Examensarbete vid Institutionen för geovetenskaper Degree Project at the Department of Earth Sciences ISSN 1650-6553 Nr 472 Palaeomagnetism and Magnetic Fabrics of The Lake Natron Escarpment Volcano-sedimentary Sequence, Northern Tanzania Paleomagnetism och magnetisk anisotropi av Natronsjöns vulkano-sedimentära bergarter, norra Tanzania Gülsinem Polat INSTITUTIONEN FÖR GEOVETENSKAPER DEPARTMENT OF EARTH SCIENCES
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Examensarbete vid Institutionen för geovetenskaper Degree Project at the Department of Earth Sciences
ISSN 1650-6553 Nr 472
Palaeomagnetism and Magnetic
Fabrics of The Lake Natron
Escarpment Volcano-sedimentary
Sequence, Northern Tanzania
Paleomagnetism och magnetisk anisotropi av
Natronsjöns vulkano-sedimentära
bergarter, norra Tanzania
Gülsinem Polat
INSTITUTIONEN FÖR
GEOVETENSKAPER
D E P A R T M E N T O F E A R T H S C I E N C E S
Examensarbete vid Institutionen för geovetenskaper Degree Project at the Department of Earth Sciences
Published at Department of Earth Sciences, Uppsala University (www.geo.uu.se), Uppsala, 2019
1
Abstract
Palaeomagnetism and Magnetic Fabrics of The Lake Natron Escarpment Volcano-sedi-
mentary Sequence, Northern Tanzania
Gülsinem Polat
The East African Rift System diverges in the Lake Natron Basin of Northern Tanzania and is a major
zone of continental extension and crustal thinning with resulting in active tectonics and volcanism. The
discovery of Acheulean technology in Olduvai Gorge and Peninj as well as the presence of significant
volcanic centers, has made in the region subject to studies in various disciplines. However, lack of pre-
cise radiometric age constraints due to the complex geology of the region is a major drawback. The
basin is bordered on the western side by an escarpment that contains thick sequences of volcanic (neph-
elinites, basanites, hawaiites, alkali basalts), volcaniclastic and lacustrine strata that predates 1.2 Ma.
This thesis is based on 41 rock samples that were collected from two geological sections, the Endukai
Kete (EK) and Waterfall (WF) sections and aims to establish a preliminary geomagnetic polarity time
scale (GPTS) for the Natron Escarpment, together with establishing possible flow directions of the
volcanic lavas within these sections.
Nephelinites of EK section have an inferred NW-SE direction of flow, based on study of anisotropy
of magnetic susceptibility. They record a normal polarity that most likely correspond to the Cobb
Mountain Event (CMT; 1.187-1.208 Ma), although there is an 80-ka discrepancy between the CMT
event and the dated lavas. The most probable source is the Mosonik that erupted nephelinitic lavas 1.28
Ma ago. The palagonitic tuff layer below the nephelinites displays reverse polarity and a NE-SW direc-
tion of flow. Due to the absence of approximately 200 m strata within the basanite series of the section,
regional lithological correlation is used to constrain the GPTS pattern. Hajaro Beds of the Peninj Group
to the north of the escarpment, postdates the Olduvai Event (1.71 to 1.86 Ma) and lacustrine strata of
the escarpment for EK and WF sections are deposited over the same unconformity and share deposi-
tional similarities. Therefore, the lacustrine strata are correlative to Hajaro beds and the normal event
observed within the basanite series of both sections is attributed to the Réunion Event (2.116 – 2.137
Ma).
The establishment of a preliminary magnetostratigraphic sequence presented in this thesis demon-
strate that the rift escarpment in northern Tanzania is suitable for paleomagnetic dating. Future studies
should be conducted to establish a more detailed and constrained magnetostratigraphic section, which
will be of great use in this part of the African Rift where radiometric dating has been challenging.
Keywords: Natron Escarpment, Northern Tanzania, Paleomagnetism, Geomagnetic Polarity Time Scale, Anisotropy of Magnetic Susceptibility Degree Project E in Geophysics, 1GE031, 45 credits
Supervisor: Bjarne Almqvist
Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala
ISSN 1650-6553, Examensarbete vid Institutionen för geovetenskaper, No. 472, 2019
The whole document is available at www.diva-portal.org
2
Populärvetenskaplig sammanfattning
Paleomagnetism och magnetisk anisotropi av Natronsjöns vulkano-sedimentära bergar-
ter, norra Tanzania
Gülsinem Polat
Det Östafrikanska riftsystemet divergerar längs Natronbassängen i norra Tanzania, och är ett viktigt område med kontinental extension och bildandet av en förtunnad skorpa, vilket bidrar till en aktiv tek-tonisk och vulkanisk miljö. Fynd av verktyg från Acheuléenkulturen i Olduvai Gorge och Peninj samt vulkanavläggningar i regionen har varit föremål för studier inom många discipliner. Brist på ålderbe-stämningar har dock försvårat dessa studier på grund av den komplexa geologin i regionen. Natronbas-sängen gränsar på västra sidan till en brant riftvägg som innehåller tjocka sekvenser av vulkaniska bergarter (nefeliniter, basaniter, hawaiiter, alkalibasalter), vulkaniklastiska och sedimentära bergarter som nedlagts före 1.2 Ma. Denna uppsats bygger på 41 stenprover som samlades in från två geologiska sektioner, Endukai Kete (EK) och vattenfallsekvensen, och syftar till att skapa en preliminär tidserie av geomagnetiska polombyten (GPTS) tillsammans med bestämning av möjliga flödesriktningar för de vulkaniska bergarterna. Nefeliniter från EK påvisar en NV-SÖ flödesriktning, genom tolkning av magnetisk anisotropi. En nor-mal geomagnetisk pol motsvarar sannolikt Cobb Mountain händelsen (CME; 1.187-1.208 Ma), men en avvikelse på 80 ky noteras mellan CMT och den mest sannolika källan, Mosonik-nefeliniterna som avsattes 1.28 Ma (Foster, et al., 1997). Asklagret under nefeliniterna visar en polomkastning och NÖ-SV flödesriktning. På grund av avsaknaden av cirka 200 m strata inom sektionen av basaniter används här en regional litologisk korrelation för att begränsa GPTS (Geomagnetic Polarity Time Scale). Lik-nande avsättningsmönster av lakustrina sediment vid Hajaroavlagringarna i Peninj-gruppen längs norra delen av riftväggen är äldre än Olduvai-händelsen (1.71 till 1.86 Ma), och sista normala polombytet inom basanitserien tillskrivs Réunion-händelsen (2.116 - 2.137 Ma). Upprättandet av en preliminär mag-netostratigrafisk tidserie som presenteras i denna avhandling visar att riften i norra Tanzania är lämplig för paleomagnetisk datering. Framtida studier bör genomföras för att upprätta en mer detaljerad och begränsad magnetostratigrafisk sektion, som kommer att vara till stor nytta i den här delen av den afri-kanska riften där radiometrisk datering har varit utmanande. Keywords: Natronsluttningen (Gregoryriften), norra Tanzania, Paleomagnetism, Geomagnetisk polari-tet, Anisotropi av magnetisk susceptibilitet Degree Project E in Geophysics, 1GE031, 45 credits
