*Ancient Landscapes of Western North America: A Geologic History with Paleogeographic Maps, Ronald C. Blakely and Wayne D. Hanney. 2018.

February 2026

Contents

About the Book

This book is about the tectonic assembly, and I suspect the geomorphology, of what is now the Western United States. In particular, it will use paleogeographic maps – in which the first author is an expert – to describe the sequence of processes. Besides great maps, the book contains a lot of lovely pictures of geologic landscapes.

This book will have some overlap with the book on Canadian geology we read recently – Four Billion Years and Counting – and also with John McPhee’s Assembling California, which we read a couple of years ago. We being me and CJS.

The Book

C1. Introduction.

Reviews the aim and scope of the book.
Covers a few fundamentals like the assembly of the North American Craton, and some of its subsequent development especially outside of the regions that will be the focus of this book.
The book is focused on the Cordillera, basic the region of North American including the western mountain belt, and areas to its west.

Other interesting bits

There’s a nice map of the North American Craton on page 4. It shows three Archean cratons collided about 2Ga, and are stitched together by accretionary orogens to form the earliest version of Laurentia. The next billion years saw the accretion of various terranes to the south and east of Laurentia.
Parts of 2 of the 3 Archean cratons (and a little bit of extra crust) forms the Canadian Shield — I’m not sure what differentiates the Canadian Shield from the stitched together cratons — maybe it is the area of that region that is uncovered?.
When terranes accrete to a continent the often create compressive pressure that initiate an orogeny.
Thrust/Reverse faults occur when one block of crust is pushed up over another in response to compressional forces. It is called a thrust fault if the angle is < 45°, and reverse fault if ≥ to 45°.
Detachment/Normal faults occur when on block of crust moves down relative to the other because of extensional forces, with detachment being at a < 45°, and normal if ≥ to 45°. A detachment fault that flattens with depth is called a listric fault
Autochthon/Allochthon: The fault block that is thrust up over/covered up by the other fault block.
The Colorado Plateau has remained fixed on the North American craton except for a slight rotation during the Cenozoic. (Is it considered part of the craton or is it ‘on top’ of it — how do you tell the difference?)
The Colorado Plateau has remained fixed on the North American carton, excepting a slight rotation during the Cenozoic. Is the CP considered part of the NA craton or is it something that sits ‘on top’ of it?

C2. Principles

This chapter reviews the basic principles one needs to understand the book. These include:

Terranes. A terrane is a complete block of crust that can be shown to have completely different history, age, and origins than neighboring blocks. Terranes that are from far away are often referred to as exotic or suspect terranes. The concept was developed in the 1970’s. By the 1990’s hundreds of terranes had been described, and now the focus is more on groups of terranes. Terranes are classified in terms of their suspected origins, examples being continental fragments, volcanic arcs, mid-ocean plateaus, and so on.
Suturing and jumping. When a terrane collides with a continent the subduction zone often ‘jumps’ outward — i.e. the original terrane sutures to the continent and ‘clogs’ the existing trench.

Sediment and sedimentary processes. Sedimentary environments are divided into

continental environments: rivers, lakes, eolian settings, and alluvial fans — usually quartz rich sandstone, mudstone, and shale
shoreline environments5: beaches, barrier bars, and tidal flats – usually sandstone, mudstone, and shale
offshore marine environments: Sandstone, mudstone and limestone; may show signs of bioturbation.
Deep water marine environments: slope rise; abyssal plane; submarine canyon and fan; trench. Turbidites and slump deposits.
Carbonate deposits form in marine environments, but most prolifically in the absence of mud and sand
Volcanic sediments — ash, tephra, flows – can be found in any of the environments if volcanism is taking place.

Igneous rocks and processes. The Cordillera contains some of the most extensive deposits of igneous rocks on the planet because of the subduction of the Farallon plate (the largest tectonic plate known).

