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

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

ISSN 1650-6553

Copyright © Gülsinem Polat

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.

5

Contents

1 Introduction .................................................................................................................................... 7

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 Theoretical Background ............................................................................................................... 15 3.1 Magnetism in Solid Matter ...................................................................................................... 15

3.1.1 Diagmagnetism............................................................................................................... 15 3.1.2 Paramagnetism ............................................................................................................... 15 3.1.3 Ferromagnetism .............................................................................................................. 16

3.2 Magnetic Susceptibility and Anisotropy of Magnetic Susceptibility ...................................... 17 3.3 Magnetic Remanence .............................................................................................................. 20

3.3.1 Ferromagnetism and Ferromagnetic domains ................................................................ 20 3.3.2 Natural Remanent Magnetization ................................................................................... 21

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

5 Results .......................................................................................................................................... 29 5.1 Bulk Susceptibility and AMS .................................................................................................. 29 5.1 Natural Remanent Magnetization ............................................................................................ 35 5.2 Alternating Field Demagnetization ......................................................................................... 35

6 Discussion .................................................................................................................................... 41 6.1 Flow Directions and Possible Sources of Lava ....................................................................... 41 6.2 GPTS Interpretation ................................................................................................................. 44

7 Conclusions .................................................................................................................................. 48

8 References .................................................................................................................................... 50

APPENDIX I ......................................................................................................................................... 54

APPENDIX II........................................................................................................................................ 56

APPENDIX III ...................................................................................................................................... 60

6

Abbreviations

EARS, East African Rift System

AMS, Anisotropy of Magnetic Susceptibility

PCA, Principal Component Analysis

MAD, Maximum Angular Deviation

NRM, Natural Remanent Magnetization

ChRM, Characteristic Remanent Magnetization

SD, Single Domain

MD, Multi Domain

TC, Curie Temperature

CMT, Cobb Mountain Event

TRM, Thermoremanent magnetization

VRM, Viscous Remanent Magnetization

DRM, Depositional Remanent Magnetization

IRM, Isothermal Remanent Magnetization

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1 Introduction

Northern Tanzania is a notable intersection for geological and anthropological evolution on Earth. Ol-

duvai Gorge and Peninj in the Arusha Region of Tanzania are two of the momentous archeological sites

in the world with discoveries of earliest technology, Acheulean that are tools manufactured by hominins

(Diez-Martín, et al., 2015) (Torre, et al., 2008). The region also hosts a remarkable geological system,

the East African Rift, where continental extension resulted in crustal thinning and active volcanism that

shapes the geomorphology of the continent.

The East African Rift is a complex, multipart rift system that defines the geomorphological features

of East Africa. The Northernmost extension reaches the Afar triple junction with three intersecting ex-

tensional zones of the Red Sea, the Gulf of Aden and the East African rifts (Figure 1). The rift fractures

the continent in a semi-continuous segment that stretches from Ethiopia, Kenya and Tanzania to the

coast of Mozambique, forming major fault zones, lakes, valleys and numerous active volcanoes. The

more stable North African (Nubian) plate diverges from the Somalian plate due to plume related move-

ments (Dawson, 2008), resulting in crustal thinning and widespread volcanism.

Tanzania has been studied by various researchers to increase our knowledge about human evolution

and the tectonic configuration of East Africa. Although a conceptual geological framework has been

constructed, there are still many problems to be tackled. The lack of precise age constraints is a major

drawback due to the complex geology of the region, and different methods have been applied to over-

come this problem. Absolute dating methods have been extensively used since 1970s to assure the age

of hominin fossils over extensive volcanic rocks that also intercalates with fossil bearing strata. How-

ever, radiometric dating can involve large uncertainties, inconsistent ages together with the tectonic

complexities lead scientist to apply relative dating methods. Several paleomagnetic studies are held to

correlate the rocks of Northern Tanzania and the Olduvai Event is defined in the region (Gromme &

Hay, 1971) (Thouveny & Taieb, 1986).

Permanent magnetism is a special consideration in rock forming minerals. Since the beginning of its

development, paleomagnetism has been used to interpret movements of tectonic plates around the world.

Permanent magnetic, or ferromagnetic, minerals are fit to record the direction of Earth’s magnetic field

when they form and cool from a volcanic melt. Upon cooling the newly formed minerals have their

magnetic moment ‘fixed’ when temperature falls below a certain threshold (i.e. the Curie temperature

for magnetite at approximately 580 C). Variation in direction of magnetization reflect a stratigraphic

record of ancient paleomagnetic pole positions and directions, widely exploited to build up a global

GPTS (Geomagnetic Polarity Time Scale) timescale (Cande & Kent, 1995).

