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Lecture 1-2 continued Material balance and properties Uplift and subsidence. Topography, crustal and litho PowerPoint Presentation
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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

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slide1

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)

slide2

AIRY ISOSTASY:

Thicker crust in orogenic belts gives higher topography becausem > c

C2 = C1 + h (m/(m - c)

h = (C2 - C1)(m - c)/ m

A thick, light crust floats high.

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

slide3

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)
slide5

Some density

measurements of

rocks with near

identical geochemical

compositons

from the Bergen area

slide6

What are the implications for the

topography in Mountain Belts?

Crustal thickening => uplift

Mantle lithosphere thickening => subsidence

Lets look at some examples of modelling

where the petrophysical changes related to

metamorphic reactions and their reactions

rates have been considered.

slide7

MODELLED TOPOGRAPHIC EVOLUTION RELATED TO

THICKENING AND THINNING OF LITHOSPHERIC MANTLE

AND CRUST

Lithosphere

Delamination

No eclogitization

Eclogitization

half-time 6.4 myr

Eclogitization

half-time 3 myr

Lithospheric thickening

Tectonic denudation

No amphibolitization

amphibolitization

half-time 3 myr

No tectonic denudation

After Dewey et al. 1993

slide8

The petrophysical effect of

metamorphism.

Density changes related to

equilibrated prograde

metamorphism of hydrated

oceanic mantle lithosphere.

Notice that this diagram is

Particularly relevant for

Benioff zones.

(after Hacker et al. 2003)

slide10

Blueschist

facies

earthquakes

INTRA-PLATE

EARTHQUAKES

INTER-PLATE

EARTHQUAKES

Hacker et al. (2002)

Example, Japan

Example, Costa Rica

slide11

The petrophysical effect of

metamorphism.

Density changes related to

equilibrated prograde

metamorphism of crust with,

density

Granitic 2.74 g/cm3

Andesitic 2.84 g/cm3

Gabbroic 2.95 g/cm3

compositions.

(Calculated by Henry et al. 2001)

slide13

Here we assumed that most

of the subducted crust reacted

and achieved mantle-type density.

We were mostly concerned with

keeping the topography realistic.

The key element is the

mantle-wedge above the

subducted part of the Continent.

slide14

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

slide15

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

slide16

METAMORPHISM AND MODELLED DENSITY

STRUCTURE IN THE ALPS

CONVERGENCE RATES 8 (TOP) AND 4 MM/YR,( after Henry et al 2001)

slide17

MODELLED (DOTTED) AND OBSERVED (SHADED)

TOPOGRAPHY OF THE ALPS (Henry et al 2001)

slide18

IMPORTANT DISTINCTION BETWEEN CONSEPTS!

UPLIFT VS. SUBSIDENCE

UPLIFT => SURFACE IS RAISED RELATIVE TO

REFERENCE

SUBSIDENCE => SURFACE IS LOWERED RELATIVE TO

REFERENCE

EXHUMATION VS. BURIAL

EXHUMATION => ROCKS APPROACH THE SURFACE

BURIAL => ROCKS MOVE AWAY FOR THE SURFACE

(IRRESPECTIVE OF UPLIFT OR SUBSIDENCE)

slide19

Some important points brought out by the first lectures:

  • Pro- and retrograde metamorphic reactions play important
  • roles 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 to
  • estimate shortening