Tectonics and climate: impacts on the evolution of fluvio -...

Institut für Erd- und Umweltwissenschaften, Universität Potsdam Berkeley Geochronology Center *

University of Nairobi** Geothermal Development Company of Kenya***

Tectonics and climate: impacts on the evolution of fluvio - lacustrine environments in

the Kenya Rift

Manfred R. Strecker *Alan Deino

Yannick Garcin Daniel Melnick **Lydia Olaka

***Peter Omenda Veronica Torres

Martin Trauth

(I)  Tertiary and Early Pleistocene basins may have repeatedly hosted large lakes – vestiges are limited

(II) Oblique extension and magmatic processes in Central Kenya Rift have compartmentalized the rift, ultimately causing disparate sedimentary environments (III) Holocene lake-highstands caused fluvial connectivity between isolated basins and left shorelines that can be used to quantify extension on 103yr timescales (IV) CRN erosion rates document that transitional environments are important in forcing erosion during climate change

Key topics

Co

mp

ilatio

n o

f Ke

nya

n g

eo

log

yBa

ker e

t a

l., 1

988;

Ge

ol.

Surv

ey

of K

en

ya, 1

989

(I) Early sedimentary basins – we don’t know much

Basement anisotropies, fracture propagation and basins

Hetzel & Strecker, 1991, J. Structural Geology

Kerio sediments Kabarnet trachytes Uasin Gishu phonolites Tambach sediments Marble Gneiss Granulite

2 km

Stamps et al., 2008 GRL

(II) Pleistocene basins in central Kenya: changing obliquity of plate-slip vectors along the rift

En échelon zones of extension and semi-independent basins

91±2

60±2

88±493±3

107±4

113±2

146±2161±4

176±2

322±20

92±5

59±4

72.7±1.873±381±4

108±7

Presentlake level ±0

+5

-5

-20

-15

-25

-35

-30

-40

-50

-45

-55

-60

Change frompresent lake level

(m)

40Ar/39Arages(kyr)

TuffsSiltstone bedsDiatomitesFluvial depositsVolcanic rocks

141±3

Decreasing alkalinity

Fresh glassSm

ectitesC

habaziteC

linoptiloliteAnalcim

eSubarial

III

II

I

IV

V

Trauth & Strecker, 2000, E African Lakes; Trauth et al., 2002, Geology; Trauth et al., 2005, Science

A. Bergner et al., 2009, Quat. Sci. Reviews

Contrasting sedimentary environments: orographic barriers, volcano-structural barriers, and forcing of fluvial networks

Bergner et al., 2009, Quat. Sci. Rev., V. Torres, ongoing PhD work

Oblique slip along border faults: earthquakes with ground ruptures Subukia M 6.9 earthquake in 1928 Alluvial fan and landslide deposits at escarpments

Zielke & Strecker, 2009, Bull. Seism. Soc. America

(III) Holocene rift-normal extension in N Kenya and the ups and downs of lakes

during the African Humid Period (AHP)

Paleo lake levels during the AHP present-day lakes

Ethiopian! Plateau! East African!

Plateau!

N S

modified after Junginger, 2012, PhD thesis U Potsdam

Melnick et al., 2012, EPSL; Garcin et al., 2012, EPSL

Strecker, Melnick & Olago, work in progress

Owen et al., 1982, Nature; Garcin et al. 2009, Quat. Sci. Rev.; Garcin et al., 2012, EPSL; Junginger, 2012, PhD thesis, Potsdam

Present-day lakes

Overflow sill

Rivers

250 50 km

Late Pleistocene-early Holocene lakes

Turkana basin (148 586 km2)

Suguta basin(13 035 km2)

Baringo-Bogoria basin (8005 km2)

Lobat Gap

Kamuge

Ker

io R

iver

36°E 37°E

0°

1°N

2°N

Sugu

ta R

iver

Protracted lake-highstand and formation of shoreline during

African Humid Period – an ideal marker to assess tectonism

Melnick et al., 2012, EPSL

Stamps et al., 2008, GRL

Linking different timescales of extension

(IV) Were these humid periods characterized by greater erosivity, sediment production, and mass transfer?

Erosion rates in different precipitation/vegetation compartments: effects of vegetation on cosmogenic nuclice (10Be,26Al)

basin-wide erosion rates

Veronica Torres, PhD thesis in preparation, Potsdam University

Below vegetation threshold

Above vegetation threshold

Slide showing shifting climate and vegetation

(IV) Can we learn about past humid episodes and sediment production from climate modeling?

IPCC 2007 Synthesis Report Drier Wetter

Predicted changes in annual runoff, 2090-2099 compared to 1980-1999

(0)( ) PN tρε

λ=

+Λ

Anima'on by Kevin Norton 2010

Basin-wide erosion rates using CRN (10Be, 26Al)

Concentration of nuclides (N (t)) and erosion rate (ɛ) are inversely related:

Cosmic rays

•  GPS measurements agree with long-term extension

rate (3.2 Ma) and geologic rates

•  Kinematic model predicts

~4 mm/a of extension in North Kenya between

Victoria and Somalia plates

•  Strain localized along narrow rift zone that isolates

large lithospheric blocks

•  Oblique rifting generates en échelon zones of

extension

Stamps et al., 2008 GRL

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Tectonics and climate: impacts on the evolution of fluvio -...

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