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Tectonic deformation and geologic structures Mountain building • Anatomy of a convergent mountain belt Blue ridge Valley & ridge Mountain building • Extensional horsts and grabens Horst Graben Continental rift; opening of an ocean basin? Deformation Deformation: 1) translation 2) rotation 3) distortion (shape change) Deformation results from stress, Stress = force per unit area Deformation - types of stress Compressive stress Tensional stress Lateral shear stress Deformation - tectonic Uniform pressure from all directions Compression in a principal direction Extension in a principal direction Lateral shear Rock deformation: When does it fold and when does it fault? depends on rock type, crustal depth, rate • Brittle: shallow crust; with increased stress (breaks) little change until failure • Ductile: deeper crust; with increased stress (squishes) gradual change Products of deformation Folds Faults Types of folds • Anticline: opens downward; oldest rocks in center • Syncline: opens upward; youngest rocks in center Deformation - folds Strike and dip measurements along the limbs of a fold Types of folds dome basin Plunging folds • fold appears to die out on ground surface http://www.fault-analysis-group.ucd.ie/papermodels/models/syncline_anticline_lowres.jpg Types of faults Thrust faults: Normal faults: • • • • • • Compression HW up shallow dip Extension HW down steep dip Types of faults Strike-slip faults: • • • lateral shear no vertical change vertical dip (map view) Thrust, strike-slip or normal faults? Interrelated Folds & faults 10 km • Cross-section through western Virginia • imbricate thrusts Gum Spring Charlottesville Rockfish Valley Blue Ridge Waynesboro Staunton fault North Mtn fault Virginia West Virginia • Southern Appalachians - fold and thrust belt Mountain building 10 km • Cross-section through eastern Virginia Richmond Basin = continental rift; related to opening of the Atlantic ocean Cape Charles Chesapeake Bay Richmond Basin Yorktown Richmond Gum Spring • Extensional horsts and grabens
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