Supervisor: Bjarne Almqvist
Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala
ISSN 1650-6553, Examensarbete vid Institutionen för geovetenskaper, Nr 472, 2019
Hela publikationen finns tillgänglig på www.diva-portal.org
3
Steffen için
4
Acknowledgements
First, I am grateful to my supervisor Bjarne Almqvist for his effort and support during all the
stages of research and writing. Additionally, I would like to thank to Ian Snowball for his help
with the magnetometer and reviewing of the study. Ann Hirt (ETH Zurich) is thanked for
providing the samples and material used in the study. I also thank to Hannes Mattsson for his
help and opinions about the study.
I want to thank to all my friends in the University for their valuable support, help and consid-
eration; Maria for our unforgettable moments and sharing the thesis journey with me, Martina
for her help and friendship, Frida for helping me as an opponent and a friend, Alvarro, Teegan
and Mohsen for their friendship and all the other people who helped me during my studies.
I am also grateful to my roommates and friends in Uppsala. I would like to thank to Nicole,
Pauline, Antonin, Madde, Melissa and Pepi for their companion, friendship and support while
I stay in Uppsala.
Finally, I would like to thank to my family, Ayşe, Emin, Duygu and our cat Mıncık for every-
thing, I felt their presence while they are away and Steffen who is the beginning of this story.
2 Geological and Paleomagnetic Background .................................................................................. 9 2.1 Tectonic History and Development of Rift System ................................................................... 9 2.2 Volcanism ................................................................................................................................ 11 2.3 Previous Paleomagnetic Studies and Stratigraphy .................................................................. 12
3.2 Magnetic Susceptibility and Anisotropy of Magnetic Susceptibility ...................................... 17 3.3 Magnetic Remanence .............................................................................................................. 20
4 Methodology ................................................................................................................................ 23 4.1 Study Area and Sampling ........................................................................................................ 23 4.2 Anisotropy of Magnetic Susceptibility .................................................................................... 24 4.3 NRM and AF Demagnetization ............................................................................................... 25
4.3.1 Alternating Field Demagnetization ................................................................................ 26 4.3.2 Rotation of NRM and AF Demagnetization Data .......................................................... 27 4.3.3 ChRM and Principal Component Analysis .................................................................... 28
APPENDIX I ......................................................................................................................................... 54
abolically with respect to the applied field, however, there is a saturation magnetization that a substance
might reach. Increasing the applied field beyond this value would not cause significant additional mag-
netization, as all permanent moments will be aligned with the applied field. Saturation magnetization
diminishes as a function of raising the temperature and then reaches zero at the Curie temperature (TC).
Figure 5. Magnetization J versus magnetizing field H, (a) for a diamagnetic substance, where susceptibility χ is negative and constant, (b) for a paramagnetic substance, where susceptibility χ is positive and constant, (c) for a ferromagnetic substance, where the magnetic material experiences hysteresis (irreversible) and magnetic susceptibility, χ is not constant as a function of the applied field (Butler, 1992)
17
At temperatures higher than the Curie temperature, ferromagnetic solids act paramagnetically. The Curie
temperature is an important characteristic that is useful to identify types of ferromagnetic minerals.
In a strict sense, ferromagnetism defines substances with coupling of adjacent atomic magnetic mo-
ments, usually organized in layers. However, if the layers have opposing direction of magnetic moments
they are referred as antiferromagnetic, and if the layers have parallel and non-equal atomic magnetic
moments, they are called ferrimagnetic (Butler, 1992). The most common natural ferromagnetic miner-
als are (titano-) magnetite, partially oxidized (titano-) maghemites and fully oxidized (titano-) hematite.
Generally, rocks contain a mixture of minerals that display all three types of magnetism, either dia-
magnetic, ferromagnetic or paramagnetic. Each individual minerals magnetic susceptibility contributes
to the magnetization of the assemblage of minerals in the rock. In case of abundance, if ferromagnetic
minerals exceed 0.1% of the total rock volume, they likely dominate the observed magnetic properties
(Figure 6) (Tarling & Hrouda, 1993).
Typical applications of magnetic susceptibility use weak applied fields where the response between
applied field and magnetization is linear, but this assumption is generally only valid in weak field appli-
cations of less than 1 mT (Tarling & Hrouda, 1993).
3.2 Magnetic Susceptibility and Anisotropy of Magnetic Susceptibility
There are two specific versions of magnetization, induced magnetization and remanent magnetization.