8

Magnetism and magnetic properties of rocks also provide clues regarding movement of lava during

volcanic eruptions. Anisotropy of magnetic susceptibility, and magnetic fabric gives clues about the

spreading extent and directions of volcanic and plutonic rocks, depositional conditions of sedimentary

rocks and deformational history of metamorphic rocks (Tarling & Hrouda, 1993) (Bascou, et al., 2005)

(Hrouda, 1982). It is an advancing branch of magnetism for use in Earth Sciences due to its practicability

and wide range of application.

This thesis is based on 41 rock samples that were collected over two geological sections in a field

excursion to Northern Tanzania in 2013. All samples were subjected to various paleomagnetic and mag-

netic susceptibility experiments to provide better understanding of the geological context of the region.

Figure 1. Plate Tectonic Configuration of East Africa (USGS, http://pubs.usgs.gov/gip/dynamic/East_Africa.html.)

500km

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2 Geological and Paleomagnetic Background

The study region is located in the Arusha region of Northern Tanzania, and the study is based on the

samples of two stratigraphic sections from the escarpment that belongs to Gregory rift, at the southwest-

ern side of Lake Natron near the currently active Ol Doinyo Lengai volcano. Since the first discovery

of hominin fossils and artifacts within the Pleistocene strata during excavations in the 1930s, the region

has been extensively studied by researchers from different scientific branches. The following section

aims to present a brief summary about to stratigraphic, tectonic, volcanic and paleomagnetic studies that

have been conducted in the region.

2.1 Tectonic History and Development of Rift System

The “East African Rift Fracture” statement was first expressed by Suess in 1891 after explorations in

the 19th century (Chorowicz, 2005). The “Great Rift Valley of East Africa” was named by Gregory in

1896 and defined to be series of graben basins that connected with the Red Sea and Dead Sea, forming

the greater Afro-Arabian rift system. Continental rift zones form in response to tectonic forces that

stretches the crust in addition to the adhesive friction between asthenosphere and lithosphere boundary

induced by the flow of the mantle (Ring, 2014).

Today, this major zone of crustal extension, rifting and magmatic upwelling, accompanied with fault-

ing and seismic activities, is called the East African Rift System (EARS). Crustal extension generally

started in the Afar triangle, where the Gulf of Aden and the Red Sea are already in oceanic spreading

stages, and deformation has propagated continuously towards the south as the rift system has grown (Le

Gall, et al., 2008). The EARS is composed of two branches, the Western and Eastern rift valleys over

the continent. The Western branch is located in the more inland part of the Craton, following a line of

lakes and depressions with a well-structured symmetric rift system. The Eastern branch extends over

2200 km length from the Afar triangle in the north, through Ethiopian and Kenyan rifts and terminates

in northern Tanzania prior to diverging further to the south. This segment of the rift in Northern Tanzania

is known as Gregory Rift. In comparison to the Western branch of the EARS, Gregory Rift is differen-

tiated with more pronounced volcanism. Rifting developed over the Mozambique Belt which is a Cam-

brian aged orogen structure formed under subduction/collision mechanisms (Ring, 2014).

The rifting process has not been synchronous across the region, and overall rifting in the EARS

started in the Late Cenozoic but for sub-branches of the rift, the timing is quite irregular and poorly

known. Rifting initiated in the Ethiopian branch during the Oligocene around 40 Ma and in Kenya

around 30 Ma, whereas Northern Tanzanian sector started to form around 8 Ma (Dawson, 2008).

10

In Northern Tanzania, the Gregory Rift valley comprises an asymmetric half graben with the absence

of major bounding fault development on the eastern side, unlike Ethiopia and Kenya where sub-parallel

rift structures are well established (Dawson, 2008).

A broad geologic overview of the region displays superimposition of two main pulses of volcanism

triggered by major faulting phases that deformed the Archean Tanzanian Craton.

Rifting dismantled the craton into a number of half grabens bounded by major faults where various

lakes and small basins formed and where lacustrine sediments intercalated with lava flows and tuffs

deposited. The most important basins formed adjacent to the Pliocene escarpment are the Natron, Man-

yara, Eyasi and Balangida half grabens (Figure 2).