Andesite is commonly found in arc complexes.
The granite batholiths of the Sierra Nevada used to have andesite above them, but it has eroded away.

Ophiolites are a suite of rocks representing oceanic crust that has been shoved to the surface in a process called obduction.

Metamorphic rocks.

Blueschist is produced during high pressure metamorphism of oceanic crust during subduction.

Cordilleran Margins

There are active plate boundaries characterized by interaction between oceanic and continental crust. The two types of active boundaries are subduction boundaries (which may or may not involve accretion) and transform boundaries — both are commonly found together.

C3. The Cordillera’s Foundations: Paleoproterozoic and Mesoproterozoic Periods: Ca. 1800-1000 Ma.

“fleeting snapshots in the long cavalcade of time…”

The mantel is in motion, at about the same pace as fingernail growth, because of the heat of radioactive decay near the core-mantle boundary.
The Cheyenne Belt is the southern edge of the ancient Archean craton — it runs roughly along the line demarcated by the Wyoming-Colorado border. The craton, in place by 2.5 Ga, was where further material began to be accreted at about 1.8 Ga. Metamorphic rocks produced during that period of accretion are, among other places, found in the inner gorge of the Grand Canyon.
There are three phases to the accretion of material along the Cheyenne Belt.
- 1. The Archaean craton in Montana, Wyoming and Northeast Utah
- 2. Accretion of basement rocks of Colorado, Arizona and the rest of Utah.
- 3. The western half of the cordillera (Nevada, California, Oregon, Washington, Idaho and British Columbia) was underlain only by ocean plate until various younger pieces of crust are attached piecemeal starting in 325 Ma.
xxx

C4. The Cordillera’s Long-Lived Passive Margin: Neoproterozoic to Middle Devonian Periods: Ca. 1000 Ma-400 Ma

dropped down basins
passive margins persist for 200 Ma following precambrian — on the south
Wasatch line: SLC to Las Vegas — tectonic hinge
1Ga–4000 Ma –600 Ma of passive margins — on the west?

C5. The Antler Orogeny and the First Suspect Terrane: Middle Devonian to Late Pennsylvanian: Ca. 400-300 Ma

arrival of terrains from west starting at 400 Ma
Antler orogeny
confused about subduction dipping to the west and then the east, page 74
Alamo impact — 367Ma
Trench rollback — collapsed mountains; created back arc basins as west of Japan

C6. The Amalgamation of Pangaea and the Sonoma Orogeny: Early Permian to Early Triassic – Ca. 300-240 Ma.

C7. The Arrival of Wrangellia and the Nevadan Orogeny: Late Triassic to Late Jurassic: Ca. 240-145 Ma

C8. The Continental Arc, Sevier Orogeny, Western Interior Seaway and Flat-Slab Subduction: Cretaceous Period: Ca. 145-65 Ma.

Pangea-induced overheating leads to rifting and faster motion of the subducting Farallon plate, with various terranes being sutured onto the W coast and arc volcanism occurring in the Cordillera and compression east of that.

I found this chapter difficult to follow, and am not sure of the timing and interrelationship of many of the events following the breakup of Pangea. Later: I put the earlier draft of this into Claude, and it assisted in ordering the events covered in mynotes.

Breakup of Pangea (300Ma—~150Ma). The Pangean supercontinent had the effect trapping core/mantle heat beneath it, leading to rifting and volcanism. As Pangea broke up, new rifts created the central Atlantic, the Gulf of Mexico, The Canadian Basin, and the Indian Ocean. The pent up energy also increased plate motion, and thus subduction (e.g., the Farallon plate beneath the North American plate. More igneous material per unit time may have been created during the Cretaceous than any other period of time in the Phanerozoic.

