part 8: fold types
DESCRIPTION
Part 8: Fold Types. Compressive Stress. Shear Stress. Tensional Stress. Orientation of stress leads to different folds. Ductile Strain. Earthquakes. Stanford, 1906. Compressive Stress. Tensional Stress. Shear Stress. Earthquakes occur on faults. Active Fault. Faults. - PowerPoint PPT PresentationTRANSCRIPT
Part 8: Fold Types
Tensional Stress
Compressive Stress
Shear Stress
Orientation of stress leads to different folds
Ductile Strain
Earthquakes
Stanford, 1906
Tensional Stress
Compressive Stress
Shear Stress
Earthquakes occur on faults
Active Fault
Faults
Faults are fractures with movement
Faults
Faults are fractures with movement
STRESS BUILDS UNTIL ITEXCEEDS ROCK STRENGTH
Local rock strength
Stress
Earthquakes
Time
STRESS BUILDS UNTIL ITEXCEEDS ROCK STRENGTH
Local rock strength
Stress
Earthquakes
Time
Earthquakes arethe result ofstress that buildsup over time.
STRESS BUILDS UNTIL ITEXCEEDS ROCK STRENGTH
Local rock strength
Stress
Earthquakes
Time
Earthquakes arethe result ofstress that buildsup over time.
Stress gradually builds as tectonic forces deform rocks.
STRESS BUILDS UNTIL ITEXCEEDS ROCK STRENGTH
Local rock strength
Stress
Earthquakes
Time
Earthquakes arethe result ofstress that buildsup over time.
Stress gradually builds as tectonic forces deform rocks.
When the stress exceeds the strength of the rocks…
STRESS BUILDS UNTIL ITEXCEEDS ROCK STRENGTH
Local rock strength
Stress
Earthquakes
Time
Earthquakes arethe result ofstress that buildsup over time.
Stress gradually builds as tectonic forces deform rocks.
When the stress exceeds the strength of the rocks…
…the faultslips, causingan earthquake.
STRESS BUILDS UNTIL ITEXCEEDS ROCK STRENGTH
Local rock strength
Stress
Earthquakes
Time
Earthquakes arethe result ofstress that buildsup over time.
Stress gradually builds as tectonic forces deform rocks.
When the stress exceeds the strength of the rocks…
…the faultslips, causingan earthquake.
The process repeats againand again.
Focus0 SecondsRupture expands circularly on fault plane, sending out seismic waves in all directions.
5 SecondsRupture continues to expand as a crack along the fault plane. Rocks at the surface begin to rebound from their deformed state.
10 SecondsThe rupture front progresses down the fault plane, reducing the stress.
20 SecondsRupture has progressed alongthe entire length of the fault.The earthquake stops.
Fault cracksat surface
Fault crackextends
World seismicity from 1976 to 2002
EUROPE
AFRICA
ASIA
INDIAN OCEAN
AUSTRALIA
SOUTHAMERICA
NORTHAMERICA
ANTARCTIC OCEAN
PACIFIC OCEAN ATLANTICOCEAN
≤ 50 km deep (shallow focus)
50–300 km deep
≥ 300 km deep (shallow focus)
Plate Boundaries and Faults• Strike-slip faults along transform boundaries• Reverse faults along convergent boundaries• Normal faults along divergent boundaries
Strike-Slip Faults: Transform Boundary
• San Andreas Fault– Several magnitude 7
earthquakes recently
– usually occur in clusters
– arrows show motion/yr
Faults not always parallel to boundary
Results in uplift of mountains
Denali fault, AlaskaMt. McKinley
Offset streams along the San Andreas Fault in the Carrizo Plain, CA
Southern California fault traces
San Andreas fault San Gabriel Mountains
North AmericanPlate
PacificPlate
Los AngelesMotion of Pacific Platerelative to motion of North American Plate
Southern California fault traces
San Andreas fault San Gabriel Mountains
North AmericanPlate
PacificPlate
Los AngelesMotion of Pacific Platerelative to motion of North American Plate
Here, the San Andreas fault is parallel to plate motion, and the faulting is right-lateral strike slip.
Southern California fault traces
San Andreas fault San Gabriel Mountains
North AmericanPlate
PacificPlate
Los AngelesMotion of Pacific Platerelative to motion of North American Plate
The “Big Bend” causes the Pacific Plate to compress against the North American Plate, causing thrust faulting.
Here, the San Andreas fault is parallel to plate motion, and the faulting is right-lateral strike slip.
Southern California earthquakes(July 1970-June 1995)
Northridge 1994Magnitude 6.9
San Fernando 1971Magnitude 6.7
Landers 1992Magnitude 7.3
July 1970–June 1995
Key:5+<5
Subduction = biggest quakes
– 1960 Chile, magnitude 9.5– 2004 Sumatra, magnitude 9.2– 1964 Alaska, magnitude 9.2– 1868 Peru, magnitude 9.0– 2001 Peru, magnitude 8.4– 2007 Sumatra, magnitude 8.4– 2007 Peru, magnitude 8.0
Lithosphere
Asthenosphere
Deep-ocean trench (convergence)
Large shallow earthquakes occur mainly on thrust faults.
Further from trenchdeeper quakes
Long Recurrence IntervalLarge earthquakes occur in NW U.S. every few hundred years
Lithosphere
Asthenosphere
Transform fault(lateral shearing)
Rift valley(divergence)
Normal faultingMid-ocean ridge (divergence)
Shallow earthquakes coincide with normal faulting at divergent boundaries and with strike-slip faulting at transform boundaries.
Rifting Also Occurs on Land
• Basin and Range of Nevada• Rio Grande Rift
Basin and Range
Province
New Mountains-Active Fault –Basin and Range Province
Sandbox- Block rotation
Sand above plasticine
Normal Fault Zone
Active Fault