Figure 2. Main structural and magmatic features of the south Kenya, north Tanzania rift system (Dawson, 2008)

Figure 3. Schematic illustration of Natron Basin and the Escarpment faults (Foster, et al., 1997)

The Lake Natron Basin is the southernmost part of Gregory Rift (Rodrigo, et al., 2009). The basin is

bordered on the western side by a 120 km long fault system and hosts the Lake Natron and Engaruka

depressions that contains thick sequences of volcanic, volcaniclastic and lacustrine strata (Foster, et al.,

Study Region

11

1997). The Natron Basin is near the Northern Tanzania Divergence Zone, which splits into three direc-

tions (Figure 2). This complicated part of the rift zone contributed to volcanism in the Crater Highlands.

The main escarpment contains thick lava flows (Rodrigo, et al., 2009).

Main escarpment faults of the rift (Figure 2) are oriented in N-S direction with associated NE-SW

transform faults. The 3 Ma old Sonjo-Eyasi faults are separating the Serengeti plains where the Craton

is situated from Salei Plain and Crater Highlands to the west (Figure 3). The Sonjo Eyasi fault systems

shows en échelon pattern and joins with the major Nguruman Fault to the north of Lake Natron (Dawson,

2008) (Foster, et al., 1997). The Sambu faults is another normal fault that creates 400-600 m relief

difference and lies parallel to the Sonjo faults and present-day Lake Natron (Figure3) (Rodrigo, et al.,

2009). The major phase of faulting in this main fault advance between 1.15 to 1.2 Ma from potassium

argon age determinations (Macintyre, et al., 1974) and the present-day outline of the rift valley has been

mostly formed.

2.2 Volcanism

Volcanoes of northern Tanzania were first classified by Guest in 1953 as older, mainly basaltic extru-

sives, and younger more alkaline, ultrabasic and silica poor extrusives (Dawson, 2008). This nomencla-

ture is still widely used with some modifications.

Table 1. Main features of Older Extrusives (Dawson, 2008) (Le Gall, et al., 2008) (Mana, et al., 2012) (Manega, 1993)

Name Type Timing Ma Composition

Shombole stratovolcano 2.0, 1.96 nephelinite/carbonatite

Oldoinyo Sambu shield volcano 7.7, 3.5-3.1, 2.02 up to 0.7

mainly olivine basalt

Mosonik stratovolcano 3.12 nephelinite/carbonatite/phonolite and basanites

Gelai shield volcano 0.99, 1-1.5 alkali basalt/trachyte

Ketumbeine shield volcano 1.8-1.6 alkali olivine basalt/trachyte

Lemagrut shield volcano 1.9-1.8, 2.7-2.4, 3.1, 5.5-4.3

Olivine basalt/nephelinitic

Olmoti shield volcano 2.01, 1.07-1.65, 1.09

trachybasalt/trachyte

Ngorongoro shield volcano with caldera

3.7, 2.40, 2.25, 1.94

olivine basalt/trachybasalt/trachyte

Sadiman stratovolcano 4.5, 3.7-3.3 nephelinite/phonolite/mellilite/carbonatite

Essimingor stratovolcano 5.76-5.91 nephelinite/phonolite

Tarosero lava cone 2.4-1.9 alkali olivine basalt/hawaite/trachyte

Elanairobi shield volcano 0.81, 1.16, 1.05, 1.52

Limburgite/trachy basalt/nephelinite/peralkaline phonolite

12

Older extrusives date back to Pliocene until middle Pleistocene times. Older volcanoes, such as Mo-

sonik and Shombole, have ultrabasic composition including nephelinitic lava and tephra (Isaacs &

Curtis, 1974) and relatively younger basaltic and trachy-basaltic rocks are represented by volcanoes

such as Gelai and Kitumbeine. Table 1 summarizes the main features and ages of older extrusives in the

study region.

A second phase of volcanism and tectonic activities initiated around 1.2 Ma and is still continuing

today. Younger extrusives are dominated by silica poor nephelinites, phonolites and carbonatites at sev-

eral localities. Ol doinyo Lengai, located near the study region is a unique active natro-carbonatite vol-

cano.

2.3 Previous Paleomagnetic Studies and Stratigraphy

The oldest rocks exposed in the Northern Tanzania are Precambrian quartzites, quartz mica schists and

gneisses of Tanzanian Craton in the west of the study region across the Serengeti plains (Figure 2) (Isaac,

1967). The Mozambique Belt is separated from the Plio-Pleistocene rift related basins by the Sonjo

Nguruman fault line, which is 3.5 to 7 Ma old (Rodrigo, et al., 2009).

Initiation of the rift system is marked by deposits of Mosonik tuffs and nephelinites of Late Tertiary

age that are in contact with Precambrian rocks. Sambu lavas overly this sequence and are mostly basaltic

with some trachytic and phonolitic lava flows and they contain several magnetic polarity reversals. Ra-

diometric dating estimates with their ages to 2.02 and 2.99 Ma (Isaacs & Curtis, 1974) (Manega, 1993).

