Geologic History of the Appalachians Dr Stephen Crabtree

Geologic History of the Appalachians Dr. Stephen Crabtree November 11, 2019

Today’s Topic: The Appalachians Complicated History Appalachians are not from just a single mountainbuilding event

Appalachians Basics – aka “Eastern Cordillera” – Mix of sedimentary and igneous – Strongly folded/faulted – Many smaller parts to form the whole – Very Old, – Heavily eroded – Minimal tectonic activity today – Key Characteristics: OLD, NOW STABLE MOUNTAIN RANGE

Appalachians Basics – Many different “subprovinces” within the Appalachians • • Alleghenies Blue Ridge Mtns. Labrador Canadian maritimes Valley-and-Ridge Etc… Not Adirondacks

Smoky Mountains, NC/TN

Allegheny Mountains, PA

Bay of Fundy, NB/NS Canada

Produced by a series of collision and rifting events Must understand the idea of “Wilson Cycles”… Long-term Plate Tectonic cycles

Wilson Cycles

Wilson Cycles 1 1. Continent thins and begins rifting 2 2. Rifting continues, forming an ocean basin

Wilson Cycles 3. Convergence begins; an oceanic plate subducts, creating a volcanic chain at an active margin 2 3

Wilson Cycles 4. Terrane accretion from the sedimentary wedge welds material to the continent 3 4

Wilson Cycles 5 5. As two continents collide, orogeny thickens the crust and builds mountains 4

Wilson Cycles 6 5 6. The continent starts to spread and thin under its own weight, and erodes

Wilson Cycles 1 6 6. The continent spreads, thins, and erodes 6 1. The thinned continent rifts and begins to break up

Wilson Cycles: Key Points • Long, long cycles of increasing and decreasing “continentality” • Going from supercontinents to widely spaced continents, and back • Dramatic effect on global currents • Currently a period of Low continentality in the Southern Hemisphere; • Currently a period of High continentality in the Northern Hemisphere

Focusing on the Appalachians… A whole series of cycles of collision, divergence, and collision again Emphasize Orogenies: Mountainbuilding Events

Appalachian Mountain Building Collision phase of first Wilson Cycle: Grenville Orogeny Mid-to-Late Proterozoic age ~1 billion years ago Ancient blocks of Europe and Africa crashing into the east coast of North America

Appalachian Mountain Building Divergence phase of first Wilson Cycle: Extension & Spreading Late Precambrian thru Ordovician age ~1 billion to ~450 million years ago Breaking apart a supercontinent: Rodinia – Make new “Tethys” ocean basins and continental shelves Subduction to make new volcanic arcs

Appalachian Mountain Building Convergence phase of second Wilson Cycle: Taconic Orogeny Ordovician-Silurian age ~450 -420 million years ago Subduction of spreading ridge Collision of North America with smaller Piedmont, and Carolinia volcanic arcs and microcontinents

Appalachian Mountain Building Convergence phase of second Wilson Cycle: Acadian Orogeny Devonian-Mississippian age ~420 -320 million years ago Brief period of stability, followed by more collisions Close Tethys ocean basin Also sediment making the Appalachian Plateau Collision of North America with Avalon microcontinent – Build mountains of northern Appalachia

Appalachian Mountain Building Convergence phase of second Wilson Cycle: Alleghanian Orogeny Pennsylvanian-Permian age ~320 -250 million years ago Collision (again) of North America with Africa and Europe – Build mountains of central Appalachia – Forming supercontinent of Pangea – Also built Caledonide Mtns. in Europe, cited in Wegener’s theory

Appalachian Mountain Building Divergence phase of second Wilson Cycle: Mesozoic Rifting Triassic-Cretaceous age ~250 -65 million years ago Splitting (again) of North America from Africa – Really splitting up all of Pangea – Atlantic opened like a zipper from North to South – Lots of extensional faulting; left behind “Valley and Ridge” – Part of Africa left behind as chunks of modern NC/VA

Appalachian Mountain Building Modern Day Deposition on the Continental Shelf Lots of sediment just flowing Eastward (and Westward) off of the modern-day Appalachians Erosion of the Appalachians has turned them into much lower-relief mountains compared to earlier heights Uplift continues due to isostacy. It’s why the Appalachians haven’t fully eroded away – Both Grenville and Alleghanian orogenies made mountains taller than the Rockies

Remember Stress and Strain • Collision forces not equal along length of boundary • Some areas severely bent inwards, others compensate by bending outwards • Normal and Reverse faulting VERY common in mountain-building events

Appalachian Topography

Lots of Structurally Strained Areas

Lots of Structurally Strained Areas

…still simplified as basic “units” Important to note: These units of the Appalachians were not formed in a simple West East sequence Oldest shown above is the Blue Ridge (part of Precambrian Grenville Orogeny) Youngest is the Valley & Ridge (formed by Mesozoic Rifting)

…and that late rifting generated some igneous/volcanic material N A I H C A L AU A P AP LATE P VA Y LLE E G ID A R D N T E N G O ID M R A D T E E I S P A BLU CO L N I A PL Igneous Intrusions

So what are the key points? • The Appalachians are not one singular mountain range • The Appalachians were formed by a series of (at least) four collision events between the Precambrian and the end of the Paleozoic – Grenville, Taconic, Acadian, Alleghanian orogenies • Some orogenies were separated from eachother by rifting events – opening of ocean basins

Next Time… The Rocky Mountains