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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