Microscopic aspects of rock deformation (Part I) ESCI 302

Deformation mechanisms Operative mechanisms depend on: Temperature (T) Lithostatic pressure (plitho) Fluid pressure (pf) Differential stress (σd = σ1- σ3) Strain rate ( ) Porosity (Φ) Grain size (d) Chemical conditions Rock type

Deformation mechanisms Tell us about bulk rheology (e.g., power-law creep) Result in the grain-scale fabric & microstructures of metamorphic rocks (e.g., crystallographic preffered orientations) Tell us about physical conditions during deformation (e.g. T, plitho, pf etc.)

Deformation mechanisms Upper crust: Brittle deformation dominant (also elastic deformation) Mid to lower crust: Elastic deformation, ± brittle failure (minor), Dislocation creep (“crystal plasticity”) Other diffusion controlled “ductile” mechanisms

Two classes of def. mechs. Elastic-recoverable (ε < 2-3%) Recoverable Not permanent Instantaneous No breaking of bonds Non-recoverable (ε > 2-3%) Permanent Time-dependant, non-instantaneous (rock strength related to ) Bonds broken

DM: early to pre-diagenesis Intergranular flow (particle flow) rolling and sliding of rigid particles past each other in unlithified sediments low confining pressures and/or high fluid pressures no microstructures

DM: fault zones near brittle-ductile transition Cataclastic flow: high effective pressures (e.g., depths >6 km) involves continuous brittle fracturing of grains in a rock produces progressive decrease in grain size different from granular flow in that small-scale fracturing plays an integral role little/no imprint on microfabric (no actual deformation of mineral grains except fracturing)

cataclasite Alpine Fault

Example: compaction due to sediment burial (increases load) involves partial removal of fluid phase from porous solid +/- removal of void space in response to sediment load process yields porosity decrease with burial depth (non-linear) May involve significant inter-granular flow and/or cataclastic flow Little microfabric evidence (except fissilty and pencil structures in shale)

Example: compaction Porosity: fraction of void space in rocks void solid

Example: compaction Fluid pressure Effective pressure works against lithostatic pressure reduces effective normal stress (impedes compaction) Effective pressure plitho plitho pf plitho plitho

Compaction in wet rocks Terzaghi’s spring analogue Karl von Terzaghi (1883-1963) “Father of soil mechanics” 1 2a 2b 3 underconsolidated consolidated time

Compaction in wet rocks Terzaghi’s spring analogue fluid supports extra weight extra fluid pressure fluid at rest A 1 2a 2b 3 underconsolidated consolidated time

Compaction in wet rocks Terzaghi’s spring analogue Weight transferred from fluid to load-bearing grains. Extra weight supported by compacted grains 1 2a 2b 3 underconsolidated consolidated time

Compaction in dry rocks Terzaghi’s spring analogue 1 2

Compaction – reality check actual rock behaviour is not spring-like, i.e. not elastic Real compaction leads to permanent deformation (intergranular flow, fracturing etc) time scale of compaction: 10s – 100s m.y. pore closure by deformation and / or cementation of sediments yields “plugged system.” This causes retained high fluid pressures (Pf> Phydro) at depths of several km (i.e., pressurized fluid cannot escape).

Fluid pressure gradients Lithostatic vs. hydrostatic Pf Fluid pressure transition zone between two end-members State of undercompaction / under-consolidation (problem in petroleum industry) Fluid Pressure ratio l = Pf/Plitho CD/Ch 8/depth curves/hydro-litho

Sources of fluids Compaction-related pore fluids: connate brines Water incorporated into a sediment during sub-aqueous deposition Fluids released from minerals by dehydration during metamorphism Adsorbed water (e.g. low grade smectite clays)

DM: diffusion and dislocation creep

Steady-state creep Remember me? activation pre-exponential energy factor activation energy stress exponent strain rate differential stress universal gas constant = 8.314 J K-1 mol-1 temperature

Deformation mechanism maps conditions of stress and temperature for some flow mechanisms can be plotted on a deformation mechanism map deformation mechanism maps… … are lab-derived … are often monomineralic (e.g., “pure qtzite”) … are either “dry” or “wet” … are derived for 1 grain size

Deformation mechanism maps. show fastest operative mechanism Deformation mechanism maps show fastest operative mechanism contours of strain-rate Flow-stress Flow-stress (normalized) Flow-stress (normalized) wet quartzite d = 100 µm calcite d = 100 µm Crystal plasticity Crystal plasticity Diffusion creep Diffusion creep Passchier & Trouw, 2005

Deformation mechanism maps Problems: only approximate, lab-derived incorrect in detail (e.g. evidence from natural rocks suggests different def. mech. at certain stress or temperature level) not all known mechanisms shown- only plot the fastest one despite others possibly operating together not all conditions that natural rocks encounter are accounted for (e.g. hydrolytic weakening) may not indicate effect of changing grain size best to do: consider maps as a qualitative illustration of the relative importance of the different deformation mechanisms

DIY: deformation mechanism maps Passchier & Trouw 2005