More stable yet bimodal geodynamo during the Cretaceous superchron?1

Comportement plus stable quoique bimodal durant le superchron du Crétacé ?1

F. Lhuilliera, S. A. Gildera, M. Wacka, K. Hea, N. Petersena, B. S. Singerb, B. R. Jichab, A. J. Schaenb, and D. Colonb

a: Department für Geo- und Umweltwissenschaften, Ludwig-Maximilans-Universität, Munich, Germanyb: Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin, USA

Abstract: We report palaeomagnetic and 40Ar/39Ar dating results from two sequences of basaltic lava flows deposited at the same locality in western China, yet separated in time by ~50 Myr: one set lies within the Cretaceous normal superchron at 112–115 Ma and a second at 59–70 Ma spanning the Cretaceous-Palaeogene boundary. We find that magnetic field directions during the superchron exhibit bimodal populations: one with inclinations representative of a dipolar field and another with

shallow inclinations that could reflect a more complex, multipolar field. However, the time-dependent variability in field directions was 50% lower during the superchron than after, which implies greater field stability during the superchron. Our results suggest that episodes of less dipolar field behavior occurred within the Cretaceous superchron and raise the question whether a second, more multipolar, field state is more persistent than previously thought.

Résumé : Nous rapportons des résultats paléomagnétiques et radiométriques (40Ar/39Ar) provenant de deux séquences de coulées de lave basaltique déposées au même emplacement à l'ouest de la Chine, quoique distantes dans le temps d'environ 50 Ma: une première déposée durant le superchron du Crétacé vers 112–115 Ma et la seconde enjambant la frontière Crétacé-Paléogène. Nous trouvons que les directions du champ magnétique durant le superchron présentent une distribution bimodale: l'une avec des inclinaisons représentatives d'un champ dipolaire, l'autre avec des

inclinaisons plus faibles reflétant un champ plus complexe et plus multipolaire. En dépit de cela, la variabilité du champ magnétique demeure 50% plus faible durant le superchron qu'ultérieurement, ce qui confirme une plus grand stabilité durant le superchron. Notre étude suggère en conclusion que des épisodes de moindre dipolarité puissent être survenus durant le superchron du Crétacé et soulève la question d'un état secondaire du champ, multipolaire, qui pourrait être plus persistant que précédemment conçu.

I- MotIvatIons :

from Pavlov and Gallet2 (2005)

0 50 100 150 200 250 300 350 400 450 500 550

2

4

6

8

10

Rev

ersa

l rat

e (/

Myr

)

0

Cre

tace

ous N

orm

al S

uper

chro

n

Kia

man

Rev

erse

Sup

erch

ron

Moy

ero

Rev

erse

Sup

erch

ron

No

data

?

0

50

100

150

200

Polarity interval length (My)

Virtu

al (a

xial

) dip

ole

mom

ent (

ZAm

2 )

CNS

C26rC13n,C27nC2An.3n, M0r

1001 100.1

Age (Myr)

a.

b.

c.

d. e.

Magnetic polarity reversals occured several times per Myr during the Phanerozoic, yet there are prolonged periods of more than 20 Myr of stable polarity. One of these periods is the Cretaceous Normal Superchron (CNS) from ~83 to ~121 Ma.

Why the CNS did occur and what the characteristics of the field reveal in terms of morphology and paleointensity are still largely debated issues.

II- Methods / Méthodes :

70°E 80°E  90°E  100°E 

 

110°E 30°N

 

35°N 

40°N 

45°N 

50°N 

0

100

200

300

400

500

−50050

→

→

→

→

0

50

100

150

200

−50050

←

←

0

50

100

−50050

←

←

115.17 Ma ±0.56

114.92 Ma ±0.64

113.00 Ma ±1.60

111.93 Ma ±0.24

a.

b. 59-70 Ma112-113 Ma inclination [°]

inclination [°]

stra

tigra

phy

[m]

stra

tigra

phy

[m]

K−Section

L−Section

114-115 Ma

P−Sectionc.

59.44 Ma±0.33

59.73 Ma±0.77

61.62 Ma ±0.19

69.88 Ma ±0.63

d.

e.

