Transcript Faults: Basics

Faults: Basics
Goal: To understand and use the basic
terminology for describing faults.
Basic Terminology
Hanging wall and footwall: Come from 18thcentury English coal mines
• Dip-slip faults: Slip up or down the dip.
– Normal fault: Hanging wall down — indicates
extension
– Reverse fault: Hanging wall up — indicates
shortening
Reverse
Normal
Strike-slip faults
• Slip parallel with earth’s surface
• Typically have subvertical dip
Sense of motion
• Dextral = right-lateral = righthanded
• Sinistral = left-lateral = lefthanded
Oblique-slip faults
• Strike-slip and dip-slip components
• Most faults are oblique-slip, but are often
dominantly strike-slip or dip-slip
Slip vs. Separation
• Slip: Total movement along fault surface.
– Vector lying in fault surface
– Direction of vector (slip-line) expressed as
trend and plunge or rake in fault plane
• Separation: Total apparent offset along
fault when viewed in 2-D (either map or
cross section).
Same separation, different slip
Dip-slip fault
Strike-slip fault
To determine slip, you need a piercing point
– Piercing point: Line that intersects fault
surface and is off-set by fault
– Match hanging-wall cutoff with footwall cutoff
Character of faults
a) Discrete, single plane
b) Zone of anastomosing, closely spaced faults
(fault zone)
c) Wide zone of penetrative, plastic deformation
A
B
C
Fault zone showing
separation
Near Sheep Creek, Utah
Fault Rocks
• Frictional/brittle fault rocks: Mechanical
disaggregation and “grinding”
• Plastic fault rocks: Plastic flow of minerals
at atomic scale
– grain-size reduction due to deformation-driven
dynamic recrystallization
Watch deformation movies
Frictional/brittle fault rocks
Fault gouge: Clay-sized particles
Fault breccia: Angular chunks surrounded by
gouge and/or vein material
Cataclasite: Indurated version of fault gouge
Pseudotachylyte: Glass formed from frictionally
generated melt
Breccia/gouge zone
Plastic fault rocks
Protomylonite: Up to 10% dynamically
recrystallized material
Mylonite: 10–90% dynamically recrystallized
material
Ultramylonite: 90–100% dynamically
recrystallized material
1
3
2
Brittle-Plastic transition
Recognizing faults
• Truncation of rock units
• Visible off-set of rock units
• Omitted or repeated stratigraphy or
biostratigraphy
• Juxtaposition of seemingly unrelated rock
units
Visible off-set and damage zone