Induced magnetization is the magnetic response of the material to an applied magnetic field, presumably
H, such that
𝑀 = 𝑘 . 𝐻
Figure 6. Bulk susceptibility as a function of volume fraction for selected minerals (Butler, 1992)
18
Where M is the sample magnetization and H is the applied magnetic field. Magnetic susceptibility is
a ratio, and it is usually volume (volumetric susceptibility, k) or mass (mass susceptibility, χ) normalized
(Rochette, 1992).
The equation for magnetic susceptibility above presume a scalar quantity, and holds for isotropic
materials; however, the induced magnetization in most materials is not directly parallel to the applied
field. Only a small portion of rocks in nature would yield the same intensity of magnetization regardless
to the direction of the field. Such materials are isotropic. On the contrary, for most rock samples, mag-
netization in weak applied fields relate to the direction of the field and therefore they are anisotropic
(Tarling & Hrouda, 1993).
Anisotropy is expressed as variability depending on directions of magnetization (Hrouda, 1982).
Thus, magnetic susceptibility is more appropriately described as a tensor:
𝑀𝑖 = [𝑘𝑖𝑗] × 𝐻𝑗
𝑀1 = 𝑘11𝐻1 + 𝑘12𝐻2 + 𝑘13𝐻3
𝑀2 = 𝑘21𝐻1 + 𝑘22𝐻2 + 𝑘23𝐻3
𝑀3 = 𝑘31𝐻1 + 𝑘32𝐻2 + 𝑘33𝐻3
Where constants 𝑘𝑖𝑗 = 𝑘𝑗𝑖 are the elements of a second rank, symmetric susceptibility tensor in Car-
tesian coordinate system (Hrouda, 1982).
Favored directions of magnetic constitutes within a rock body is portrayed as anisotropy of magnetic
susceptibility, AMS. Therefore, AMS is the preferred orientation of magnetic minerals in rocks. AMS
results from two main contributing factors, crystalline and shape anisotropy (Tarling & Hrouda, 1993).
Magnetocrystalline anisotropy results when for a specific crystal, electron spins are aligned on cer-
tain crystallographic directions, resulting in an increase or decrease in magnetization with respect to
these crystal directions. Shape anisotropy occurs when electron spin alignments produces north and
Figure 7. The magnetic susceptibility ellipsoid. Tensor defining the anisotropy of susceptibility can be visual-ized as an ellipsoid where orthogonal axes corresponds to Kmax, Kmin and Kint. (Retrieved from Tarling and Hrouda 1993)
19
south magnetic poles according to the shape of crystals (Tarling & Hrouda, 1993). Therefore, when an
external magnetic field is applied, magnetization will generally be higher along the long axes of grains
and smaller along n short axes. Magnetocrystalline anisotropy is generally small in natural ferrimagnetic
minerals, such as magnetite, so that the sample anisotropy is usually controlled by its shape anisotropy.
However, magnetocrystalline anisotropy is distinguishing for some minerals like hematite.
Ferromagnetic minerals, particularly titanomagnetites dominate the magnetic susceptibility, and thus
are main contributors to AMS. The anisotropy roughly imitates the orientation of titanomagnetites
(Tarling & Hrouda, 1993).
Susceptibility measurement results are compiled to establish a magnetic susceptibility ellipsoid that
defined by three principal axes 𝐾1 ≥ 𝐾2 𝐾3, which are the three eigenvectors and eigenvalues of the
susceptibility tensor.
If all three axes are distinct, the susceptibility ellipsoid is triaxial neutral. If two principal suscepti-
bilities are same but the third is different, the magnetic susceptibility ellipsoid is either rotationally ob-
late or prolate. In case of three components are the same, it is an isotropic medium and the ellipsoid is
effectively a sphere (Jelinek, 1977).
Mean susceptibility is the arithmetic sum of the principal susceptibilities:
• 𝐾𝑚𝑒𝑎𝑛 = (𝐾1 + 𝐾2 + 𝐾3)/3.
• Magnitude of the Anisotropy 𝑃 = 𝐾1/𝐾3
The lineation and foliation of the ellipsoid are defined by
• L (also referred as lineation) = 𝐾1/𝐾2
• F (also referred as foliation) = 𝐾2/𝐾3
There are different parameters that merge both lineation and foliation to deliver unique ratio for both,
termed as shape factor T (Hrouda, 1982) (Jelinek, 1977) (Tarling & Hrouda, 1993);
• T = [2(η2−η3)
(η1−η3)] − 1
Where η1 = lnK1, η2 = lnK2 and η3 = lnK3.
If the shape parameter is between 1 and 0, (i.e. 1>T>0) the susceptibility ellipsoid is oblate (planar)
and if T is between 0 and -1, (i.e. -1<T<0) the susceptibility ellipsoid is prolate (linear) (Figure 8).
Corrected degree of anisotropy or Pj or Pα is after (Jelinek 1981) is denoted as intrinsic anisotropy
where:
• α = √(1+T2)
3
20
Anisotropy of magnetic susceptibility (AMS) is a significant characteristic for various rock types. In the
scope of this study AMS is used to analyze directions of flow in lavas, and consistency of different
sections in terms of anisotropy parameters.
3.3 Magnetic Remanence
Magnetization of a rock is the vector sum of two components, induced and remanent magnetization.
J = Ji + Jr
While induced magnetization is acquired when the present magnetic field applied, remanent magnet-
ization is another type of magnetization that results due to previous magnetic fields experienced since
the formation of the rock. Remanent magnetization is the permanent magnetization recorded by rocks.
3.3.1 Ferromagnetism and Ferromagnetic domains
In the perspective of paleomagnetism or geomagnetism, ferromagnetic minerals become the main target
of interest. Butler (1992) describes the rock as assemblages of fine grained-ferromagnetic minerals
spread throughout the matrix of paramagnetic and diamagnetic minerals.
Single ferromagnetic mineral that holds uniform magnetization can be described by a pair of mag-
netic charges, where adjacent forces cancel each other, while generating a surficial magnetic charge
Figure 8. Shape parameter, T versus degree of anisotropy Pj (After Tarling and Hrouda 1993).
