Mountain building and crustal deformation PDF

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Mountain building and crustal deformation...

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Tectonics mountain building and crustal deformation What is a mountain? • Geologically speaking = where the crust is thicker…. • Crust made thicker and shorter = deformation Deformation = structures Mountains • Mountain Building => orogeny or orogenesis • Isostacy - crust floating in gravitational balance Orogeny = uplift Crustal Loading => subsidence Rock Deformation and Geologic Structures • Deformation=>rocks subjected to stresses that exceed their strength...... • controlling factors ->1) Pressure and Temperature & 2) Time/Rate Brittle versus Ductile Deformation Geologic Structures • types of deformed rock • 1) Faults, 2) Folds, and 3) Joints • Faults => fractures along which there has been movement..... • parts => fault plane, upthrown block, downthrown block, hanging wall and footwall Faults Dip-slip = vertical movement a. Normal Fault => hanging wall moves down relative to the footwall b. Reverse Fault =>hanging wall moves up relative to the footwall

low-angle reverse fault = thrust fault Faults Strike-slip = horizontal movement Transform Fault => the result of shear forces Folds • Folds => rocks bend due to compression • parts = axis and limbs • Anticline => convex upward fold • => oldest beds towards the middle • Syncline => concave upward fold • => youngest beds towards middle Nonplunging Folds Plunging folds..... Basins and Domes What are these folds? • Joints => fractures in the crust with no movement Types of Mountains 1) Fault Block Mountains 2) Upwarped Mountains Faulblock and Upwarped mountains Mountains at Convergent Boundaries Subduction=> magmatic/Andean Arc The Andes 1) 200 my ago-> South America and Africa part of the same plate: Western edge of South America = passive margin

2) 140 my ago-> South America and Africa begin to break apart -> South America moves west....

The Andes 3) Oceanic crust fractures to accommodate movement => subduction..... 4) partial melting of subducted crust => Andean Arc - forearc basin, - accretionary wedge, and - backarc basin 5) Subduction Angle Decreases -> volcanic activity moves westward => Andean Arc -> old volcanoes eroded

Continental Collision the Himalayas...... the Himalayas 1) 120 my ago -> plate with India breaks away from Antarctica => moves north at ~20 cm/yr the Himalayas 2) Leading oceanic part of the plate subducted along southern Tibet -> partial melting => Andean Arc the Himalayas 3) 50 my ago -> oceanic crust entirely subducted => continental collision -> igneous activity stops... => thrust faulting and folding => crustal shortening and thickening.… => suture zone -> plates welded together Thrust Faults The Himalayas * Results in mountain chain of highly deformed sedimentary rocks plus associated igneous and metamorphic rocks...

=> compression still active = continued uplift Appalachians • formed the same way as the Himalayas, but compression not active for over 250 my = erosion........ Appalachians Step 1 = subduction (Taconic Orogeny)

Appalachians Step 2 = continental collision (Acadian and Alleganian Orogenies) => erosion begins to outpace uplift ........ Especially when uplift stops Influence of Orogeny on the interior of North America • Compression - subtle features • Basins & Domes...