Geologic History of
Southcentral Alaska
Alaska is made up of numerous
geologic terranes. Terranes are
blocks of continental crust that did not originate at the same time or place as
the continent to which they are now attached. Almost all of Alaska originated elsewhere and over time
these bits and pieces were moved by plate tectonics until they accreted onto
the North American Craton; the ancient continental core that formed over 3.5
billion years ago.
Most of Southcentral Alaska’s terranes originated far
to the south as either volcanic island arcs, or sedimentary material that was
eroded off of these island arcs.
These island arcs journeyed northward along the coast of North America,
first being carried by the Farallon Plate, then by the Kula Plate, which was
formed when the Farallon plate split along an east-west axis. The Kula Plate has almost entirely been
subducted underneath Alaska and its successor, the Pacific Plate, continues to
carry materials northwards.
In Southcentral Alaska, we find the oldest terranes
located against the North American Craton, and generally the farther from the
Craton, the younger the age of the terrane. The youngest terranes, the Prince William and Yakutat
terranes, are all still in the process of accreting onto Alaska.
The edges of these various terranes are often major
geologic faults such as the Denali Fault and the Border Ranges Fault.
Terranes of central Alaska such as the Yukon-Tanana, Dillinger, Nixon Fork, and Ruby Terranes had been in the vicinity of the northwest part of the North American Craton since Precambrian times. These terranes are collectively known as the Central Composite Terrane (CT). In the late Precambrian, the Alexander Terrane began forming through volcanic activity far to the south in the proto-Pacific ocean. By the end of the Paleozoic Era, the Wrangellia Terrane, another equatorial island arc, had been joined to the Alexander Terrane to form what is known as the Wrangellia Composite Terrane.
By the end of the Triassic Period, the Peninsular Terrane, yet another southern island arc, had joined with the Wrangellia CT and the three journeyed eastward on the Farallon Plate towards North America, which at that time lay in equatorial regions.
A subduction zone lay on the west side of Wrangellia. The trench it created accumulated sediments in two ways: by sediments eroding off of the Wrangellia CT and settling in the trench and by material scraping off the oceanic plate that was subducting into the trench. Much of the material on the oceanic plate was too light to be subducted into the mantle. Instead it was compressed against the edge of Wrangellia forming an accretionary wedge that we now call the Chugach Terrane. This began approximately in the Late Triassic Period. The subduction zone was forced to move to the seaward side of the Chugach Terrane as it slowly accreted, but there is still a zone of weakness between the Chugach Terrane and the Wrangellia CT known as the Border Ranges Fault.
The Chugach Terrane has two major parts. The landward side is the older McHugh Complex. It is a convoluted formation known as a mélange which formed from the Late Triassic through the Jurassic and into the mid-Cretaceous. It consists of fragments of ocean crust, limestone, and shales that were mixed with sandstone and volcanic tuff (ash) that eroded off of the Wrangellia CT seaward into the Border Ranges Trench. Much of this rock has been metamorphosed. The seaward side of the Chugach Terrane is the more recent Valdez Group, which is discussed below.
As the Chugach Terrane was forming on the seaward side of the Wrangellia CT, large amounts of sediment were being eroded into the ocean between the landward side of the Wrangellia CT and North America. The sediments were deposited from rivers onto the ocean floor in great fans of sand and silt that lithified into flysch. Flysch forms from finer sediments that erode off a continental shelf. When Wrangellia eventually collided with the older terranes emplaced along the North American Craton, this mass of flysch was faulted and folded to form the Kahiltna Terrane.
Towards the end of the Jurassic Period, the northern edge of Canada began to rift apart, forming the Arctic Ocean. This began to rotate the Arctic Composite Terrane, which would form northern Alaska, southward in a counter-clockwise motion centered just north of the present Yukon Territory.
By the Early Cretaceous Period, the Wrangellia CT had come into contact with North America in the vicinity of eastern Oregon and northeast California. Some of the northern reaches of the Peninsular Terrane had come into close proximity to the already existing parts of Alaska. By the mid-Cretaceous, the Kula Plate had rifted apart from the Farallon Plate and began to move the Wrangellia CT northward at a relatively rapid rate. Along the way, some parts of the Wrangellia CT were scraped off. Blocks of it are found today in northeast Oregon and Vancouver Island in British Columbia.
By the Late Cretaceous, the Wrangellia CT and the Chugach Terrane finally docked onto Alaska in its entirety. This docking clogged up the subduction trench which had existed between the two and a new trench formed south of the Chugach Terrane.