21
distribution (Figure 9a). When a particle is very small, (<100nm) coupling of atoms uphold uniform
magnetization over the complete crystal; these kinds of particles are called single domain, SD. When
the crystal becomes larger, energy savings divides the crystal into two or more magnetic domains where
magnetization vectors might be antiparallel or angled to each other depending on the magnetocrystalline
easy axes (Dunlop and Özdemir 2007). These larger grains are defined as multidomain, MD. For in-
stance, magnetite grains that have sizes larger than 10 m are accepted as MD particles (Figure 9 b). SD
grains are composed of only one domain and they have distinct magnetic characteristics compared to
MD grains, carrying a stable remanent magnetization (Butler 1993).
3.3.2 Natural Remanent Magnetization
Magnetic remanence retains the memory of ancient paleomagnetic fields and this is the main interest
for paleomagnetic studies. Natural remanent magnetization (NRM) is the contemporary magnetism of
the rock before any laboratory experiment is applied. NRM, is the sum of the primary remanence that
was generated during rock formation process and potential secondary remanent magnetization acquired
by the rock later through processes that alter or overprint the original or primary remanence.
Primary NRM is developed either during cooling from high temperatures (thermoremanent magnet-
ization) or during the growth of ferromagnetic minerals below the Curie temperature (chemical remanent
magnetization) or during the deposition of sediments that contains ferromagnetic grains and minerals
(depositional remanent magnetization).
Thermoremanent magnetization, TRM is the process through which igneous rocks acquire a rema-
nence when cooled below the Curie Temperature, TC. Magnetic moments of the individual grains would
be stable below blocking temperatures, which are distributed under TC, for different minerals. Magnetic
moment of SD ferromagnetic grains is locked at their own blocking temperatures in alignment with the
geomagnetic field at that time and space. However, acquisition of TRM is acknowledged only for SD
grains, smaller grains acquire TRM by later magnetic fields apply and for MD grains gaining of TRM
might be inefficient (Butler, 1992).
Figure 9. (a) Uniformly magnetized, SD ferromagnetic grain. (b) MD ferromagnetic grain subdivided into vari-ous domains. (c) Rotation of atomic moments inside the domain walls. (Butler, 1992)
22
Depositional remanent magnetization (DRM) is acquired during the deposition of the sedimentary
rock. Ferromagnetic grains align in the same direction with the applied magnetic field while settling
from water table to the sediment interface and alignment is finally secured with further compaction.
Rocks can acquire a secondary NRM through a variety of processes. For example, chemical weath-
ering can alter or dissolve primary magnetic minerals such as (titano-) magnetite and grow secondary
authigenic minerals with a different magnetization.
Moreover, there are several other ways to gain natural remanence. For instance, exposure to the weak
magnetic fields might cause attainment of remanent magnetization by the rock, which is termed as Vis-
cous Remanent Magnetization (VRM) or short-term exposure to the strong magnetic fields at constant
temperature such as lightning strike might also cause gaining of secondary remanence to rocks which
defined as Isothermal Remanent Magnetization (IRM).
23
4 Methodology
4.1 Study Area and Sampling
The thesis work is based on 41 samples acquired from volcanic strata in the Natron escarpment of the
Gregory Rift. Rock specimens were collected from two geological sections, the Waterfall sequence and
Endukai Kete, within the escarpment. Sample localities and a detailed geological map of the region is
shown in Figure 10 and a stratigraphic column of the measured sections in Figure 11.
Figure 10. Detailed geological map of the study region based on (Sherrod, et al., 2013)
The section known as Waterfall sequence (WF) is exposed next to Engaro Sero Canyon waterfall.
The sequence has approximately 50m thickness, consisting of 10 m alkaline basalt at the bottom, fol-
lowed by 25 m scoria agglomerates, divided by 2m thick green tuff, containing fragments of green ae-
girine augite and apatite, which implies a more alkaline magma suite Green colored lacustrine sediments
containing fossil fragments are separating the sequence from overlying basanites (Neukirchen, et al.,
2010).
24
Figure 11. Vertical Geological Profiles of Endukai Kete and Waterfall Sections.
Endukai Kete Section (EK) represents most of the southern escarpment. Alkalinity increases in the
sequence upwards, from alkali olivine basalts at the bottom to basanites and nephelinites at the top. The
section starts with basaltic rocks that are overlain by lacustrine sediments with sands and clays contain-
ing fossil fragments. These sediments are comparable to WF sediments. Deposition of lacustrine sedi-
ments might imply a time gap between different lava flows (Neukirchen, et al., 2010).
Basalt layers are followed by basanite series, which makes up most of the escarpment. Previously,
various volcanic layers have been differentiated with respect to its composition and divided into; 1)
basalts, 2) picrobasalts and 3) hawaiites (Neukirchen, et al., 2010). The uppermost layers contain a thin,
yellow colored palagonitic tuff level. Thick nephelinites are deposited over the basanite series
4.2 Anisotropy of Magnetic Susceptibility
Magnetic anisotropy of volcanic rocks generally lower than the sedimentary rocks, however it is still
actively used for interpretation of magnetic fabric (give reference, Hrouda and Tarling, 1993). During
the expansion of magma in the plastic or fluid form, many ferromagnetic minerals escorted within a
25
paramagnetic and diamagnetic matrix, aligns with the flow. However, since the temperature of the
magma is much higher than the Curie temperature, recording of the geomagnetic field is ineffective
while the magma is still moving in hot molten state. Therefore, the minerals are merely nucleations
within the magma body are still in solid solution phase. As the magma cools down, minerals start to
crystallize from solid solution. The most likely scenario is that magnetic fabric of ferromagnetic miner-
als within the magma is representing the fabric of paramagnetic minerals (Tarling & Hrouda, 1993).
