Joints & Shear Fractures

Remember: Three “directions” of stress Compression Extension Shear How are these stress conditions created? What are their effects? Focus on Extension Today

Modes of Fracture - Definitions

33 11 MODE I - TENSILE FRACTURE JOINT

Straight up tensile failure (direction of displacement is perpendicular to fracture plane, displacement is parallel to  3. Fracture on  1-  2 plane.) 33  =90 MODE I - TENSILE FRACTURE

Fracture plane (ideally) tangent to failure envelope. 2  = 180

33 11 MODE II - SHEAR FRACTURE

Shear fracture (all displacement parallel to fracture surface) Much more on this when we talk about faults! 33 Ideal  =60

MODE II - SHEAR FRACTURE  = 60 2  = 120

33 11 MODE I/II - TENSILE & SHEAR

Tensile failure with some shear component - motion components both parallel and perpendicular to fracture. 33  >60

MODE I/II - TENSILE & SHEAR Fracture plane (ideally) tangent to failure envelope.  > > 2  > 120

Focus on Joints - Mode I & I-II VEINS are just joints filled with something (either mineral cement, soft sediment, or even liquid hot magma). Veins are more common than empty joints in many environments!

Mud-filled joints in siltstone, Panther Beach (JCM)

Columnar joints, Devil’s Postpile Basalts (JCM)

Exfoliation Joints, Sierra Batholith, “onion peels”

Systematic & Asystematic - Purisima Fm. at Pt. Reyes

Joint spacing controlled by layer thickness

Mud cracks - two sets - two layers

MODE I - TENSILE FRACTURE Criteria for falling in realm of jointing:  3 < 0  1 is small (differential stress is therefore small, mean stress is also small.) Coulomb failure doesn’t apply here!

How do we drive  3 into negative values? Remember “negative”  3 means pressure acting outward from within a body of rock. 1.Unroofing - pressure “frozen” into a pluton or metamorphic rock is released when overburden eroded away 2.Cooling/drying compaction - i.e. columnar jointing in a basalt flow or mud cracks in a puddle 3.Fluid pressure - pore pressure pushes out from inside, drives all forces more negative…

Fluid Pressure is homogeneous - Cannot support directionality!

Tectonic stresses haven’t changed - but Effective Stress (  *) is reduced! 11  1*

You find a fracture in the field… How can you tell whether it’s a joint (mode I), a shear fracture (mode II) or has components of both?

1. Surface decoration Joints often have “plumose structure”

Anatomy of Joint Surface features

1b. Surface Decoration on shears Shear fractures often have linear striations - either grooves (slickenlines) or “antigrooves” where fibers have grown (slickenfibers) which record direction of shear motion on fracture face

Angles of intersection Joints may form along  1-  2 plane and along  1-  3 plane, therefore are often in perpendicular sets.

May be combo of systemic and non-systemic - which set came first?

Shear fractures ~60° to  3 33

Joints: Sandstone (lighter gray) was extended, joints filled with Qtz veins. 33

Natural Bridges Introduction Print your own for reference: es.ucsc.edu/~crowe/structure/natbridges.html