Historical Geology Notes

The Breakup of Pangea and Deformation in the Western Cordillera

 

Pangea Breakup

- rift basins formed parallel to the eventual lines of breakup; Triassic rift basins in eastern North America ran from present Carolinas to Nova Scotia and included the Newark Rift Basin

- Separation of Laurasia (future North America and Eurasia) from Gondwana at around 165 m.y. in the Middle Jurassic resulted in the opening of the central Atlantic between North America and Africa and opening of the Tethys Ocean by the separation of Africa and Europe

- The North Atlantic between North America and Europe didn't open until the Cenozoic

- Gondwana breakup began about 175 to 160 m.y. with the separation of Africa and South America as a unit from Antarctica and continued with the sequential separations of India, then Australia, and finally New Zealand from Antarctica between about 130 and 85 m.y.; The South Atlantic Ocean opened between Africa and South America about 120 m.y.

 

Western Cordillera

- had been a passive (rifted) margin since the Proterozoic

- by Devonian it became an active (subduction) margin (Devonian Antler Orogeny in Nevada)

- small orogenies (Sonomian and Nevadan) occurred during the Triassic through Cretaceous as the result of the accretion of exotic terranes

- Jurassic and Cretaceous granitoid (granodiorite) plutons emplaced in a magmatic (volcanic) arc that included the Sierra Nevada etc. near the western margin of North America as subduction continued; this was part of a more or less continuous subduction zone extending from Antarctica through South America and all of North America

- because the Atlantic began opening in the mid Jurassic North America was moving westward toward a midocean ridge in the Pacific; as North America neared the ridge it had to subduct progressively younger, hotter, more buoyant ocean crust; this resulted in the angle of subduction becoming shallower and perhaps finally flat, greatly increasing the friction and deformation and resulting in the following two events

- Late Cretaceous Sevier Orogeny - thin skinned thrusting of sheets of sedimentary strata and igneous intrusion to the east of the Jurassic/Cretaceous magmatic arc

- Latest Cretaceous -> Eocene Laramide Orogeny, deep-seated, basement-involved thrusts and folds far inland (to Colorado, Wyoming)

faults and folds

The Big Picture

The breakup of Pangea and Gondwana during the Jurassic and Cretaceous may have resulted from the release of an excessive amount of heat from the mantle; this drove rapid seafloor spreading, which drove rapid subduction that resulted in the Laramide Orogeny; rapid seafloor spreading increased worldwide sea level and flooded the continents;

rapid seafloor spreading also outgassed additional carbon dioxide (a greenhouse gas) from the mantle which warmed up the earth's climate; dinosaurs lived in Antarctica during the Cretaceous

 

Cenozoic Trends in the Western Cordillera

- subduction of Pacific/Farallon midocean ridge under western North America began around 30 million years ago resulted in the San Andreas Fault system; the San Andreas Fault is a long transform (strike-slip) plate boundary linking the portions of the spreading ridge that have not yet been subducted

faults and folds

- with the subduction of the midocean ridge Laramide compression gave way to Basin and Range extension; normal faults cut across older thrust faults throughout the western cordillera; the Basin and Range province is characterized by long narrow subsiding basins separated by long uplifting ridges; these are grabens and horsts (hanging wall blocks and footwall blocks) separated by normal faults