Lava flows fabrics tend to be more foliated than lineated and minimum susceptibility axes are per-
pendicular to foliation plane. Therefore, maximum and intermediate axes form a belt over the stereo-
graphic projection. Maximum susceptibility axes can be either parallel or perpendicular to the flow di-
rection (Figure 12). Tuffs fabrics are more often oblate, where the foliation plane display similarities
with the direction of flow. Lineations can be either parallel or perpendicular to flow.
Figure 12. Conceptual model for imbrication of magnetic foliation (Giordano, et al., 2008)
In order to determine the AMS characteristics of the samples, the second rank susceptibility tensor is
determined with measured with a Multifunction MFK1-FA Kappabridge. Field used is 200 A/m and
frequency of alternating field is 976 Hz.
4.3 NRM and AF Demagnetization
NRM was measured at the Laboratory for Experimental Paleomagnetism at Uppsala University. A Cry-
ogenic magnetometer was used to measure intensity of magnetization of each specimen in x, y and z
directions. Cryogenic magnetometers run with superconducting quantum interference device (SQUID)
sensors that are hosted at extremely low temperatures, at which superconductivity can operate, thus
enabling very sensitive measurement capability. Specimens are first inserted into the sensor coil, which
is connected to the transfer coil, where the flux induced in this coil is measured by the SQUID sensor.
26
The inserted specimen rotates 90˚ stepwise about its vertical axis for measurement. The procedure pro-
duces determines the x, y and z components of remanent magnetization, which are analyzed together
with holder and background measurements (Turner, et al., 2007).
Rock specimens are rarely obtained from flat lying surfaces, and bedding orientations and tilt of the
strata must be considered to determine original position of the rock samples. Rock magnetometers are
rather precise devices which allows us to measure magnetization of rock samples. Therefore, sampling
must be carried out carefully. Since the magnetization data is described in 3 dimensions, it is critical to
record in situ orientation of the rock sample to be able to re-construct the geographic reference frame
(Turner, et al., 2007).
4.3.1 Alternating Field Demagnetization
Determination of the direction and intensity of the ancient geomagnetic field is the main target of pale-
omagnetic studies. A successful demagnetization technique must overcome different generations of
NRM acquired by the rock since its formation. Various components of NRM overprint on the rocks,
which can be differentiated or cleaned in a way by using stepwise demagnetization techniques.
Alternating field (AF) demagnetization is based on the principle of exposing the rock sample to in-
creasing alternating magnetic fields, step by step, thus demagnetizing the individual grains with larger
microcoercivity (Dunlop and Özdemir 2007). Although individual grains are not truly demagnetized, an
almost equal number of grains will be magnetized in opposing orientations, therefore effectively reduc-
ing the net magnetic moment to zero.
The AF demagnetization process of samples is performed using the SQUID magnetometer. The re-
maining remanence of the sample is measured after each step of the demagnetization. All samples have
been demagnetized fields of 0 to 10 mT in 5 steps and from 15 mT to 50 mT in 8 steps. When a sample
Figure 13. Alternating Field Demagnetization Scheme. (a) Magnetic field versus time (Note the decay in the amplitude of sinusoidal waveform. (b) Detailed examination of an AF Demagnetization waveform (retrieved from Butler 1993).
27
was not sufficiently demagnetized, i.e. when more than 1-5% of the NRM remained, additional steps of
60 mT to 120 mT in 5 steps were performed (Table 2).
Table 2. AF Demagnetization Steps
Demagnetization
Steps
0th to 5th 6th to 13th 14th to 16th 17th to 18th
Frequency (mT) 0-10 15-50 60-80 100-120
4.3.2 Rotation of NRM and AF Demagnetization Data
The orientations of the samples in terms of azimuth and dip as collected in the field. Azimuth (ψ) is the
angle of the sample surface from north, while dip (λ) is the angle to the horizontal plane. All magnetic
moment components/magnetization (M), as measured by the magnetometer, were rotated on the hori-
zontal and vertical surfaces to achieve azimuthal and tilt-correction (Figure14).
Figure 14. Rotation of Orthogonal Coordinate Systems
The Euler rotation matrix (R) for Cartesian coordinates was used to perform rotations of magnetiza-
tion matrices:𝐌 = [ Mx MyMz]
𝐌′ = [ Mx′ My′Mz′] is the rotated version of M
(𝐌′)T= R (M)T
Rotation of λ, ψ angles about the z and y axis is defined as (Figure 14):
Rz(λ) = [cos λ −sin λ 0sin λ cos λ 0
0 0 1] Ry(ψ) = [
1 0 00 cos ψ sin ψ0 − sin ψ cos ψ
]
General rotation matrix
R(λ, ψ) = [
cos λ cos ψ − sin ψ sin λ cos ψcos λ sinψ cos ψ − sin λ sin ψ
sin λ − sin ψ cos λ]
Instrument runs on a software that stores all the measured steps together with the magnetization com-
ponents in Cartesian coordinates. However, to achieve structural corrections, magnetization components
28
had to be rotated to obtain horizontal orientations of specimens. The same procedure for the rotation of
NRM data was also applied to the data obtained from each AF demagnetization step.
4.3.3 ChRM and Principal Component Analysis
Progressive demagnetization techniques aim to remove secondary components of NRM. Less stable
components are eliminated easily during partial demagnetization, while stable components are isolated.
Isolated characteristic components of NRM is termed as Characteristic Remanent Magnetization
(ChRM) that represents a stable magnetization of the rock.
The remanent magnetization vector changes during demagnetization, the final vectoral component
that forms a straight line pointing towards the origin is considered to be the ChRM. Principal Compo-
nent Analysis (PCA) is applied on the demagnetization data to obtain the most probable ChRM orienta-
tion. The theory of PCA is linked to computing the moment of inertia of a set of data to a reference
point (Love, 2007). It can be described as a linear transformation of the orthogonal coordinates to a new
orthogonal reference frame that matches with the geometry of the data set (Kirshvink, 1980).
