Lecture 1-2 continued Material balance and properties Uplift and subsidence. Topography, crustal and lithospheric thicknesses, LATERAL TRANSPORT OF MATERIAL (tectonic extrusion)2)VERTICAL TRANSPORT OF MATERIAL (fundamental change in physical properties (SUBDUCTION AND EDUCTION)

Surface elevation is described by the concept of isostasy which relates vertical distribution of mass with elevation, where the lithosphere floats on the asthenosphere. Isostasy is a stress balance za = zb that equates the stress () at the compensation depth (z) of two areas (a, b).We can discriminate between (1) hydrostatic- and (2) flexural isostasy.In 1, all vertical columns are independent; in 2 shear stresses between columns are considered. qqHydrostatic- and (2) Flexural isostasy q is load

mcHZB AVertical normal stress thickness z2 columns of density a, b as function of depth zEquation 2 solved for figure; by intergrating left and splitting right half of (2) we get: Hmat elevation of column above dence material; Cancelling g, air density negligible compared to crust and lithosphere gives: A comparison of column A and B: Elevation (H) is only a reflection of density (material)diffence between A and B.For further expl: see Stwe Geodynamics of the Lithosphere

AIRY ISOSTASY:Thicker crust in orogenic belts gives higher topography because m > cC2 = C1 + h (m/(m - c)h = (C2 - C1)(m - c)/ mA thick, light crust floats high.Pratt isostasy --> density varies laterally with topography

What happens if the crust and/or mantle density change? Example:Partial eclogitization of orogenic crust, (100% below C2n km)

C2 = C2n + [C1 (c - m) - m h + C2n (m - c)] / (e- m) = C2n + [C2n - C1) (m - c) - m h ] / (e - m)

h - Elevation (above sea level) C1 - Normal crust thickness ( 30 km)C2 - Orogenic crust thickness C2n- Orogenic crust without eclogitization - Densities of (m) mantle, (c) crust and (e) eclogitized crust

(for details see: Fowler: The Solid Earth)Airy Pratt

Metamorphic reactions change mineral assemblages:

New minerals => different density, rheology and petrophysical properties

Dilation related to Gabbro => eclogite transition is - 15 %Amphibolite => eclogite transition is - 18 % Peridotite => serpentinite transition is + 35 % Metamorphic reactions break down minerals and so may enhanced deformation (increased strain / strain-rate)

From Hacker 2004

Some density measurements of rocks with nearidentical geochemicalcompositons from the Bergen area

What are the implications for the topography in Mountain Belts?

Crustal thickening=> uplift

Mantle lithosphere thickening => subsidenceLets look at some examples of modellingwhere the petrophysical changes related tometamorphic reactions and their reactions rates have been considered.

No eclogitizationEclogitization half-time 6.4 myrEclogitization half-time 3 myrNo amphibolitizationamphibolitizationhalf-time 3 myrNo tectonic denudation

After Dewey et al. 1993MODELLED TOPOGRAPHIC EVOLUTION RELATED TO THICKENING AND THINNING OF LITHOSPHERIC MANTLEAND CRUST Lithospheric thickeningLithosphereDelaminationTectonic denudation(Dewey et al. 1993)

The petrophysical effect ofmetamorphism.

Density changes related toequilibrated prograde metamorphism of hydratedoceanic mantle lithosphere.

Notice that this diagram isParticularly relevant for Benioff zones.

(after Hacker et al. 2003)

From Hacker 2004

Example, Costa RicaExample, JapanHacker et al. (2002)

The petrophysical effect ofmetamorphism.

Density changes related toequilibrated prograde metamorphism of crust with,densityGranitic 2.74 g/cm3Andesitic2.84 g/cm3Gabbroic2.95 g/cm3compositions.

(Calculated by Henry et al. 2001)

(Henry et al. 2001)

Here we assumed that most of the subducted crust reacted and achieved mantle-type density.We were mostly concerned withkeeping the topography realistic.The key element is the mantle-wedge above the subducted part of the Continent.(Andersen et al. 1991)

METAMORPHISM, DENSITY STRUCTURE and TOPOGRAPHY(from unpublished thesis by M. Krabbendam 1998)AIRY ISOSTASY:

C2 = C1 + h (m/(m - c)

C1 - Normal crust thickness ( 30 km)C2 - Orogenic crust thickness ( 30 km)h - Elevation (above sealevel) - Densities of (m) mantle and (c) crust

METAMORPHISM, DENSITY STRUCTURE and TOPOGRAPHY(from Krabbendam 1998)AIRY ISOSTASY:

C2 = C1 + h (m/(m - c)

C1 - Normal crust thickness ( 30 km)C2 - Orogenic crust thickness ( 30 km)h - Elevation (above sealevel) - Densities of (m) mantle and (c) crust

METAMORPHISM AND MODELLED DENSITYSTRUCTURE IN THE ALPSCONVERGENCE RATES 8 (TOP) AND 4 MM/YR,( after Henry et al 2001)

MODELLED (DOTTED) AND OBSERVED (SHADED)TOPOGRAPHY OF THE ALPS (Henry et al 2001)

IMPORTANT DISTINCTION BETWEEN CONSEPTS!

UPLIFT VS. SUBSIDENCE UPLIFT => SURFACE IS RAISED RELATIVE TO REFERENCESUBSIDENCE => SURFACE IS LOWERED RELATIVE TO REFERENCE

EXHUMATION VS. BURIALEXHUMATION => ROCKS APPROACH THE SURFACEBURIAL => ROCKS MOVE AWAY FOR THE SURFACE

(IRRESPECTIVE OF UPLIFT OR SUBSIDENCE)

Some important points brought out by the first lectures:

Pro- and retrograde metamorphic reactions play importantroles for the dynamics in orogenic belts in that they give Changes in petrophysical properties (density structure and hence evolution of topography) Reaction enhanced deformation(increased strain (strain-rate) in zones of reaction) Material balance and cross-sections, which in turn is used toestimate shortening