PART II. Deformation of plates 1. Overview : Questions ! 2. Feedbacks and localization : 1) grainsize, 2) temperature, 3) segregation of melt/ fluids / weak phases. (This week !) 3. Strength Envelopes: Background, successes, failures... Alternatives? 4. Viscoelasticity: Cheese/Some thermodynamics/Earth ToDo 4: 1-D Thermal Part II: Rheology & Dynamics profiles 1. Localization(4-6) 2. Thermal structure & strength profiles (4-13) ToDo 5: Strength 3. Viscoelasticity, Thermodynamics (4-15) Profiles OVER AMBITIOUS: Part II: Rheology & Dynamics ToDo 3: 0D shear 1. Stress, strain, forms of suffering (4-1) zones/ wires 2. Viscoelasticity, Thermodynamics (4-6) 3. Shear Zones (4-8) ToDo 4: 1-D Thermal 4. Thermal structure & strength profiles (4-13) profiles 5. Strength profiles 2. (4-15) ToDo 5: Strength Profiles
1. Overview: Questions 1. How does the lithosphere deform? How does the response to forces depend on the cause of the forces? i.e. the time scale of the change in the force? 2. What are the appropriate ways to think about the boundary conditions of regional deformation (gravitational or constant force, constant velocity, constant energy) ? 3. How do we extrapolate micro-mechanisms to plate-scale processes ? At what length scale is deformation “homogeneous” ? How, why and where does deformation localize? 4. What are the principles that determine the patterns of lithospheric deformation (at different depths/thermodynamic conditions) ? To what extent is understanding these patterns a deterministic problem? 5. How do we describe (mechanically, not kinematically) deformation in the Earth ? 6. What are the structures and dynamics that lie beneath the seismogenic regions? How do we infer processes at depth from the patterns of seismicity?
Why does Earth have plates? potential uniform and moderate yield stress: feedbacks d. c. in viscosity: temperature η ( T ) 3 �������� ��������������� � �� �������������� ������� � ������������ 3 Geochemistry Geosystems G G Geophysics stress η ( σ ) strain rate weakening, melt weakening + asthenosphere: grain size η ( d ) melt η ( φ ) VISCOSITY DOWNWELLING + WATER!
PAGEOPH, Vol. 137, No. 4 (1991) 0033-4553/91/040439 2251.50 + 0.20/0 9 1991 Birkh~iuser Verlag, Basel Shear Localisation in Upper Mantle Peridotites MARTYN R. DRURY, 1 REINOUD L. M. VISSERS, 2 DIRK VAN DER WAL, 2 and EILARD H. HOOGERDUIJN STRATING 2 PAGeoph, 1991 442 M. R, Drury et al. PAGEOPH, my/onites Lanzo ~ ~ Sierra Alpujata Abstract--Upper mantle peridotite bodies at the earth's surface contain relict structures and microstructures which provide direct information on the role and the mechanisms of shear localisation in the upper mantle. Deformation which occurred at high temperatures (T > 950 _+ 50~ is relatively ~ 4km C homogeneous within domains ranging in scale from a few kilometres to a few tens of kilometres. Below 950 + 50~ strain is localised into centimetre to several hundred metre wide shear zones which commonly contain hydrated mylonitic peridotites. The microstructures developed in the peridotites suggest there is a correlation between the occurrence of shear localisation and the occurrence of strain softening and brittle deformation processes. The most important strain softening processes are inferred mylon/#es I 4 km to be structural and reaction induced softening. Structural softening processes include dynamic recrys- I tallisation and strain-induced transitions from dislocation creep to some form of grain-size-sensitive (GSS) creep. Reaction induced softening is related to the formation of fine grained polyphase reaction ~i Bouchera products which deform by GSS creep and the formation of weak sheet silicates such as phlogopite, chlorite, talc and antigorite. From experimental studies these softening processes and brittle deformation km , Z~ processes are inferred to occur mainly at temperatures less than about 910 _+ 160~ This temperature ~i ,~.'-~ Balmuccla Erro-Tobbio range is inferred to be a significant rheological transition in the upper mantle. Below 910 _+ 160~ deformation during orogenesis may be accommodated by an anastomosing network of hydrated mylonitic shear zones with a distinct, perhaps weak, rheology. At higher temperatures strain is accommodated in much wider deformation zones. On the scale of the lithosphere the degree of localisation may be different to that determined at the scale of the peridotite massif. An anastomosing network of hundred metre wide mylonitic shear zones forming 0.05-0.3 by volume fraction of the mantle lithosphere at T < 950~ could accommodate d ~ f 2kin inhomogeneous or homogeneous bulk deformation depending