Linear section of the vector component plot defines the direction of magnetization, so it can be meas-
ured directly from the plot. However, data is mostly scattered and predicting the best straight line is not
straightforward and might be subjective (Turner, et al., 2007). PCA analysis aims to find the best fitting
line to the chosen vector that describes the ChRM objectively. The vectors that points towards the origin
are chosen from demagnetization steps (r1 and r2). They are indicated with green squares on the
Zijderveld plots. A PCA is performed by 1) calculate the covariance matrix of the chosen steps for each
sample is calculated, and 2) find Eigenvalues and Eigenvectors of the covariance matrix. Based on the
results, the maximum angle of deviation, (MAD)p that is the approximate maximum angular deviation
from the major axis, is calculated.
29
5 Results
5.1 Bulk Susceptibility and AMS
Susceptibility parameters, AMS and average susceptibility are presented in Table 3 (EK section and
WF section). EK and WF sections are composed of various rocks types, including solidified lava flows,
pyroclastic deposits and sedimentary layers. In order to interpret the magnetic fabric, rocks are classified
with respect to their depositional and compositional commonalities. From top to bottom, nephelinites
(EK 1206, EK 1179, EK 1179C, EK 1177, EK 1177C, EK 1168) of EK section are shown in Figure 15,
and Jelinek-type plots for the same samples is displayed in Figure 21. Nephelinites have the highest
degree of anisotropy (P̅j= 1.06) of all samples and their mean susceptibility is also considerably higher
(K̅mean =18.7×10-3 [SI]̟)
Confidence ellipsoids of Kmax and Kint for the nephelinite samples lie along a great circle, perpendic-
ular to Kmin. This fabric represents a lava flow from NE-SW direction if the flow plane is parallel to
Kmax. Although nephelinites have higher anisotropy, their shape parameter is not consistent.
Figure 15. Equal area Projection of Susceptibility ellipsoid for nephelinites
There are several lava flows in the middle layers of the EK section, however, in this study they are
represented only as basanites. The specimens from these layers include EK 1108, EK 1090, EK 0847,
EK 0823, EK 0823C, EK 0822, EK0820, EK 0814, EK 0813C and EK 0813, and their AMS is presented
in Figure 16, and shape parameters in a Jelinek-type plot of anisotropy in Figure 21. Basanite lavas
exhibit the highest Km values (K̅mean =39.1×10-3 [SI]) with lower anisotropy (P̅j = 1.01).
N
90
180
270
Geographic
coordinate
system
Equal-area
projection
N=7
K1
K2
K3
5.47E-03 2.84E-02Km [SI]
1.000
1.155
P
1.000 1.160 Pj
-1
1
T
30
The inferred main direction of flow is similar to nephelinites, parallel to Kmax direction, NE-SW. The
Figure 16. Equal Area Projection of Susceptibility ellipsoid for basanites
Palagonitic tuff layers (samples EK 1118T and EK 1118) within basanite series are represented sep-
arately due to their different magnetic fabric. They show oblate and elongated fabric ellipsoids (Figure
21). Susceptibility is high (K̅mean = 9.9×10-3 [SI]) but samples are very weakly anisotropic (P̅j = 1.00).
Figure 17. Equal Area Projection of Susceptibility ellipsoid for tuff
Furthermore, the lava flow (EK 0805 and EK 0801) over the lacustrine sediments are presented sep-
arately due to its alternating characteristics (Figure 16). Their susceptibilities are lower (K̅mean = 2.8×10-
3 [SI]), and weak anisotropy is similar to basanites (P̅j = 1.01).
N
90
180
270
Geographic
coordinate
system
Equal-area
projection
N=2
K1
K2
K3
8.20E-03 1.18E-02Km [SI]
1.000
1.003
P
1.000 1.004 Pj
-1
1
T
N
90
180
270
Geographic
coordinate
system
Equal-area
projection
N=12
K1
K2
K3
3.42E-03 1.22E-01Km [SI]
1.000
1.038
P
1.000 1.041 Pj
-1
1
T
31
Tuff sections are good indicators of flow direction (Tarling & Hrouda, 1993). K1 and K2 directions
are in the line, K3 is perpendicular to it which implies a flow direction from NW to SE (Fig. 17). Alt-
hough directional analysis is not very efficient with such small amount of data, lower basalt layers (EK
0801 and EK 0805) shows NW-SE direction of flow (Figure 18).
Figure 18. Equal Area Projection of Susceptibility ellipsoid for lower basanite
Lacustrine samples of both EK and WF and the underlying alkali basalt level (EK 0802, EK 0797, EK
0789, EK 0787, EK 784, EK 0774, WF 757, WF 756 and WF 739) are plotted in equal area projections,
and presented together in Figure 19, alkali basalts alone in Figure 20 and anisotropy plot in Figure 21.
Lacustrine sediments have lower susceptibility (K̅mean =1.4×10-3 [SI]), but underlying alkali basalt levels
have higher values (K̅mean = 5.0×10-3 [SI]). They both have similar average anisotropy (P̅j = 1.02) for
sediments and (P̅j = 1.02) for alkali basalts.
N
90
180
270
Geographic
coordinate
system
Equal-area
projection
N=2
K1
K2
K3
2.01E-03 4.99E-03Km [SI]
1.000
1.011
P
1.000 1.011 Pj
-1
1
T
32
Figure 19. Equal Area Projection of Susceptibility ellipsoid for alkali basalt and lacustrine sediments of EK and WF section.
Figure 20. Equal Area Projection of Susceptibility ellipsoid for alkali basalt levels EK 0774 and WF739
N
90
180
270
Geographic
coordinate
system
Equal-area
projection
N=16
K1
K2
K3
-3.76E-10 6.09E-03Km [SI]
1.000
1.145
P
1.000 1.166 Pj
-1
1
T
N
90
180
270
Geographic
coordinate
system
Equal-area
projection
N=2
K1
K2
K3
3.91E-03 6.09E-03Km [SI]
1.000
1.027
P
1.000 1.027 Pj
-1
1
T
33
Figure 21 Jelinek Plot of the Samples
34
Table 3 AMS parameters of nephelinites of Endukai Kete and Waterfall sections
Name Km [SI] L F P Pj T K1dec K1inc K2dec K2inc K3dec K3inc
(EK 0774-WF739) reaches the peak magnetic intensity with ((J̅a.basalt = 4.82×10-2 A/m).