Tuoyun

Pamir Tarim

Tibet

Tianshan

Sampling area: Tuoyun village (75.30°E; 40.23°N) ~30 km south of the border of Kyrgyzstan in the Chinese Tianshan Mountains

Lower Cretaceous (112-115 Ma) K section: ~100 m thick 22 lava flows L section ~200 m thick 35 lava flows

Palaeocene (59-70 Ma) P section ~500 m thick 76 lava flows + 5 red beds

total:

Thermal and alternating field (AF) demagnetisation on pilot samples yielded analogous remanent magnetisation directions, so the remaining samples were AF demagnetized using a minimum of 12 demagnetization steps: principal component analysis on the 30–70 mT segment for all samples from the K and L sections; no set rule for the P section due to a larger directional variability.

1 Lhuillier et al. (2016), More stable yet bimodal geodynamo during the Cretaceous superchron?, Geophys. Res. Lett., 43, 6170–6177, doi:10.1002/ 2016GL069303.2 Pavlov and Gallet (2005), A third superchron during the Early Paleozoic, Episodes, 28(2), 78–84

III- PalaeoMagnetIc dIrectIons / dIrectIons PaléoMagnétIques :

0 100 200 300 400 500

−50

0

+50 → → →

→

0 50 100 150 200

← ←0 50 100

← ←

115.17 Ma ±0.56

stratigraphy [m]stratigraphy [m]

K−Section L−Section

P−Section

b.

incl

inat

ion

[°]

−50

0

+50

incl

inat

ion

[°]−50

0

+50

incl

inat

ion

[°]

a.

c.

111.93 Ma ±0.24

113.00 Ma±1.60

114.92 Ma ±0.64

59.44 Ma±0.33

59.73 Ma±0.77

61.62 Ma ±0.19

69.88 Ma ±0.63

Key result: bimodal distribution of directions?

N=56k=19.43α95=4.42SD=18.53E=2.74

N=75k=12.76α95=4.77SD=23.01E=2.58

0

1

2

3

4

5

6

occu

rrenc

es

c.

EW

N

K−Section + L−Section

EW

S

P−Section

a.

b.

0 20 40 60 800

1

2

3

4

5

6

inclination [°] (abs. value)

occu

rrenc

esd.

range of directions compatible with previous studies3,4 in the same area

3 Gilder et al. (2003), Earth Planet. Sci. Lett., 206(3–4), 587–600.4 Huang et al. (2005), Tectonophysics, 409(1–4), 107–124.

v - relatIve PalaeoIntensItIes / PaléoIntensItés relatIves :

Pseudo-Thellier analysis5,6 on the 30-70 mT segment

0 1 2 3 4 5x 10−5

0

1

2

3

4

5

6

7

8 x 10−5

ARM aquired

NR

M re

mai

ning

m=−1.817±0.004r2=0.991

g.

0 20 40 60 80 1000

1

2

3

4

5

6

7

8 x 10−5

NRM demag.pARM acquis.

ARM demag.ARM 90 mT

c.

Mom

ent [

Am2 ]

0 10 20 30 40 50 60 70 800.5

1

1.5

2

2.5

3

inclination [°]

pseu

do−T

hellie

r slo

pe (a

bs. v

alue

)

μ=1.85 σ=0.69 ε=0.37 μ=1.41 σ=0.35 ε=0.25

K−Section (11/22)L−Section (17/34)

relative variability in palaeointensityε=σ/μ= 0.32 VS ε=0.50 during the Oligocene7

compatible with sediments8 during the CNS

vI - dIscussIon :

0 30 60 90−45

0

45

90

latitude [°]

incl

inat

ion

[°]

K+L−Section

0 30 60 90−45

0

45

90

latitude [°]

incl

inat

ion

[°]

P−Section

a. GAD + octupole

b. GAD + octupole

GAD+10%+20%+30%+40%+50%

• The geometry of the field is in some aspects similar during the CNS and the Palaeocene…

– same elongation of directions;– same ability to produce low inclinations;

• … but also display differences:- dispersion of directions higher during

Palaeocene & CNS;- population of low inclination more

dominant during Palaeocene & CNS.

• An open issue is the interpretation of the two populations of inclination:

- Do they reflect two stable geodynamo states?

- Do they reflect a global or local feature just in Western Asia?

- How continuous / brutal is the transition between the two states?

5 Tauxe et al. (1995), Geophys. Res. Lett., 22(21), 2885–2888, doi:10.1029/95GL031666 de Groot et al. (2013), Nat. Commun., 4, 1–7, doi:10.1038/ncomms3727.7 Tauxe and Hartl (1997), Geophys. J. Int., 128(1), 217–2298 Cronin et al. (2001), Earth Planet. Sci. Lett., 190(1–2), 13–30

5 cores per flow

5 cores per flow

~700 cores