Contrasting Geodynamics of Small

Cold and Giant Hot OrogensC. Beaumont

R. Jamieson P. Fullsack Dalhousie University, Canada B.Lee

M. Nguyen

Susan Ellis New Zealand

Adrian Pfiffner University of Bern, Switzerland

Institute of

GEOLOGICAL

& NUCLEAR

SCIENCES

limited

GLOBE project (NOAA)1. Indentation (3D)

2. Pre-existing weakness

3. Lower lithosphere dynamics

0

500

1000

1500

2000

2500

3000

0 500 1000 1500 2000 2500

convergence (km)

wav

elen

gth

(km

)Orogen wavelength vs. convergence

HT

VA

NZ, EA

Modified from Ellis (1995)

Eastern Swiss Alps

NARROW -- < 250 km

•moderate crustal temperatures (< 700 C)

•little crustal melting

•most intense crustal deformation intense mantle lithosphere deformation

•three-dimensionality generally not a first-order control on wavelength

“Small Cold (SC) Orogens”

New Zealand Southern Alps

Wellman, 1979

•high crustal temperatures and extreme crustal thickening

•crustal melting

•most intense crustal deformation most intense mantle lithosphere deformation

•three-dimensionality may be a first-order control on wavelength

“Giant Hot (GH) Orogens”

WIDE-- > 1000 km

X

Small vs. giant orogens: (1) strength of ductile lower crust?

x=100 km

Viscosity

Ellis et al, 2000

Small vs. giant orogens: (2) localisation by strain weakening?

Ellis et al, 2000

Small vs. giant orogens: (3) localisation by erosion?

Tectonic processes affecting the Himalayan-Tibetan System“Giant Hot” (GH) orogens:

= B * ex p (Q * /n R T )

H T R L -2 5 : R eferen ce M o d el, In itia l C o n d itio n s

02 03 5

km

T = 1 3 5 0 o Ca

q s = 7 0 m W /m 2

R h eo lo g ica l P ro p erties:

U p p er c ru s t: = 1 5 o (W et Q u a rtz ite x 5 ) L o w er c ru s t: = 1 5 o (D ry M ld D iab ase )

T h erm a l P ro p ertie s:

K = 2 .0 W /m o K = 1 .0 x 1 0 m /s-6 2A = 2 .0 W /m1

3 A = 0 .7 5 W /m23

Tem p era tu re a n d Ve lo c ity

C p( T / t + v T ) = K T + AA d v e c tio n D iffu s io n H ea t

P ro d u c tio n

E ffectiv e V isco sity : H ea t B a la n ce:11

n

2 cm /y

D en u d ation ra te = S lop e f(t) g (x )

M o h o = 7 0 4 C0

A 1

A 2

0

6 0 0 k m

3 52 5

1 4 0 0 k m

km

M a rker G rid

2 cm /y

...... ........ 2 0 o

q m = 2 0 m W /m 2

K inem a tic Subduc tion

C o nd uc tive Stea dy-Sta te

E ro s io n fu n c tio n g (x )

1.0 0.0

Beaumont et al, 2001

Beaumont et al, 2001

Beaumont et al, 2001

Channel flow only occurs for cases where partial melt ( 1019 Pa.s) is present

Extrusion only occurs for cases with high erosion

Beaumont et al, 2001

Beaumont et al, submitted

“Small Cold (SC) Orogens”

New Zealand Southern Alps

Walcott, 1998

• Extrusion of mid-lower crust (Grapes, 1995; Walcott, 1998)

Batt and Braun, 1999

Felsic crustTotal erosionConvergence 10mm/a

6 Ma

Retro shear zone

Secondary pro-shear

Primary pro-shear

0 C

600 C

30

km

shortening = 40 km

-104090

140

12 62 112 162distance (km)

Vertical velocity

20C/km geotherm

1 cm/yr

Gerbault and Ellis, submitted

1021 1024

x

Structural Geology Transect

Little et al. in press

Arthur’s Pass: Campaign data sets

Beavan and Ellis, in prep.

Post-seismic signal

VISCOELASTIC MODEL: Lower crustal channel =1018 Pa s

OBSERVATIONS

“Small Cold (SC) Orogens” “Giant Hot (GH) Orogens”

High non-linear viscosities make extrusion less likely

Inherited material heterogeneities can influence dynamics

Low linear viscosities (partial melt)+erosion can cause extrusion

Hertzsprung-Russell diagrams for stars

Excess volume

H-R diagram for orogens