5.2 Alternating Field Demagnetization
Results of AF Demagnetization experiments are displayed using Zijderveld diagrams for rotated (after
coordinate corrections) data (Figures 22-25). Two projections are plotted in a single graph: x (North-
South) component plotted against y (East-West) component in purple and x (North) plotted versus z
(down) component in green colors. Unrotated versions of Zijderveld plots for individual sample are
displayed in Appendix I.
Table 4 presents the results of ChRM determination. Declination and inclination data obtained from
PCA analysis is represented in degrees (˚), AF demagnetization steps that were used for ChRM calcu-
lations are listed as r1 and r2 indicated as green squares on plots and (MAD)p values are listed in degrees
(˚). All the demagnetization steps are listed in Table 2 in previous section.
Figure 22. Zijderveld Plots of rotated data of the specimens (WF 756A- WF 739). Green squares rep-
resent demagnetization steps used for PCA analysis.
36
Table 3. Complete List of NRM of the samples
NAME Azimuth
(Field)
Dip
(Field)
ISD ˚ ISI ˚ J (A/m) Declination˚
(Rotated)
Inclination˚
(Rotated)
EK1206 72 31 239.1 72.9 1.80E-03 -89.1 -38.5
EK1179C 9 45 122.3 64.3 4.44E-03 151.3 -53.2
EK1179 9 45 35.5 19.2 1.55E-02 153.7 18.1
EK1177C 328 49 290 -1.5 3.81E-04 -136.4 14.1
EK1168C 239 25 54.3 29.3 6.83E-04 11.9 14.8
EK1168 239 25 319.8 -44.3 1.25E-03 -170.0 52.2
EK1177 328 49 13.4 8.5 4.51E-04 132.4 31.3
EK1118 86 33 89.7 10.9 1.34E-03 -174.7 -5.6
EK1118T 86 33 99.5 21.9 9.50E-04 -169.9 -19.4
EK1108C 149 19 104.6 -34.3 1.77E-04 -157.9 -0.8
EK1108 149 19 35.8 -84 3.95E-04 152.9 23.9
EK1090 110 80 110.2 19.2 1.17E-03 -176.6 -22.4
EK0847 75 14 90.2 22.2 1.71E-03 -173.4 -5.4
EK0824 352 40 12.8 -11.8 2.14E-03 146.5 59.7
EK0824CB 352 40 49 -9.8 1.56E-03 103.0 37.2
EK0823 74 38 307.4 2.2 3.41E-03 -42.9 27.0
EK0823C 74 38 301.2 7.9 4.61E-03 -42.5 18.6
EK0822 8 7 201.4 61.3 6.12E-03 -159.9 -33.4
EK820 185 13 21.1 -61.4 5.82E-03 -162.1 39.3
EK814 357 29 18.6 31 3.09E-02 159.1 27.5
EK813 7 13 309.3 -81.5 7.73E-03 0.1 18.3
EK0813C 7 13 322 -60.6 2.47E-03 -14.6 34.9
EK805 76 83 253.7 -41 3.20E-03 7.8 38.7
EK801 139 18 257 48.5 2.02E-03 3.1 -21.9
EK0797Z 96 11 77.3 7.9 1.43E-05 -163.7 10.8
EK789 170 16 301.8 35.6 1.51E-04 35.5 14.6
EK0789A 170 16 288.9 52.2 4.86E-04 25.5 -1.5
EK787 44 17 84.1 -7.6 3.80E-05 128.4 7.8
EK0787AC 44 17 107.4 37.4 3.86E-07 168.0 -23.9
EK784 304 3 197.9 71.5 5.51E-04 130.0 -20.5
EK0784C 304 3 59.1 -21.9 5.15E-05 10.5 29.7
EK0784C 304 3 89.7 -27.7 4.31E-05 6.3 1.6
EK802 151 2 77.8 -26.8 5.36E-06 -146.0 11.7
EK0802B 151 2 48 13.5 2.54E-07 -99.4 39.9
EK797 96 11 55.6 22.4 1.25E-05 -142.1 26.1
EK774 78 63 278.3 -34 3.96E-03 -1.8 33.6
WF757E 132 3 142.5 29.3 1.12E-07 -97.5 -45.8
WF756D 243 22 235.1 4.7 1.14E-04 162.6 -34.0
WF756D 243 22 324.9 19.5 3.00E-04 105.2 36.2
WF756A 319 32 25.1 67.2 3.94E-05 129.3 -11.0
WF739 128 75 44.9 -8 7.20E-03 -99.5 18.4
37
Figure 23. Zijderveld Plots of Rotated data of the specimens (EK 0774-EK 0802B) Green squares represent demagnetization steps used for PCA analysis.
38
Figure 24 Zijderveld Plots of rotated data of the specimens (EK 0805- EK 1090) Green squares represent demagnetization steps used for PCA analysis.