Sediments overlying the Sambu lava are covered with thin flow of streaky vesicular lava that display

paleomagnetic reversals (Isaac, 1967), which are called Hajaro lavas or alternatively Hajaro beds. Hajaro

beds mark the beginning of the Peninj group and they are characterized by reversed paleomagnetic po-

larity and dates to 2.1 -2.0 Ma. Reported ages for Hajaro lava above the sedimentary beds ranges be-

tween 1.7-3.4 Ma, presented in various studies (Figure 4) (Isaacs & Curtis, 1974) (Manega, 1993)

(Foster, et al., 1997). They are distinct from Sambu lavas with erosional indicators.

The Peninj Group is defined by a type section defined on the west of present day Lake Natron. It is

composed of the deltaic Humbu formation and the lacustrine Moinik formations that are intercalated

with several lava flows and tuff layers. Achulian Industry tools are found in the Humbu formation which

includes coarse grained alluvial sands and clays and layers of basaltic tephra and olivine basalt

(McHenry, et al., 2011). The Moinik formation comprises sands and clays deposited in the ancient lakes

that are interbedded with trachytic tuffs and tuffaceous shales (Isaac, 1967).

The base of the Humbu formation is dated to 1.72 Ma (Manega, 1993), Wa Mbgu Lava in Humbu

tuff has been dated to 1.91 Ma or between 0.96-1.21 and 1.19 (Deino, 2012) Ma and the base of the

Moinik formation to 1.37 Ma and 1.33 Ma (Isaacs & Curtis, 1974). Nephelinite tuff within the series

dates to 1.28-1.31 Ma (Foster, et al., 1997) (Figure 4).

13

The Olduvai event, a normal polarity subchron, is hosted within the reverse Matuyama polarity chron,

and is stated to have taken place between 1.71 to 1.86 Ma. The event was originally observed to the

south of the study region in the lava sequences overlain by pyroclastic rocks and alluvial sediments of

Olmoti. This section is comparable to the Humbu Formation in the north with respect to age and lithol-

ogy (Gromme & Hay, 1971). In later studies, the exact timing of the Olduvai subchron was tuned to

1.945-1.778 Ma, however the lower limit of the event is inconsistent with the new ages obtained to 1.918

(Deino, 2012).

Figure 4. Composite Stratigraphic Section of Lake Natron (Greenwood, 2014) (McHenry, et al., 2011).

14

Five different normal and reverse polarity zones are defined for the Moinik and the Humbu for-

mations (Thouveny & Taieb, 1986). R1 represents the deposits prior to the Olduvai events with reverse

polarity. 1.91 Ma aged Wa Mbgu Basalt within the Humbu formation is magnetized with normal polar-

ity, which might have deposited during the Olduvai Event named N1. The Matuyama-Brunhes boundary

is thought to be representing N2, which is in the upper Moinik formation. The rest of the upper Moinik

formation is divided into three more normal and reverse polarity events. Nephelinites and various ba-

saltic layers within the Moinik sequence have been demonstrated to record reverse polarity (Isaacs &

Curtis, 1974).

The sequence including the Main Tuff and upper parts of the Humbu formation might be equivalent

to the Cobb Mountain paleomagnetic event (1.25-1.10 Ma) with ages of 1.3 Ma and 1.2 Ma (Rodrigo,

et al., 2009). The composite stratigraphic section of the Peninj Group and underlying units are repre-

sented in Figure 4.

There is a consensus that the studied escarpment predates 1.2 Ma. The new tectonic phase beginning

at the end of the Early Pleistocene ceased the deposition of Peninj Group sediments. Eruption of younger

volcanoes started around 0.6 Ma with Kerimasi followed by Ol Doinyo Lengai (Rodrigo, et al., 2009).

Deposits of younger extrusives, and lacustrine sediments of Holocene age also deposited in the area.

15

3 Theoretical Background

The magnetic moment M, or specifically the magnetic dipole moment, can be explained by the combi-

nation of two magnetic charges with opposite sign or by a loop of electrical current. It is equal to mul-

tiplication of the magnitude of charge (m) with the infinitesimal distance vector (l) (Butler, 1992):

𝑀 = 𝑚 · 𝑙 .

The magnetic field, H in a region, is the force that influences a positive unit charge that is in that

region. It can be observed through the aligning torque on a magnetic moment placed in the presence of

the magnetic field.

