Contrasting Geodynamics of Small Cold and Giant Hot Orogens C. 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

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:

Beaumont et al, 2001

Channel flow only occurs for cases where partial melt (   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 crust Total erosion Convergence 10mm/a 6 Ma Retro shear zone Secondary pro-shear Primary pro- shear 0  C 600  C 30 km

shortening = 40 km Vertical velocity 20  C/km geotherm 1 cm/yr Gerbault and Ellis, submitted 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  =10 18 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