Wranglia suturing (~175–100 Ma). Wranglia originally formed as an oceanic plateau, the collided with the Alexander terrane (which had also collided with the Peninsular terrane), and then a subduction trench to Wranglia’s west sutured rocks from the Chugatch terrane onto Wranglia. Coast Plutonic Zone intrusion (~160–45 Ma). The Coast Plutonic Complex intruded into the suture zone and produced plutons called ‘stitching plutons.’ Once Wrangellia docked, the subduction trench was pushed westward, and that’s where the Franciscan Assemblage then began accumulating. So Wrangellia’s accretion is essentially the precondition for the Franciscan/forearc/Sierra Nevada system that dominates the rest of the chapter.

By 100 Ma (Cretaceous) there are a series of parallel belts running NW-SE:

Trench — where Farallon plate subducts; Franciscan Assemblage accumulates here as an accretionary wedge/mélange.
The Franciscan Assemblage accumulates in the Cordilleran trench (~150–65 Ma).
1. Accretionary wedge => melange/coast range.The accretionary part of the Franciscan melange piled up forming an early iteration of the Pacific coast ranges. It generally takes the form of a melange—a large-scale brecciated rock in a mud-sand (greywracke) matrix with unsorted fragments of all sizes. The Franciscan assemblage may be up to 16,000 meters thick, though subsequent tectonics make it difficult to be sure. Franciscan material can be found from Santa Barbara to at least Oregon. Later strike-slip faulting moved some of the Franciscan melange north where they are exposed in the Straits of Georgia in British Columbia.
1. Blueschist metamorphism. Some of the Franciscan Melanage was subducted undergoing blue schist (high pressure, low temperature) metamorphism. I believe some of the subducted material can be seen in ophiolites on the California Coast 50-100 miles north of the bay area.
Forearc basin. A basin formed to the east of the Coast Range — it is termed a forearc basin because it sits on the sea-ward side of the magmatic arc (Sierra Nevada) produced by subduction. Some of the best exposures of forearc basin rock is found from La Jolla to Point Loma, California.
Magmatic arc (peak: ~120–80 Ma). The ongoing subduction of the Farallon plate during the Cretaceous produced a vast magmatic arc from SE Alaska to Baja California. The produced extrusive felsic rocks on the surface, and granitic rocks below; the surface material has eroded, but the granitic material form the plutons and batholiths that make up the Sierra Nevada and the granitic spine of Baja.
Sevier fold-and-thrust belt (140–55 Ma). As various terranes attached to the west coast, compression continued to dominate the central and eastern cordillera. Thin layers of shale and mudstone were deformed and thrust over one another, forming the mountains in eastern Nevada, Western Utah, and in Wyoming and Montanna. The process is called thin-skinned deformation.

Flat Slab Subduction (circa 85 Ma). An oceanic plateau riding on the Farallon plate subductes giving the subducting plate more buoyancy and decreasing the subduction angle. This event reorganizes the whole system with these consequences:

The magmatic arc shuts down or migrates eastward (no longer the right depth for melting above the slab)
Compression propagates far inland — this drives the Laramide orogeny (block uplifts in Colorado, Wyoming, New Mexico), which is distinct from the Sevier thin-skinned belt
The Western Interior Seaway axis shifts east
The Franciscan Assemblage continues accumulating, but the geometry changes

C9. Flat-Slab Subduction, the Laramide Orogeny, Uplift of the Colorado Plateau and Rocky Mountains: Paleocene and Eocene: Ca. 65-35 Ma

C10. Changing Tectonics, Cooling Climates and the Dawn of Crustal Extension: Late Eocene to Early Miocene (ca. 35-20 Ma)

As the Farallon plate subducts, the Pacific plate encounters the North American plate and a transform margin is formed with the Pacific Plate moving North relative to the NA plate.

The Transform Margin. The Pacific plate is moving WNW and the North American plate is moving WSW. Over time, the Pacific plate will begin ‘stealing’ blocks from the NA plate in a process called “Plate Capture.” The northward movement of the Salinia block is one example of this.

