GEOL 101 1nd Edition Lecture 33 Outline of Last Lecture I Evidence of Plate Tectonics II Force Driving Plate Tectonics Outline of Current Lecture I Types of Stress II Folds III Faults IV Andean type Orogenesis V Continental Collision Current Lecture Deformation general term that refers to all the changes that happen as a result of stress and strain of a rock Most crustal deformation occurs along plate boundaries There are three types of stresses that a rock may be subjected to o Compressional Rocks are pushed and squeezed together Results in folded rock layers and thrust faulting Produces large quantities of metamorphic rocks Occurs at convergent plate boundaries o Extensional Rocks are stretched and pulled apart Results in normal faults Produces basin and range provinces Occurs at divergent plate boundaries o Shearing Rocks are displaced in a lateral motion Card deck analogy Results in strike slip faults These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Occur at transform plate boundaries Folds One product of deformation is folding Folds result from compressional forces that warp flat lying sedimentary strata into wavelike bends There are two basic types of folds Anticlines folded upwards like an arch o Based on the law of superposition the oldest rocks are in the middle of the fold Synclines folded downwards like a trough or a U shape o Based on the law of superposition the youngest rocks are in the middle of the fold Very often anticlines and synclines occur together in an undulating pattern Special kinds of folds some features are folded upwards or downwards on all sides rather than just two sides Domes o Circular or slightly elongated up warping of rocks o Oldest rocks are in the core o Ex Black Hills SD Basins o Circular or slightly elongated down warping of rocks o Youngest rocks are in the core o Ex Michigan Basin Faults Another product of deformation is faulting Faults fractures breaks in the rocks along which significant displacement has occurred Three types of faults Normal o Hanging wall moves down along the fault plane o Occur as a result of extension stretching of the crust o Prevalent at sea floor spreading centers and continental rift basins o May produce fault block mountains o Examples Basin and range province of the western US Grand Tebous WY Reverse thrust o Hanging wall moves along fault plane o Occur as a result of compression squeezing of the crust o Prevalent at subduction zones and continent continent collision o Reverse faults with dip angles of less than 45 are called thrust faults o Older rocks are pushed up and over younger rocks o Examples in the Himalayas Rocky Mts Glacier National Park Strike Slip o Occur as result of shearing o Displacement is horizontal lateral rather than vertical o Displacement may be to the right or to the left Right lateral strike slip Left handed strike slip o Example San Andreas Fault CA Telling right lateral vs left lateral If you were following some feature such as a road or a fence that was offset by a fault you can tell if it is right or left lateral by the way you would turn to follow the feature Right lateral shearing is to the right Make right hand turn Left lateral shearing is to the left Make left hand turn Anatomy of faults Fault plane surface along which sliding occurs Hanging wall the rock surface directly above the fault plane o The fault block you would hang on Footwall the rock surface directly below the fault plane o The fault block you would put your foot on Fault scarp line or cliff created by a fault dangle Mountain Building The geologic term for mountain building events is Orogenesis from the Greek oro mountain and genesis birth Most orogenic events occur along convergent plate boundary Compressive forces result in folding and faulting mostly thrust faulting Two types of mountain building may occur at convergent boundaries Continent continent collision Himalayas Andean type subduction Andean Type Orogenesis 1 Prior to the formation of a subduction zone The continental plate is said to have a passive margin No mountain building is occurring yet Thick sequences of sedimentary rocks are being deposited off of the coast Reg East coast of North American 2 During the formation of a subduction zone Compressional forces force the oceanic plate so it gets pushed down underneath the continental plate The oceanic plate is denser than the continental plate so it gets pushed down underneath the continental plate This initiates a subduction zone and decompressional deformation 3 After the initiation of a subduction zone The continental margin is said to be an active margin The subducting plate causes partial melting in the overriding plate initiating volcanism o Continental volcanic arcs EX Cascades Sierra Madres Andes The compressional forces squeeze the edge of the continent causing the rocks to be folded and faulted The sedimentary rocks that were deposited at the passive margin become faulted and folded This makes up the accretionary wedge Rocks in the accretionary wedge are highly deformed Post subduction zone The subduction zone may become inactive and the continental plate may be uplifted The core of the volcanic arc may become expanded as is it the accretionary wedge o Examples Sierra Nevadas and coastal ranges in California Continental Collision As subduction continues the oceanic plate gets smaller and smaller Eventually it may disappear entirely as two continents draw closer together When there is no more oceanic plate to subduct the two continents may collide The classic modern example is India colliding with Eurasia to form the Himalayas The Appalachian Mountains were also formed during continental collision as Africa collided with North American during the assembly of Pangea During subduction and collisional mountain building smaller blocks of crust called terranes may also collide with the continental margins These are not full continents but usually islands or bits of less dense oceanic crust This process is called accretion This has occurred extensively along the Pacific coast of North America as the farallon plate subducted o The Farallon Plate was an oceanic plate that was being subducted beneath North America about 56 Ma o The plate has almost entirely disappeared except for two or three small remnants The Juan de Puco and the Cocos plate are the very last remnants As the Farallon plate