Lab #3: Strat Columns: Draw to scale Lab #3: Strat Columns: Draw to scale Includes: (left to right) Age (era and period) Name of unit Thickness of unit Skip the color column Column indicating rock type (by pattern) and relative grain size (by column width) All unconformities (wiggly line that crosses name, unit thickness, and rock-type column)

This week: Lectures: finish rheology viscous deformation; stress and strain rate effect of pore fluid on strength of rock Brittle deformation types of brittle deformation tensile cracking, Griffith cracks Crack Modes faulting Cataclasis Shear fractures This week: Lectures: finish rheology viscous deformation; stress and strain rate effect of pore fluid on strength of rock Brittle deformation types of brittle deformation tensile cracking, Griffith cracks Crack Modes faulting Cataclasis Shear fractures

Real rock elasticity

Nature rocks and deformation Deformation experiments

Nature rocks and deformation Role of strain rate and rock strength Increasing strain rates causes increased rock strength- The faster you push on it, the stronger it gets At 400° C, differential stress is 20 mpa at /s At 400° C, at /s, differential stress is 160 mpa Deformation experiments: plastic deformation Plastic deformation: stress is a function of strain rate in a solid Viscous deformation: stress is a function of strain rate in a fluid Plastic deformation: stress is a function of strain rate in a solid Viscous deformation: stress is a function of strain rate in a fluid Viscosity for real video!

Nature rocks and deformation Deformation experiments Pore Pressure Pore-fluid pressure Acts in all directions Increase of pore-fluid pressure = drop in rock strength Rocks are weaker with high pore-fluid pressure Vary pore- pressure Remember effect of confining pressure- Effective pressure equals confining pressure – pore-fluid pressure P e = P c - P f

Nature rocks and deformation Deformation experiments Pore-fluid pressure Effective pressure is less than confining pressure. Effective pressure equals confining pressure – pore-fluid pressure P e = P c - P f

Some elastic deformation then… Eventually the sample starts to deform non-elastically, by small deformations in the crystal lattice (plastic deformation). Its elastic behavior is surpassed, and non-recoverable deformation begins to accumulate in the rock. After even more strain, it may rupture The point of departure from elastic behavior is called the elastic limit. Its value is known as yield strength. Below its yield strength the rock behaves as an elastic solid. How do we relate Rock experiments To real-world observations, like brittle and ductile How do we relate Rock experiments To real-world observations, like brittle and ductile

Brittle deformation: ruptures soon after yield strength is reached Ductile deformation: strain is distributed, appears to flow like a viscous fluid

Rheologic stratification in the lithosphere Brittle-ductile transition Strength: stress that a material can support before failure Competency: Resistance of rocks to flow. Interplay of lithospheric strength, rock composition, and depth (temperature) Deformation in the lithosphere

Brittle Deformation Terminology (see table 6.1) Brittle deformation: permanent change in material due to growth of fractures, or sliding on fractures. Joint: fracture without measurable shear displacement (cracks or tensile fracture) Shear fracture: fracture with small shear displacement Fault: fracture with measurable displacement

What is brittle deformation? Atomic structure of materials Elastic strain: recoverable Brittle deformation: non-recoverable Breaking bonds releases elastic strain accumulation Rocks cannot accumulate large elastic strains.

Brittle deformation Four categories of brittle deformation processes 1. Tensile cracking 2. Shear fracture 3. Frictional sliding 4. Cataclastic flow Note: tensile stress is not tension

Table 6.2 Cataclastic flow: macroscopic ductile flow as a result of grain-scale fracturing and frictional sliding over a band of finite width Frictional sliding: Sliding on a pre-existing fracture surface with significant plastic deformation Shear rupture: Initiation of a macroscopic fracture at an acute angle to  1 – growth and linkage of microcracks Tensile cracking: Propagation of cracks into previously unfractured rock, when the material is subjected to tensile stress. Tensile cracks typically form parallel to  1 and perpendicular to the least principal stress.

Brittle deformation Tensile cracking Break chemical bonds across a crack surface Theoretical strength of a rock is greater than actual values measured Strength paradox! Griffith Crack Theory: The tips of the cracks are “stress risers”. Stress is magnified here, much greater than  d, The longer the crack, the more the stress is magnified