Division of the sum of each individual magnetic moment to the volume of a material yields the

magnetic intensity, or magnetization (J). The strength of magnetization in different directions can be

interpreted as the net shape of individual grains or degree of their crystalline alignment (Tarling &

Hrouda, 1993).

The magnetic susceptibility (k) is a physical property that relates an applied magnetic field to mag-

netization and it is material dependent.

3.1 Magnetism in Solid Matter

Magnetic moment emerges from the precession of the electrons when applied to a magnetic field. Rock

forming minerals are classified into three categories with respect to their susceptibility manners, as dia-

magnetic, paramagnetic and ferromagnetic.

3.1.1 Diagmagnetism

In the presence of a magnetic field the electron magnetic moment will oppose the direction of the

applied field resulting in a negative magnetic susceptibility, this is a characteristic for all the materials

and it is termed as diamagnetism (Figure 5a). However, for most of the other substances diamagnetism

is overcame by the atomic magnetic moments (Butler, 1992). Magnetization is linearly related to the

applied field and is zero when the applied field is removed. Quartz, most feldspars and calcite are the

most common diamagnetic minerals.

3.1.2 Paramagnetism

Paramagnetism is the result of unpaired electrons in an atom’s electron shell. Every electron has a spin

that gives rise to a magnetic moment, but when paired with another electron the net magnetic moment

16

is zero, because their spins cancel each other out. In the presence of an external field, the unpaired

moment will align with the direction of the applied field, thus resulting in a positive net magnetic sus-

ceptibility (Figure 5b). Magnetic susceptibility is directly proportional to the subjected magnetic field.

The magnetization would disappear if the applied field is removed. Most iron-bearing minerals are par-

amagnetic in nature. Well-known examples include, amphiboles, pyroxenes, Fe-bearing olivine and bi-

otite mica.

For paramagnetic solids, the applied magnetic field and thermal energy creates independent reactions

on the atomic magnetic moments and follow the so-called Curie-Weiss law (Dunlop and Özdemir,

1997). In other words, if the absolute temperature is above zero, an applied magnetic field would orient

atoms in the applied field direction. In the absence of an applied field all the atomic moments would

align randomly, and total magnetic moment would be zero.

3.1.3 Ferromagnetism

Ferromagnetism is the consequence of Heisenberg exchange or superexchange coupling between the

spins of 3D orbit electrons in adjacent or almost adjacent atoms, displaying electrostatic interaction

between electrons (Dunlop & Özdemir, 2007).

For ferromagnetic solids, neighboring atomic moments from unpaired electrons interact strongly with

each other resulting in higher magnitudes of magnetizations compared to paramagnetic solids with the

same strength of applied fields. In the presence of a magnetic field the moments will try to align with

the direction of the applied field and add their moment to the field strength, resulting in a strong positive

magnetic susceptibility (Figure 2c) (Butler, 1992). Ferromagnetic substances acquire magnetization par-