Subduction and collision led to much volcanism and metamorphism along the plate boundary between the Yukon CT and the Wrangellia CT. The present Denali Fault later formed near this collision zone.
At this time the Valdez Group of the Chugach Terrane formed as an assemblage of flysch. The Valdez Group consists mostly of metamorphosed shales and sandstones that eroded off the Wrangellia CT and an island arc that formed seaward of the Wrangellia CT after its emplacement against North America.
Subduction and continued collision between the Wrangellia CT and the existing parts of Alaska caused much crustal thickening and heating. Many of the sediments that had accumulated between the two were melted into great plutons of granite, which were eventually uplifted as the great peaks of the Alaska Range such as Mount McKinley. Most of the lower peaks in the southwest Alaska Range are crumpled and uplifted blocks of the Kahiltna Flysch Basin.
By the Paleocene Epoch, the Prince William Terrane was beginning to form in much the same way as the Valdez Group flysch of the Chugach Terrane. Large amounts of sediment were being washed off of the continent and deposited into the trench. This sediment was too light to subduct and therefore was plastered onto the inner wall of the trench, eventually to be uplifted out of the ocean. Because the trench was slowly being filled with sediments, the actual subduction zone was forced to progressively migrate seaward of those sediments and away from the continent.
One of the most interesting features that differentiates the Prince William Terrane from the Chugach Terrane is the fact that the Prince William Terrane contains ophiolite sequences. These are sequences of rock that formed in spreading centers, the great rifts in the ocean floors. They are characterized by upper layers of pillow basalt, which are globular formations of lava that spilled onto the ocean floor and lower layers of gabbro, which is solidified lower crust rock. In the Eocene Epoch, about 50 million years ago, the spreading center that separated the Kula Plate from the Farallon Plate came into contact with the trench that lay seaward of the Prince William Terrane. Usually heavy oceanic rock will sink beneath lighter continental rock, but in this case, the rock at the spreading center was too hot and buoyant to be subducted, and so instead was accreted onto the Prince William Terrane. This ophiolite sequence makes up the Resurrection Peninsula across Resurrection Bay from Seward, and Knight Island in Prince William Sound.
As this hot region of the spreading center began to accrete, some of the surrounding rock was melted to form granitic plutons that have since been uplifted and can now be seen in much of Prince William Sound, such as on Culross Island.
By the Eocene Epoch most of the
terranes of Southcentral Alaska were at their present latitude. The Yakutat Terrane was being formed to
the south at this time. It is
thought to be a block of basaltic ocean floor that separated from the Pacific
Plate, was added to an accretionary wedge and subsequently was buried by
continental sediments as it moved northward along the Pacific coast of North
America.
By about 43 million years ago, the Aleutian trench was well developed. Subduction into the trench initiated the volcanism along Cook Inlet, on the Alaska Peninsula, and on the Aleutian Islands, which is still occurring today.
It was also about this time that the spreading between
the Kula and Pacific plates died out and the two fused together. The direction of the plate movement
changed as well. These plates had
been moving northward, but now they shifted to a northwesterly direction,
creating many of the slip-strike faults (such as the Denali Fault) that now
characterize Southcentral Alaska.
This faulting is thought to be the origin of the granite plutons of the
Talkeetna Mountains. Eventually,
the Kula Plate was almost entirely subducted underneath Alaska. It is now the Pacific Plate that contributes to the accretion of
terranes along the southern margin of Alaska
By the Oligocene Epoch, about 25 million years ago, the Yakutat Terrane began to subduct underneath eastern Alaska and Prince William Sound. This led to upwarping and downwarping in Prince William Sound and the Kenai Peninsula, forming many of the mountains seen there today. Its subducted portions have melted and created the volcanic Wrangell Mountains, which are active today. Other portions have smashed into the southern Chugach Terrane and uplifted the towering eastern Chugach and Saint Elias Mountains. Because the Saint Elias Mountains have only been rising for 14 million years, some of them bear layers of Wrangell lava that flowed across them when they were still plains sloping towards the sea.
Today the Pacific Plate still continues its slow journey to the northwest at a rate of a bit more than 2 inches per year. The terranes of Southcentral Alaska continue to be pushed in the same direction, creating a region characterized by great mountain ranges, volcanoes, and frequent earthquakes. The Alaska Range will continue to rise as Southcentral Alaska pushes into and scrapes along the edge of the Interior Alaskan terranes. Southcentral Alaska will also grow southward as material that is now undersea is brought to the surface by the movement of the Yakutat and Prince William Terranes. What is now Prince William Sound will likely one day be just another part of mainland Alaska.