39
Figure 25. Zijderveld Plots of rotated data of the specimens (EK 1108- EK 1206 and WF756,WF 757E) Green squares are demagnetization steps used for PCA analysis
40
Table 4 ChRM results of the samples
NAME Declination˚ Inclination˚ r1 r2 (MAD)p˚
EK1206 269 -40 5 12 1.97
EK1179 337 18 6 12 5.74
EK1179C 132 -51 4 12 3.81
EK1177C 39 4 11 17 5.16
EK1177 309 33 4 9 3.65
EK1168 11 39 4 9 6.52
EK1168CC 4 -23 4 9 3.38
EK1108 7 27 10 16 11.72
EK1118 180 -7 10 16 3.46
EK1118T 180 -9 10 16 2.13
EK1090 184 -14 10 16 8.26
EK0847 188 6 8 13 6.19
EK0824CB 284 42 10 15 20.98
EK0824 324 61 10 15 1.85
EK0823CB 139 18 10 15 12.29
EK0823 139 35 10 15 10.29
EK0822 189 34 10 15 17.7
EK820 20 32 10 15 11.3
EK814 339 30 10 18 6.06
EK0813C 148 26 10 15 10.37
EK813 172 15 10 15 6.38
EK805 182 29 7 15 24.73
EK801 6 -22 7 12 8.59
EK0789A1 27 -3 11 16 11.63
EK789 220 10 8 14 6.28
EK0787BB 303 19 4 19 8.32
EK787 85 11 2 10 3.77
EK0787AC 298 -41 6 11 3.73
EK0802B 70 26 8 12 15.64
EK0802 41 18 8 10 10.49
EK0784C3 169 4 8 13 2.49
EK0784 169 6 6 19 2.49
EK797 192 -35 5 12 9.77
EK0797ZD 191 -11 5 12 2.45
EK774 94 -11 8 16 14.08
WF757EE1 345 -17 10 14 19.13
WF756D 104 9 8 12 31.03
WF756A 128 -56 8 12 4.67
WF756 165 -36 5 10 2.75
WF739 82 20 8 14 5.71
41
6 Discussion
6.1 Flow Directions and Possible Sources of Lava
General expectancy is that the K1 (magnetic lineation) matches with the flow direction,
while K3 is perpendicular to the plane of flow (i.e., pole to the flow plane). Experimental
flow models reveal that elongated grains align with lower angles to the flow direction
(Bascou, et al., 2005). However, there is an ambiguity of 180˚ in the flow direction since
AMS represent axes of the ferromagnetic grains, but not the exact direction of flow.
The majority of the nephelinites of Endukai Kete section have NE-SW oriented maxi-
mum susceptibilities (K1), which matches with the distribution of volcanic sources and the
general rift lineament. The most likely source is the Mosonik volcano, which is approxi-
mately 5 km northeast of the study section.
Neukirchen et al. studied a similar section from the region as that studied here and made
geochemical analyses of the lava flows in 2010 (Fig. 26). Results yielded a similar chemi-
cal composition with previously studied Mosonik and Gelai nephelinites (Paslick, et al.,
1995). Mosonik nephelinites were dated using the K/Ar dating method (Foster, et al., 1997)
to 1.28 Ma (1σ = 0.05). Therefore, another possible source for nephelinites like Embagai,
was eliminated due to the difference in composition and age (Greenwood, 2014).
Middle lava layers that include basanites, hawaiites and picrobasalts exhibits two main
directions of flow, either NW-SE or NE-SW. Several flows could not be differentiated in
the scope of this study, but their AMS fabric indicates two main possible directions with
the same ambiguity of 180˚. However, the presence of several feeder dikes, of the position
of the Mosonik volcano relative to the sample location and also the compositional charac-
teristics imply Mosonik as an important feeder for the lavas. Alternatively, the NW/SE
42
direction might imply the Gelai volcano as a source, which is to the west of the study
section.
Chemical imprints also suggest Gelai as one of the possible sources (Figure 34)
(Paslick, et al., 1995). However, age data from the nephelinite extrusion of Gelai indicates
an age of 0.96 (1σ = 0.03) to 0.99 (1σ = 0.03) Ma (based on K/Ar dating) (Foster, et al.,
1997), which is younger than expected and nephelinites are not comparable to the basanite
layers in terms of chemical compositon.
43
Figure 26. Total Alkali versus Silica Diagram (retrieved from:
https://pubs.usgs.gov/of/2013/1306/)
Palagonitic tuff and basanite layers just above lacustrine sediments indicates NE-SW
direction of flow that is distinguishable from the rest of the sequence.
The lower parts of the sequence, including the lacustrine sediments and underlying
alkali basalts might have NE/SW direction of flow. However, their anisotropy degree is
44
quite low, making it harder to conclude a directional prediction with the scarce amount of
data.
6.2 GPTS Interpretation
Directions obtained from PCA analysis were individually compared to the normal (decli-
nation, 360˚ and inclination -6˚) and to the reverse (declination, 180˚ and inclination +6˚).
Samples that lie within +/- 50˚ were defined as normal (reverse) directions and considered
for interpretation (summary of data is given in Table 6). Samples with MADp values higher
than 15˚ were furthermore eliminated from the interpretation because of increased uncer-
if Inew <0 figure polarscatter(deg2rad(Dnew),abs(Inew)); else figure polarscatter(deg2rad(Dnew),Inew); end
%%Zidgerveld Plot Rotated
figure
plot(X1,X2,'-dm');
hold on plot(X1,X3,'-sc');
hold on
axis equal
plot(X1(r1),X2(r1),'gs','MarkerSize',10,'MarkerFaceColor','g'); hold on plot(X1(r2),X2(r2),'gs','MarkerSize',10,'MarkerFaceColor','g'); hold on plot(X1(r1),X3(r1),'gs','MarkerSize',10,'MarkerFaceColor','g'); hold on plot(X1(r2),X3(r2),'gs','MarkerSize',10,'MarkerFaceColor','g')
APPENDIX II
PCA Results
Principal Component Analysis
57
Figure 28 PCA Results EK0774 to EK0820B
58
Figure 29 PCA Results EK0802 to EK0847
59
Figure 30 PCA Results EK1090 to WF 739
60
APPENDIX III
Unrotated Zijderveld Plots of the Samples
Figure 31 PCA Results of WF 756A-WF756D
Figure 32 Zijderveld Plots of samples WF 756A-WF 739 prior to rotation.
61
Figure 33 Zijderveld Plots of the samples (EK 0774-EK0802B) prior to rotation.
62
Figure 34 Zijderveld Plots of the samples (EK 0805 – EK1090) prior to rotation.
63
Figure 35 Zijderveld Plots of the samples (EK 1108 – WF757E) prior to rotation.
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