Presentation on theme: "Faults: Basics Goal: To understand and use the basic terminology for describing faults."— Presentation transcript:
Faults: Basics Goal: To understand and use the basic terminology for describing faults.
Basic Terminology Hanging wall and footwall: Come from 18th- century 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 = right- handed Sinistral = left-lateral = left- handed
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 faultStrike-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 ABC
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
Plastic fault rocks Protomylonite: Up to 10% dynamically recrystallized material Mylonite: 10–90% dynamically recrystallized material Ultramylonite: 90–100% dynamically recrystallized material
1 2 3
Recognizing faults Truncation of rock units Visible off-set of rock units Omitted or repeated stratigraphy or biostratigraphy Juxtaposition of seemingly unrelated rock units