on the spatial distribution and ordering of the mylonite zones. The higher temperature deformation at deeper levels in the mantle could be Figure l markedly inhomogeneous being concentrated in shear zones with widths in the range of 2-20 kin, Schematic maps of structural domains and foliation trajectories in upper mantle peridotite massifs. alternatively these zones may widen significantly during deformation, resulting in a decrease in the Arrows show azimuth of stretching lineations. Three main types of structural domain can be recognised, degree of localisation with increasing bulk strain. coarse-granular domains, tectonite domains and mylonile domains. Tectonite domains form the largest fraction of most massifs. (a) Shamah massif, Oman, after CEUI-ENEER et al. (1988); (b) Lanzo, W. Alps, after BOVDIER (1978); (C) Sierra Alpujata, Spain, after TUBIA and CuEvAs (1987); (d) Beni Bouserra, Key words: Deformation, localisation, softening, mantle, peridotite, olivine. Morrocco, after REUBER et al. (1982); (e) Balmuccia, W. Alps, after BOUDIER et al. (1984); (f) Mount Tugello block of the Erro-Tobbio peridotite, Ligurian Alps, after VISSEI~S et al. (1991). Introduction are not always the "wall rock" of tectonite shear zones. In the case of the Ronda It is well established that shear localisation is an important process during massif (VAN DER WAL and VISSERS, 1991) the coarse-granular microstructure in the upper and middle crust (WroTE et al., 1980; BREWER et al., deformation overprints high-pressure tectonite and mylonite shear zones. This composite domain of high-pressure tectonites, mylonites and granular peridotites then forms the wall rock to a later tectonite shear zone developed at lower pressures (VAN DER WAL Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra, ACT 2601, Australia. and VISSERS, 1991). 2 Department of Geology, Institute of Earth Sciences, State University of Utrecht, PO Box 80.021, Mylonitic peridotites occur as sub-planar zones in massifs and xenoliths with 3508 TA Utrecht, The Netherlands. widths from 0.005 to 1000 metres (NICOLAS and POIRIER, 1976; BROD~E, 1980; REVBER et al., 1982; KRUHL and VOLL, 1978/79; BOUDIER et al., 1988; NORREL and HARPER, 1988; DRURY et al., 1990; HOOGERDUIJN STRATING et al., 1991). In kimberlite xenoliths mylonitic microstructures are developed in two distinct rock types (HARTE, 1983). "High temperature" xenoliths deformed at 1200-1600~ are homogeneously deformed, although, most of these xenoliths are only tens of centimetres in diameter. "Cold xenoliths" deformed at lower temperatures of 800-1100~ often contain very narrow shear zones (BoULIER and NICOLAS, t973;
(McKenzie, Jackson & Priestley, EPSL 2005) Continental geotherms, constrained by P-T estimates of xenolith equilibration )'%% )$%% /+61+7:/87+34; <34)4=034$%4=0 <4:/4>" ! ?"@" (from Equations of state and thermal conductivity) )&%% 1. Thermal structure and strength profiles )%%% % &% $% '% (% )%% )&% )$% )'% )(% &%% ' Density, pressure and temperature 17+--87+3491: $ & % % &% $% '% (% )%% )&% )$% )'% )(% &%% $ *+,-./0 !"# ! % &% $% '% (% )%% )&% )$% )'% )(% &%% *+1/23456
Strain localisation in the subcontinental mantle — a ductile alternative to the brittle mantle J. Precigout a, ⁎ , F. Gueydan a , D. Gapais a , C.J. Garrido b , A. Essaifi c Tectonophysics 445 (2007) 318 – 336
Return to this image:
1. Localization (grain size, temperature, other...) 1. Very basic (my level) introduction to thermodynamics of irreversible processes (non-equilibrium thermodynamics) (and a note on the wattmeter?) 2. Localization by grain size reduction: Observations of the Stack of Glencoul (Kohlstedt & Weathers, 1979), application and interpretation of the wattmeter and piezometer. 3. Localization by thermal feedback. Simple model, more complex models... 4. Localization by melt segregation: rocks, experiments, theory. 5. Interactions of feedbacks... (amplifying and damping situations)
1. Intro to non-equilibrium, irreversible thermodynamics dU = d p Q rev + d p W d p Q rev = TdS d p W = − PdV + ( σ d ε ) Heat System dU = TdS − PdV Work Surroundings U = internal energy Q = heat flow S = entropy W = work System d p = path-dependent, inexact di ff erential T, P = intensive state variables S, V, U, H, F, G = extensive state variables
The thermodynamic potentials (whose gradients are forces) Enthalpy: H = U + PV dH = TdS + V dP H = H ( S, P ) Helmholtz Free Energy: F = U − TS dF = − SdT − PdV F = F ( T, V ) Gibbs Free Energy: G = H − TS ( for mechanical dG = − SdT + V dP problems, use G, F ; because the entropy is G = G ( T, P ) subtracted ? )
Recommend
More recommend
