Depositional age determination

The main sedimentary depositional age determination of this work has been realised through magnetostratigraphy, for both Dungsam Chu and Sibo-Remi-Siang section (chapters 1 and 3). Therefore, general notions of magnetism and its application to the magnetostratigraphic dating of sedimentary rocks are presented here, followed by a brief description of complementary dating methods used in this thesis. The main descriptions of magnetism principles of this section are issued from Butler (1992).

4.1.1. Magnetic field

The magnetic field H (Note that the variables formatted in bold are vectors throughout the section) is characterized by intensity and direction has a fundamental unit of A/m (SI). The magnetic moment is noted Mof fundamental unit A.m2 and tends to align the axis of a dipole parallel to the magnetic field such as:

E = −M. H = −M. H. cos⁡(θ) where E is the potential energy and θ the angle between M⁡and H.

The magnetic intensity, J, or magnetization of a material is the sum of the magnetic moments per unit of volume enclosing those magnetic moments:

J = (∑ Mi

i ) /volume where⁡Mi is the constituent magnetic moment.

In paleomagnetism, the direction of a vector such as the surface geomagnetic field is usually defined by the angles shown in Fig. 10a. The vertical component, Hv, of the surface geomagnetic field, H, is defined as positive downwards and is given by:

H_v= H. sin⁡(I)

where H is the magnitude of Hand I is the inclination of H from horizontal, ranging from –90° to +90° and defined as positive downward.

42 The horizontal component, Hh, is given by:

H_h= H. cos(I) and the geographic north and east components are respectively:

H_n= H. cos(I). cos⁡(D) H_e= H. cos(I). sin⁡(D)

where⁡D is the declination, i.e. the angle from geographic north to horizontal component, ranging from 0° to 360°, positive clockwise. The determination of I and D completely describes the direction of the geomagnetic field.

A concept that is central to many principles of paleomagnetism is that of the geocentric axial dipole (GAD), shown in Fig. 10b. In this model, the magnetic field produced by a single magnetic dipole at the centre of the Earth and aligned with the rotation axis is considered. In this model, the inclination of the field can be determined by:

tan⁡(I) = 2. tan(λ)

where⁡λ is the geographic latitude, ranging from –90° at the south geographic pole to +90° at the north geographic pole. This relationship between⁡I and⁡λ is often referred to as “the dipole equation” and is essential to understanding most of paleogeographic and tectonic applications of paleomagnetism.

Fig. 10: (a) Principal components of the Earth magnetic field H. (b) Geocentric Axial Dipole model (after Butler, 1992 and references therein).

43 4.1.2. Natural Remanent Magnetism (NRM)

In situ magnetization of rocks is the vector sum of two components: J = J_i+ J_r

where Ji is the induced magnetization and Jr is the natural remanent magnetism. The magnetic susceptibility, χ, can be regarded as the magnetizability of a substance. Presence of the local geomagnetic field, H, produces the induced magnetization:

J_i= χ. H

Where the bulk susceptibility, χ, is the susceptibility resulting from contributions of all minerals. This induced magnetization Ji usually parallels the local geomagnetic field and can be the dominant component for many rock types. However, acquisition of induced magnetization is a reversible process without memory of past magnetic fields. It is the remanent magnetization that is of concern in paleomagnetism.

The Natural Remanent Magnetization (NRM) is the remanent magnetization present in a rock sample prior to laboratory treatment. NRM depends on the geomagnetic field and geological processes during rock formation and during the history of the rock. NRM is typically composed of more than one component. The NRM component acquired during rock formation is referred to as primary NRM and is the component sought in most paleomagnetic investigations. However, secondary NRM components can be acquired subsequent to rock formation and can alter or obscure primary NRM. The secondary components of NRM add vectorially to the primary component to produce the total NRM:

NRM = primary⁡NRM + secondary⁡NRM

4.1.3. Detrital Remanent Magnetism

Detrital remanent magnetism (DRM) is acquired during deposition and lithification of sedimentary rocks. When a detrital rock is deposited, magnetic particles contained in the sedimentary material from the suspended load are subjected to water friction forces, gravity and a magnetic field. Suspended load will orientate parallel to the magnetic field. During deposition, the major part of the particles will conserve this orientation which is the orientation of the natural Remanent magnetisation of deposition. This orientation classically presents an error in inclination towards the horizontal plane. However, in an environment where hydrodynamic forces are important, this orientation is not perceptible. On the contrary, when the sediment is recent, the cohesion between particles is low and magnetic particles can re-orientate in the magnetic field at the time, especially if grainsize is low. It is an early post-detrital magnetisation which generally does not present bias on the inclination. This implies a time difference between the deposition and the acquirement of this

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magnetisation. However, in the Himalayan foreland basin, the high accumulation rates lead to no significant time difference in the dating. The sediment is then buried and compacted mechanically. During diagenesis, post-deposition physical and chemical processes such as dissolution, precipitation, cementation or bioturbation can affect the magnetization and the material is susceptible to acquiring a secondary magnetisation and therefore effect on the paleomagnetic record.

