Material Balance, Uplift, and Subsidence in Earth's Crust

1. 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)

2. 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 = zbthat 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. q q Hydrostatic- and (2) Flexural isostasy q is load

3. H Vertical normal stress thickness z c m 2 columns of density a, b as function of depth z Z B A Equation 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 Stüwe “Geodynamics of the Lithosphere”

4. Airy Pratt • 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. 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)

5. 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)

6. From Hacker 2004

7. Some density measurements of rocks with near identical geochemical compositons from the Bergen area

8. 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.

9. 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 (Dewey et al. 1993)

10. 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)

11. From Hacker 2004

12. Blueschist facies earthquakes INTRA-PLATE EARTHQUAKES INTER-PLATE EARTHQUAKES Hacker et al. (2002) Example, Japan Example, Costa Rica

13. 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)

14. (Henry et al. 2001)

15. 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. (Andersen et al. 1991)

16. 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

17. 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

18. METAMORPHISM AND MODELLED DENSITY STRUCTURE IN THE ALPS CONVERGENCE RATES 8 (TOP) AND 4 MM/YR,( after Henry et al 2001)

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

20. 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)

21. 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