The two Triple Junctions and continued subduction. At either end of the (lengthening) transform margin we find triple junctions –MTZ (the Mendocino triple junction) and RTZ (the Rivera triple junction) where remnants of the now broken up Farallon plate (the Gorda plate at the MTZ and the Cocos plate at the RTZ) are still subducting and creating arc volcanism inland.

The Gorda plate. The Gorda plate is relatively young – it originates at the Gorda Ridge which is only a few hundred kilometers offshore. The plate travels such a short distance before reaching the subduction zone that it simply hasn’t had time to cool much since forming. At the point of subduction it’s only a few million years old, compared to oceanic plates elsewhere that may be 50–100+ Ma when they reach a trench. This young, warm oceanic crust is less dense and more buoyant than old, cold crust and it provides less “slab pull” (the gravitational force of a dense, sinking slab dragging the plate down– one of the main drivers of subduction velocity), so subduction is slower. This partly explains why there is no well-defined trench offshore and why large interface earthquakes are infrequent — though when they do occur, they can be very large. This also explains why the Gorda plate experiences significant intraplate deformation — folding and buckling — inside its boundaries due to stresses from its neighboring plates. Some geologists do not consider the Gorda plate to be a true plate. That said, the subducting Gorda plate drives volcanism in northern California Wikipedia — specifically Mount Shasta and Lassen Peak and the southern Cascades.

The Juan de Fuca plate. To the north of the Gorda plate the Juan de Fuca plate is subducting below the NA plate at the Cascading subduction zone, producing the Cascade Range volcanos — the Juan de Fuca will be completely subducted in 1 to 2 million years and the margin will likely be a transform fault. The Cascadia subduction zone is gradually shutting down piece by piece, with slabs breaking off while the remaining plate continues to subduct— so the Juan de Fuca/Explorer system as a whole is in the process of terminal fragmentation, the same fate the full Farallon plate met earlier.

The Cocos Plate. The Cocos plate was created approximately 23 million years ago (when the Farallon plate broke off two pieces (the other being the Nazca plate) off the coasts of Mexico, Guatemala, El Salvador, Nicaragua, Costa Rica, and Panama, subducting eastward beneath both the North American and Caribbean plates along the Middle America Trench. Cocos subduction drives two distinct volcanic arcs. The Trans-Mexican Volcanic Belt (TMVB) — a chain running east-west across central Mexico, including Popocatépetl — is fed by Cocos and Rivera subduction beneath the North American plate. Further south, the Central American Volcanic Arc (CAVA) runs the length of Guatemala, El Salvador, Nicaragua, and Costa Rica, fed by Cocos subducting beneath the Caribbean plate. Subduction of the Cocos and Rivera plates has produced a series of volcanic arcs in Mexico and Central America. The most geologically interesting feature is that the Cocos plate is undergoing flat-slab subduction beneath central Mexico — an almost exact modern analogue to the Laramide flat-slab episode

Metamorphic Core Complexes. The pull of the Pacific plate will also cause extension of the NA plate that was previously thickened resulting in the Basin and Range area. Slices of crust (carapace rocks) are being pulled away along low angle detachment faults, and the removal of theses masses allow deeper metamorphic rocks to rise buoyantly, which increases slippage along the detachment faults, in a positive feedback loop. The rising metamorphic rocks create domes – the metamorphic core complexes. And the carapace rocks sliding away rotate backwards into a low-angled fault plane creating tilted layers.

Interlude: History of the Farallon Plate, by Claude

TDE: Please describe the history of Farallon plate, from its origin to its breakup and subduction under the NA plate. Give approximate times

Claude:
Here’s the history in roughly chronological order.

~250 Ma — Origin in Panthalassa
Before the Farallon existed as a distinct plate, the proto-Pacific ocean (Panthalassa) was dominated by three plates meeting at a triple junction: the Izanagi, Phoenix, and what would become the Farallon. The Farallon plate formed one of the three main plates of Panthalassa, alongside the Izanagi plate and the Phoenix plate, which were connected by a triple junction. Wikipedia The Pacific plate itself formed later at the center of this triple junction.