4.1.4. Magnetic field inversion and GPTS

Numerous inversions of the Earth’s magnetic field occurred over geologic time. These inversions allowed the establishment of a magnetic polarity stratigraphic scale, with periods of normal or reverse polarities. These inversions are worldwide documented and reported in geomagnetic polarity time scales (GPTS) where the time of the inversions has been constrained using independent dating methods. For instance, in Neogene times, geochronology on lavas and bio-stratigraphy in sedimentary rocks are the main independent dating techniques to constrain inversion times. For older periods, inversions observed in oceanic floor in both sides of oceanic ridges are used, assuming a constant extensional rate. Several GPTS have been elaborated, in this thesis the GPTS of Gradstein et al. (2012) is used.

4.1.5. Magnetostratigraphy

The principle of paleomagnetic dating is to correlate the GPTS with a succession of measured inversions in a sedimentary sequence for which the dating is unknown, unlike the stratigraphic positions of the periods. The inversion periods of the GPTS are called chrons and are of variable duration as the magnetic inversions are of aleatory occurrence. This length variability is used for the correlations; it allows the dating of sedimentary sections and to deduce accumulation rates. This approach is based on fundamental hypotheses regarding the sedimentation processes: 1) the sedimentation must be continuous in time, without significant sedimentary gaps or erosion unconformity over a period sufficiently long to identify enough inversions; 2) the sedimentation rate must be relatively constant on short time scales, as the thicknesses of the detected inversions will be correlated with the length of the GPTS inversions.

Magnetostratigraphy dating allows the dating in a “pseudo-absolute” way of sedimentary sequences, poorly or not constrained with classical absolute dating such as biostratigraphy or isotopic methods. In fluvial continental series without magmatic rocks, the isotopic dating of sedimentary deposition is very difficult.

45 4.1.6. Analytical procedure

Samples are collected and orientated at regular stratigraphic intervals in a sedimentary section. Then the samples are thermally demagnetised in an amagnetic oven. The thermal demagnetisation is based on the Curie temperature principle. Above this temperature, any magnetic body loses its magnetisation. The thermal demagnetisation of the samples is performed through temperature steps. At each temperature step, magnetic grains exposed to temperature higher than their respective demagnetisation temperature lose their magnetization orientation and re-orientate in the current local magnetic field. As the samples are cooled in a null field magnetic chamber, they present a remanent magnetisation carried exclusively by particles of higher Curie temperature than the temperature at which they have been exposed. In this way, increasing stepwise the temperature of the samples allows identification of the different components of the Remanent magnetisation and the minerals which carry them, and eventually the polarity of the period they are contained in. From the results of all samples from a sedimentary succession, a polarity column can be realised and correlated to the GPTS.

4.1.7. Maximum depositional age constraint

Since magnetostratigraphy is a “pseudo-absolute” dating method, a correlation can only be successful if independent time constrains are provided. Indeed, correlating without a required minimum knowledge on the approximate time of deposition is very difficult considering the high number of reversals in geological history. A relevant method for sedimentary sections without ash layers or biostratigraphy is detrital thermochronology. In this thesis, the apatite fission-track thermochronological system has been used to define maximum depositional ages. Unless reset, an apatite grain cannot be deposited prior to its fission-track age. Consequently, the apatite fission- track age is regarded as a maximum age for the deposition of the sediments deposited above the apatite sample in the stratigraphic section. This method is briefly described in the thermochronology section, and detailed further in chapter 1 and 3 as it has been respectively performed by I. Coutand and P. O’Sullivan.

4.1.8. Age uncertainties

The uncertainties on magnetostratigraphic ages are of several natures. They can be related to the chosen correlation. Indeed, several correlations are possible for a sedimentary sequence (see chapter 1 and 3), and arguments to discriminate between correlations are not necessary possible. The preferred correlation is the main factor of bias. The quality of a correlation is dependent on the quality of the column (sampling density, magnetic properties, lithology and continuous exposure of

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outcrop). Additionally, the hypothesis that the accumulation rate is relatively constant is also a source of uncertainty in the correlation and therefore the dating.

4.1.9. Other applications

Magnetostratigraphy offers other applications, for instance in chapter 3, it helped in the choice of a relevant “absolute” dating technique. Sedimentary rocks from the Sibo outcrop of the combined Sibo-Remi-Siang section (chapter 3) were not of sufficient exposure to allow magnetostratigraphic dating. These sedimentary rocks have been estimated to be of Late Quaternary age based on field observations. Towards the objective of dating more precisely these sedimentary rocks, paleomagnetic analysis has been carried out and the results indicate deposition during a normal polarity period. An age between 0 and 0.8 Ma has been inferred from this results, as the last and current period is normal since 0.8 Ma. Consequently, luminescence dating for this time scale has been performed. This method will not be developed in this section, as it has been carried out by S. Lowick in Bern University, but described in chapter 3 and the corresponding appendix.