acquisition
2.3 Fitting procedure of Raman spectra
3.1 Effect of temperature and pH on W speciation
3.2. Stability and reversible formation of W-species
3.3. Effect of carbonate and chloride
4.1. Identification of W-species
4.2. Tungsten speciation
4.3. Geological implications