~200–180 Ma — Subduction begins
The Farallon plate began subducting under the west coast of the North American plate — then located in modern Utah — as Pangaea broke apart, during the Early Jurassic. Wikipedia This is the subduction system that would, over the next 150 million years, build most of western North America. The subduction formed a line of volcanoes stretching all the way from Alaska to Central America.

~200–85 Ma — Terrane accretion and arc building
Through the Jurassic and Cretaceous, the Farallon plate did two major things simultaneously. First, it acted as a conveyor belt, carrying oceanic plateaus, island arcs, and continental fragments eastward and accreting them onto North America — Wrangellia, the Alexander terrane, and others. During the subduction of the Farallon plate, it accreted these island arcs and terranes to the North American plate. Much of western North America is composed of these accreted terranes. Wikipedia Second, subduction generated the enormous Cretaceous magmatic arc — the batholiths of the Sierra Nevada, the Peninsular Ranges, and the Coast Ranges of British Columbia.

~90–50 Ma — Flat-slab subduction and the Laramide orogeny
An oceanic plateau riding on the Farallon plate subducted at roughly 90 Ma, increasing the plate’s buoyancy and flattening its subduction angle. The Farallon plate developed a flat-slab segment starting at ca. 90 Ma, coinciding with subduction of the Conjugate Shatsky Rise oceanic plateau. From ca. 80 to ca. 55 Ma, the flat slab grew in length to as much as 2,000 km from the plate margin. Geological Society of America This suppressed arc magmatism along the coast and pushed deformation far inland, producing the Laramide block uplifts in Colorado, Wyoming, and New Mexico — far east of where you’d expect subduction effects.

~55–30 Ma — Slab steepening, rollback, and Basin and Range extension
As the oceanic plateau material eventually eclogitized and became denser, the slab began to steepen and roll back. The flat slab is inferred to have undergone rollback in a plane subparallel to the direction of plate convergence until ca. 30 Ma. Geological Society of America This rollback stretched and thinned the previously overthickened crust, initiating Basin and Range extension. Meanwhile the Farallon plate as a whole was shrinking rapidly through ongoing subduction.

~30 Ma — The East Pacific Rise reaches the trench; plate breakup
This is the pivotal moment. Around 30 million years ago, a dramatic segmentation occurred when the mid-ocean ridge collided with the continent, effectively splitting the ~10,000-km-long plate into two smaller plates. This gave rise to the San Andreas Fault that now connects the northern and southern portions of the former plate, the Juan de Fuca plate off Washington state and the Cocos plate off Central America. At this point the Farallon ceases to exist as a coherent entity. The Pacific plate comes into direct contact with the North American plate along what becomes the San Andreas transform system.

~30 Ma–present — Remnant fragments
The surviving pieces continue subducting but are themselves fragmenting. The Juan de Fuca broke into the Gorda (south) and Explorer (north) microplates. The Cascadia subduction zone is gradually shutting down piece by piece, with slabs breaking off while the remaining plate continues to subduct. Louisiana State University Buried remnants of the Farallon slab are still detectable today via seismic tomography — cold, dense slabs sitting in the mantle beneath the western US, some as deep as the lower mantle.

The through-line: the Farallon plate’s ~180-million-year subduction history essentially built western North America — its terranes, its mountain ranges, its batholiths, its sedimentary basins — and its death throes are still playing out in the Cascades, the San Andreas, and the Basin and Rang

C11. The End of Cordilleran Subduction and the Formation of the Basin and Range: Early and Middle Miocene: Ca. 20-10 Ma

C12. Interior Basins, Drainage Integration and Deep Incision: Late Miocene to Pliocene: Ca. 10-2.6 Ma.

C13. The North American Cordillera Today: Pleistocene, Holocene and the Anthropocene: Ca. 2.6 Ma to Present.