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

EK1206B 5.47E-03 1.01 1.01 1.02 1.02 -0.07 37 31 139 19 255 52 EK1179C 2.30E-02 1.01 1.05 1.06 1.06 0.55 24 12 290 20 143 66 EK1179A 2.69E-02 1.01 1.04 1.05 1.05 0.64 326 16 59 10 180 71 EK1177C 1.67E-02 1.01 1.01 1.01 1.01 0.14 47 5 313 39 143 51 EK1168B 2.84E-02 1.04 1.11 1.16 1.16 0.40 45 2 137 34 312 56 EK1168C 1.51E-02 1.03 1.03 1.05 1.05 -0.01 146 19 245 23 20 59 EK1177A 1.56E-02 1.02 1.01 1.03 1.03 -0.41 72 31 309 43 184 32 EK1118TB 8.20E-03 1.00 1.00 1.00 1.00 0.83 346 43 124 39 234 23 EK1118A1 1.18E-02 1.00 1.00 1.00 1.00 0.78 238 64 112 16 15 20 EK1108C 3.42E-03 1.00 1.02 1.02 1.02 0.89 37 31 186 55 298 15 EK1108B 5.01E-03 1.01 1.03 1.04 1.04 0.61 125 76 34 0 304 14 EK1090A 2.40E-02 1.00 1.01 1.02 1.02 0.50 177 68 346 21 78 4 EK0847 1.22E-01 1.01 1.00 1.02 1.02 -0.75 235 16 348 54 135 32 EK0824 4.91E-02 1.00 1.00 1.00 1.00 0.21 176 50 71 12 332 37 EK0823C 2.84E-02 1.00 1.00 1.01 1.01 -0.04 37 17 137 31 282 54 EK0823 3.57E-02 1.01 1.00 1.01 1.01 -0.20 54 27 148 7 252 62 EK0822 4.71E-02 1.00 1.00 1.01 1.01 0.92 325 30 182 55 65 18 EK0820 3.77E-02 1.00 1.00 1.00 1.00 -0.49 46 10 311 22 159 65 EK814 5.40E-02 1.00 1.00 1.00 1.00 0.19 102 59 210 10 305 29 EK0813C 2.56E-02 1.00 1.00 1.00 1.00 0.37 112 44 297 46 204 2 EK0813 3.68E-02 1.00 1.00 1.00 1.00 0.31 142 70 300 18 32 7 EK0805 4.99E-03 1.00 1.01 1.01 1.01 0.24 130 11 36 20 247 68 EK801 2.01E-03 1.00 1.01 1.01 1.01 0.64 314 3 45 21 215 69 EK789A 1.85E-03 1.00 1.00 1.01 1.01 0.01 215 31 320 23 81 50 EK0789 1.59E-03 1.00 1.00 1.00 1.00 -0.51 274 10 8 23 162 65 EK0787AC 1.10E-04 1.00 1.00 1.00 1.00 0.49 220 15 94 66 316 19 EK0787A 1.35E-04 1.02 1.02 1.04 1.04 0.01 163 3 67 58 255 32 EK0787B 8.51E-04 1.00 1.00 1.00 1.00 -0.28 100 84 227 4 317 5 EK0802 3.01E-04 1.01 1.14 1.15 1.17 0.92 103 62 236 20 333 19 EK0802C 7.71E-05 1.00 1.00 1.00 1.00 -0.17 46 73 309 2 218 17 EK0784C 5.48E-03 1.00 1.00 1.00 1.00 -0.13 192 9 297 58 96 30 EK0784 5.75E-03 1.00 1.00 1.00 1.00 0.28 330 10 235 24 80 64 EK0797Z 5.30E-04 1.00 1.00 1.00 1.00 0.14 63 54 180 19 282 30 EK0774 3.91E-03 1.01 1.00 1.01 1.01 -0.62 339 6 247 21 84 68 WF757E 2.96E-04 1.00 1.00 1.00 1.00 -0.40 211 24 2 63 116 12 WF756AC 1.96E-03 1.00 1.00 1.01 1.01 -0.13 10 23 195 67 101 2 WF756D 5.97E-03 1.01 1.00 1.02 1.02 -0.65 80 43 311 35 199 28 WF756 7.51E-04 1.01 1.00 1.01 1.01 -0.63 274 77 84 12 175 2 WF739A 6.09E-03 1.01 1.01 1.03 1.03 -0.04 153 26 29 49 259 29

35

5.1 Natural Remanent Magnetization

NRM results of 41 samples measured by cryogenic magnetometer. Field orientations as declination and

inclination in degrees (˚), volume magnetization (M) and intensity (J) are listed in (Table 4).

Measured magnetic moment in orthogonal x, y and z directions, (X, Y, Z) calculated inclination (ISI)

and declination (ISD) values, rotated magnetic moments (X’, Y’, Z’) in orthogonal directions and rotated

declinations (Dec) and inclinations (Inc) are listed in the same table.

Average magnetization intensity (J̅) of all the samples was 2.76×10-3 A/m. Nephelinites exhibit

higher intensities than rest of the section with (J̅nephelinites = 3.5×10-3 A/m). The tuff layer has lower inten-

sity than the average (J̅tuff = 1.14×10-3 A/m). Middle lava layers, basanites and other have the highest

intensity averages with (J̅basanites = 4.36×10-3 A/m). Sedimentary layers of EK and WF sections have the

lowest intensities as expected (J̅lacustrine = 3.58×10-4 A/m). Alkali basalt level below sedimentary sections

(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-

tainty.

Table 5. Magnetization Parameters

Polarity Groups

Mean Declination (˚)

Mean Inclination (˚)

mean MADp (˚)

No of samples

Normal 25.55108721 15.40890105 8.961086263 9

Reverse 172.7936199 1.186509121 7.755637643 17

Intermediate 157.1196295 -4.359791484 11.28596195 14

Magnetostratigraphic chart of the study region is prepared and correlated with the In-

ternational Chronostratigraphic chart of 2017 of International Commission on Stratigraphy

(IUGS) for last 2.7 Ma (Figure 27).

Radiometric dating limits the age of Natron sequence to be younger than 1.2 Ma (Foster,

et al., 1997) (Macintyre, et al., 1974) (Paslick, et al., 1995). Therefore, the first polarity

change from reverse to normal most likely reflects the Cobb Mountain Event (CMT, 1.187-

1.208 Ma). This would be a reasonable choice considering the duration of this particular

event, which is much longer than shorter similar age events, such as Bjorn, Gardor or Gilsa.

