Earth History GEOL 2110
The Mesozoic EraGeologic and Tectonic History of the
North American Cordilleran
Major Concepts• The geologic history of Cordilleran Orogen of North
America during the Mesozoic Era was dominated compressional tectonics that accreted exotic terranes in the early Mesozoic (Triassic and early Jurassic periods) and then became Andean-type volcanic arc in the mid- to late Mesozoic (Jurassic and Cretaceous periods).
• A flattening out of subduction in the Cretaceous is thought to squelch volcanism in the central part of the western US and bring about a basin and range-type tectonism called the Laramide Orogeny
• A major transgression in the Cretaceous created a Great Interior Seaway where great accumulation of marine and non-marine sediment were deposited in the Great Plains
Events of the Mesozoic Era
Accumulation of Thick Cambrian Sediments on the Rifted Western Margin of Laurentia
Belt Supergroup sediments preserved in rift grabens (aulocogens)
Late Devonian-Mississippian Antler OrogenyAccretion of a Volcanic Island Arc
Sediments Shed off the Antler Orogen
The Ancestral Rocky MountainsPennsylvanian-Permian Cratonic Warping
Penn-Perm Ouchita Orogen Extension of the Appalacian Orogen
Tectonic Grain of Vergence shifts in
the Mesozoic
Mesozoic Vergence
Paleozoic Vergence
Triassic Accretion of Suspect Terranes Collage Tectonics
Six Major Suspect Terranes2 mostly Paleozoic3 Late Paleozoic- Early Mesozoic1 mostly Mesozoic
Wrangellia – oceanic ridge system
Sonoma/Stikinia – volcanic arcs
Evidence for Suspect TerranesPaleomagnetic Latitudes Reconstructions and Tethyan Fossil
Assemblages
Many terranes contain fossils that suggest a source from the Tethys Sea
Timing of Suspect Terranes Vergence
Cont, TerA and TerB all have unique paleopole orienations Cover Sed I contains pebbles from Cont and Ter B onlyCover Sed II contains pebbles from all terranes and late intrusionsAge of granites sets upper age of vergence of TerA and TerB
Late Triassic Transition to Andean-type Margin
Antler Orogeny
Sonoman Orogeny
Navajo Sandstone
Sierran Orogeny
Accretion of Suspect/Exotic Terranes and Volcanic Arcs
Continental Volcanic Arc
Forearc EnvironmentAccretionary Wedge - material scraped
off descending slab Melange – complex mix of rock types –
ophiolites and deep water sedimentsForearc basin – filled with immature
graywacke turbidites
Forearc Rock Types
Great Valley Greywackes
Blueschist (Hi-P metamorphic rx)Franciscan Melange
Pillowed Basalts
Magmatic Arc EnvironmentGranite Batholiths – Feeders to Stratovolcanoes
Mt Jefferson
Foreland Basin Environment
Clastic Wedge Sedimentation
Alluvial Fan Conglomerates= Molasse
Deep water muds= Flysche
Sevier Orogeny Intense Compression in the Late Cretaceous (~80 Ma)
results in Crustal Thickening by Overthrusting
Devonian
Triassic Sentinal Mtns, British Columbia
Thrust
Fault
Laramide Orogeny Uplifted Blocks of Precambrian Crust
Latest Cretaceous – Eocene (65-35 Ma)
Laramide Orogeny Magmatic Null Zone – Shallow Slab Subduction
Magmatic Nulls in the Andean Continental
ArcGaps in volcanic activity• shallow subduction• overthickened slab
Winter (2011)
Next Lecture
Cretaceous Trangression And Mesozoic Life