Transcript File
Interpreting Ancient Tectonic Settings Historical Geology 2014 Introduction • Ancient tectonic settings – cratons and mobile belts • Craton – the stable, tectonically inactive portion of a continental interior • Mobile belts – the tectonically active portions of the continents Craton Ancient Mobile Belts • Very ancient tectonically active regions: – Mountains eroded – Valleys filled with sediment – Ocean basins subducted • Or…. Accreted to continents Mobile Belts / Pangaea •Mobile belts – the tectonically active portions of the continents Appalachian Chain Mobile Belts Philippines Mobile Belts Mobile belts via convection currents Mobile Belts Craton • Part of craton where Precambrian rock is exposed – the shield • Part of craton where Precambrian rock is covered with sediment – the platform • Sedimentary rocks of the platform – the cratonic sequences Part of craton where Precambrian rock is exposed – the shield Geologic Regions Shields Canadian Shield Ontario Canadian Shield North American Shield Wisconsin Shield Ancient Continent-continent divergent • Rifting • Creates grabens (valleys) bounded by normal faults • Results in 3 “Arms” or rift valleys – Two usually open to ocean basins – One stays a continental rift (failed rift) • A failed rift is called an aulacogen – May result in large river basins Triple Junction Aulacogen Continental rifts are commonly initiated by hot spots. The continental crust begins to extend, creating a rift valley (graben) bounded by normal faults. Typically, continental rifts will have three “arms” or three rift valleys that meet at one point (a triple junction), forming a shape like a “Y”. Aulacogen Oklahoma Aulacogen Ouachita Aulacogen Ancient ocean-ocean divergent • Rift valley may sink below sea level • Results in surge in volcanism • Creating basaltic oceanic crust between two fragments of continental crust • Fragments of oceanic crust now on land are ophiolites Ophiolite Formation Ophiolite Formation Ophiolite Formation Ophiolite Ophiolite Ancient Subduction Zones • Occurs only at ocean-ocean convergent and ocean-continent convergent • Ocean crust recycles back into the mantle • Along the trench sediments are dragged down and metamorphosed • Resulting rock is high-temp blue schist • Chaotic mixture of ig and sed rocks from both crusts is a melange Melange Melange Ancient Subduction Zones • Eventually, a microcontinent (or another large continent) will be rafted along the subduction zone • Continent-continent collision will occur • Pieces of one continent accreted to another continent are called accreted terranes Accreted terrane Accreted Terrane Accreted terranes will lead to an increase in continent size. Much of North America has formed from continental accretion. Evidence for ancient subduction zone Ancient Subduction Zones • Oceanic plate subducts under another plate • May be extension behind volcanic arc • This forms a back arc basin – Modern example: Sea of Japan • Subduction continues = back arc basin closes • Compression results in formation of fold and thrust mountains inland from volcanic arc Back Arc Basin Ancient Continent-Continent Collision • When two continents collide – subduction ceases • As continents collide - crust will be folded and thrusted from compressional stress • Large complex mountain belts develop • Geologists use adjacent sediment (clastic wedge) to estimate elevation of ancient mountain belts Clastic Wedge Oil Fields Clastic Wedge Wedge Wilson Cycle • The Wilson cycle is the cycle of opening and closing ocean basins. • Initial stages = a hotspot underneath a continent causes rifting – Example of this stage of cycle – East Africa Rift or Rio Grande Rift in New Mexico • If rifting continues, a new ocean basin and midocean ridge will form – Example - Red Sea and Gulf of California • By the mature stage of rifting, a wide ocean basin will form – Example – Atlantic Ocean Evidence of the Wilson Cycle in Canada’s Northwest Territories – Cross section on bottom, depicts a zone of intrusive igneous rocks that pass eastward into metamorphic rocks and then into folded and thrust-faulted stratified rock. Relationship of rock units following the Paleoproterozoic opening of an ocean basin along the western margin of the Slave province. Assignment • Lab # 6 - Interpreting Ancient Tectonic Settings • Information – page 79 – 83 • Activity – page 85 - 90