Jaramillo Event (0.990-1.071 Ma) is also a long term normal event, yet it is eliminated

from correlation due to conflicting radiometric ages. Thus, CMT is the most likely to rep-

resent the consistent normal polarity interval within almost 25 m of nephelinite layers.

In addition, nephelinites of EK section are indicating Mosonik to be potential source,

which also yielded similar radiometric ages (1.28 and 1.31) (Foster, et al., 1997). This is

45

comparable to N2 normal polarity zone of (Thouveny & Taieb, 1986), with respect to the

presence of nephelinites which are also sourced from Mosonik volcano.

The lower boundary of CMT is speculative for the scope of this study due to the absence

of sampling for more than 50 m between the nephelinites and the palagonitic tuff layers.

However, similar palagonitic tuff layers are also present within the upper parts of Humbu

formation, which predates the Olduvai Event (N1 of (Thouveny & Taieb, 1986))

(McHenry, et al., 2011). Therefore, the lowermost estimate for the lower boundary of CMT

is the tuff layer which also exhibits reverse polarity.

The presence of an almost 200 m gap within the basanite series of EK makes it difficult

to continuously interpret the polarity pattern. The correlation of lithologies is the only tool

to establish a geochronological basis. Peninj and Olduvai regions have lacustrine and allu-

vial sediments that deposited around 2 Ma. The Hajaro Beds that are deposited uncomfort-

ably over the Sambu basalt are similar to lacustrine sediments of WF and EK sections.

Besides, the absence of soil development in the basanite series indicate deposition over a

short period of time (Neukirchen, et al., 2010).

GPAvailable radiometric ages demonstrate that deposition of Hajaro Beds occurred be-

tween the Hajaro lava and Sambu lava. Consistent ages obtained for Hajaro lava varies

between 1.7 to 2.0 Ma and for Sambu lava around 2.0 to 2.99 Ma (McHenry, et al., 2011).

The last normal polarity event that is bound by two reverse polarity chrons within EK sec-

tion suggest that this is the Réunion Event (2.116 – 2.137 Ma). Therefore, normal polarity

interval over lacustrine sediments within basanite series might be indicating Olduvai Event

(1.71 to 1.86 Ma), however this assumption is vague since the thickness of the normal

polarity interval is less than 5 m. The closest equivalent for the Olduvai Event in the region

is the Peninj Formation, Wa Mbgu Basalt, which is also a 5 m sequence. However, there

is a strong compositional difference since Wa Mbgu is an olivine Basalt (Isaacs & Curtis,

1974), whereas EK 0789 and EK 0801 are basanites (Neukirchen, et al., 2010).

46

Figure 27. Preliminary GPTS Profile of Natron Escarpment

47

Age prediction for the underlying alkali basalt of the WD and EK is unclear. Results of

ChRM for two alkali basalt samples both yielded intermediate polarity. Besides, presence

of lacustrine sediments over alkali basalts indicate an erosional period where a depositional

hiatus is present. Since there is no correlative alkali basalt layer with age data in the vicin-

ity, any prediction would not be valid. Thus, the basalt layers can be substantially older

than the overlying basanites and sediments.

48

7 Conclusions

Rock magnetism offers numerous applications to solve geological problems. Magnetic fab-

ric of rocks based on anisotropy of magnetic susceptibility of magnetic grains can be ex-

ploited to interpret flow directions of volcanic and volcaniclastic rocks. Based on magnetic

fabric, this study suggests possible flow directions and sources for 400-450 m thick lava

flows of Natron Escarpment through two geological vertical sections, Endukai Kete and

Waterfall.

Magnetic fabric of nephelinitic rocks of Endukai Kete section designates NE-SW direc-

tion of flow. Available geochemical and radiometric data indicates Mosonik Volcano as

possible source for nephelinitic lava flows prior to second phase of volcanism. Basanites

of Endukai Kete section however include layering of alternating lava flows that originates

from different volcanoes. Majority of the samples indicate NE-SW direction for flow to-

gether with NW-SE direction of flows in some levels. Geochemical analyses might point

Mosonik (NE) and Gelai (NW) volcanoes again, however radiometric age data is not com-

patible or sufficient to prove this hypothesis. Palagonitic tuff layers within 1000-1050m of

EK section indicate a flow from NE-SW direction. Lacustrine sediments also follow a

similar trend. Alkali basalt layers below lacustrine formations demonstrate flow in the op-

posite direction, NW-SE.

