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