Note summary for:

2-1-99

rose diagrams are essentially histograms of directional properties; for planar features, half of the diagram is redundant, and thus they are often split to show different patterns in an object

en echelon (step-like) arrangement of veins - important for at least two reasons:

1. can get principle stress directions

2. opening of these veins can accumulate shear, and thus act like a fault

stylolites or "anticracks" are solution features

under compression, SiO2 and esp. CaCO3 are dissolved, leaving behind darker minerals that appear as dark seams normal to veins; water is needed to form both veins and stylolites

fractures usually begin at a definite places of inhomogeneity, nucleating features

Why does joint spacing depend on lithology and thickness of bed?

Around a crack is formed a (low) stress-shadow , and it is unlikely for a new crack to develop in such a place; since these stress-shadows scale positively with size, cracks in thicker beds will have larger stress-shadows, accounting for the bed thickness-spacing relationship.

What other properties affect crack propagation?

• amount of strain (i.e., stress history)

• lithology, including mineral orientation and Young’s modulus

• interlayer thickness (esp. in sedimentary deposits)

Assuming a perfectly elastic material, a crack should be ellipsoidal; why is this not the case?

rocks are not perfectly elastic materials!

Why do cracks change orientation?

1. inhomogeneity of material

2. change in stress pattern (e.g., unloading, intrusion), which may be temporal or spacial

3. principle stresses must always be normal or parallel to a free surface, so a fracture near the Earth’s surface must be normal or parallel to it

4. when two fractures approximate, stress fields interact: if a joint is open, each side must form a free surface

2-3-99

3 modes of fracture

I. extensional, movement normal to fracture surface

II. sliding, motion parallel to end of crack (i.e., propagation direction)

III. tearing, motion normal to propagation direction

plumose structure along joints tells something of the direction of joint propagation

ripple structures along the surfaces are "arrest lines" and record hiatuses in fracturing

Footwall block always lies beneath the hanging wall block (except in high-angle faults where these may not be distinguished). Other key descriptors of fault movement may be combined to describe more complicated motions, such as "oblique, right-lateral normal fault". Rotational faults are also known as "scissor faults".

Faults may die out in a number of ways. Displacement may simply decrease to zero in some directions, described by termination lines, or a fault may terminate against another fault.

Oblique-slip faults combine the motion of strike-slip and dip-slip. The vertical and horizontal components of displacement are known as the throw and heave, respectively.

Most earthquakes along faults occur in the zone of brittle deformation. Cataclasites, fault-brecciated rock, are commonly fault in this zone, along with polished slickensides. Deeper down, below 10-15 km, is the zone of ductile deformation, where quartz especially begins to flow. Here mylonites, which result from the recrystallization of minerals, are common.

On faults, break-off points are also evident, often in association with slickensides and toothlike structures (a type of stylolite) that result from pressure solution perpendicular to displacement. Both may be used to guage direction of motion on a fault.