GPTS of the sections are dependent on the radiometric ages from different studies. Gen-

eral consensus about the upper limit age of the Escarpment is 1.2 Ma and absolute age data

from the escarpment confirms this assessment (Paslick, et al., 1995). Therefore, upper

nephelinitic layer with normal polarity is attributed to CMT Event. The tuff level below

nephelinites also have reverse polarity like nephelinitic tuff layers of Peninj Type section.

Therefore, they might be younger than Olduvai Event like Peninj Group nephelinitic tuffs.

49

Olduvai Event is probably within lower middle basanite section that deposited just after

lacustrine sediments. However, absence of continuous sampling complicates further inter-

pretation. Yet, two normal polarity chrons within the middle basanite series might be indi-

cating Olduvai and Réunion Events since the lacustrine deposits of both EK and WF sec-

tions possess reverse geomagnetic polarity that are correlative to the Hajaro Beds of Peninj

Section in the North of Escarpment.

50

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Lengai Volcano and Surroundings, Arusha Region, United Republic of Tanzania,

Vancouver: USGS.

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Chapman & Hall.

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Thouveny, N. & Taieb, M., 1986. Preliminary magnetostratigraphic record of

Pleistocene deposits, Lake Natron Basin, Tanzania. In: L. E. Frostick, ed.

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s.l.:Geological Society of London, pp. 331-337.

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Peninj (Lake Natron, Tanzania). Journal of Anthropological Archaeology, pp.

244-264.

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Techniques. Treatise on Geophysics, pp. 93-146.

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APPENDIX I

Matlab Code for PCA num = readtable('C:\ Sample.xlsx'); [m,n] = size(num);

azi = azi; dip = dip;

l = deg2rad(dip); f = deg2rad(azi);

lambda = ones(m,1); lambda = lambda.*l; fi = ones(m,1); fi = fi.*f;

x1 = num.X; x2 = num.Y; x3 = num.Z;

%%Zijderveld Plot Unrotated

figure

plot(x1,x2); hold

plot(x1,x3); axis equal

%%Rotation of data

for i=1:1:m a = [cos(lambda(i))*cos(fi(i)) -sin(fi(i)) -

sin(lambda(i))*cos(fi(i)); cos(lambda(i))*sin(fi(i)) cos(fi(i)) -

sin(lambda(i))*sin(fi(i)); sin(lambda(i)) 0 cos(lambda(i))]; X1(i) = a(1,1).*x1(i) + a(1,2).*x2(i) + a(1,3).*x3(i); X2(i) = a(2,1).*x1(i) + a(2,2).*x2(i) + a(2,3).*x3(i); X3(i)= a(3,1).*x1(i) + a(3,2).*x2(i) + a(3,3).*x3(i); B(i) = sqrt((X1(i))^2+(X2(i))^2+(X3(i))^2); D(i)= rad2deg(atan2(X2(i),X1(i))); I(i) = rad2deg(asin(X3(i)/B(i)));

55

end

%%PCA

%%define the range (start to end)

r1 = r1; r2 = r2;

x = [X1 X2 X3];

for i = 1:1:r1-1 x(i,:) = [ ]; end

for j = 1:1:m-r2 x(r2+1-r1,:) = [ ]; end

%% Covariance matrix

a = x(:,1); b = x(:,2); c = x(:,3);

x = [a-mean(a) b-mean(b) c-mean(c)];

C = cov(x); [V,U] = eig(C); [sU,sI] = sort(diag(U),'descend'); V = V(:,sI);

MADp = atan(sqrt((sU(3)+sU(2))/sU(1))); MADo = atan(sqrt(sU(3)/(sU(2)+ sU(1))));

pca_x = V(1,1); pca_y = V(2,1); pca_z = V(3,1); pca_xy = sqrt((pca_x)^2 + (pca_y)^2); an = abs(pca_z/pca_xy); k = mean(c); if k >0

else an = -1*an; end

Inew = rad2deg(atan(an)); Dnew = 180 + atan2(pca_y, pca_x) * 180 / pi;

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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

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Figure 28 PCA Results EK0774 to EK0820B

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Figure 29 PCA Results EK0802 to EK0847

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Figure 30 PCA Results EK1090 to WF 739

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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.

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Figure 33 Zijderveld Plots of the samples (EK 0774-EK0802B) prior to rotation.

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Figure 34 Zijderveld Plots of the samples (EK 0805 – EK1090) prior to rotation.

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Figure 35 Zijderveld Plots of the samples (EK 1108 – WF757E) prior to rotation.

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