An accreted terrane is a fragment of the Earth’s crust that originated far from its current location and has since been permanently attached, or “welded,” to the edge of a larger continental landmass. This process of attachment, known as accretion, is a fundamental mechanism of planetary geology, allowing continents to grow outward. These newly added crustal blocks carry a distinct geologic history, often traveling thousands of kilometers across ocean basins before colliding with a stable continental margin. The study of these fragments reveals how major mountain belts are constructed through the assembly of smaller, disparate pieces.
Defining the Exotic Terrane
A terrane is defined as a crustal block bounded by major faults that possesses a geologic history markedly different from the surrounding continental crust. Geologists refer to these fragments as “exotic” because their origins are foreign, meaning they did not form in place next to the continent they are now part of. An exotic terrane might be:
- An ancient oceanic island
- A submerged oceanic plateau
- A remnant of a volcanic island arc
- A microcontinent that rifted from another landmass
Identifying an exotic terrane relies on tracing unique geological fingerprints inconsistent with the adjacent continental rocks. For instance, the rock types often include oceanic material like basalt and deep-sea sediments, which contrast sharply with typical continental granite. Paleomagnetic analysis reveals that the terrane’s rocks formed at a different paleolatitude than the main continent. The fossil record may also contain unique assemblages of ancient organisms, indicating a biological environment once separated by a vast ocean from the continental margin.
The Mechanics of Accretion
The collision and attachment of an exotic terrane is an outcome of plate tectonics, driven primarily by the convergence of lithospheric plates at subduction zones. Here, a dense oceanic plate slides beneath a less-dense continental plate, consuming the oceanic crust and sediments. The oceanic plate transports buoyant crustal fragments—the future accreted terranes—toward the continental margin.
When these fragments, such as thick oceanic plateaus or volcanic arcs, reach the deep-ocean trench, their lower density or greater thickness prevents them from sinking into the mantle. Instead of being subducted, the buoyant terrane resists the downward pull, leading to a “docking” event. The fragment scrapes off the subducting plate and is compressed onto the overriding continental margin, effectively extending the continent’s edge.
This kinetic process is a form of continental growth distinct from the magmatic addition that occurs above a subduction zone. The collision causes significant internal deformation within the terrane and the adjacent continental crust, resulting in intense folding and faulting. Once attached, the subduction zone may jump seaward of the newly expanded margin, continuing the cycle of oceanic plate consumption.
Identifying Geological Signatures
Long after the collision, geologists map accreted terranes by looking for specific physical and chemical evidence. The most telling feature is the suture zone, a major fault system that marks the boundary where the exotic terrane fused with the original continent. These sutures are zones of intense deformation, often containing highly sheared and fragmented rock known as mélange.
Rocks within the terrane and suture zone often exhibit metamorphic changes resulting from the pressure and temperature of the collision. The presence of high-pressure, low-temperature minerals, such as blueschist facies rocks, indicates that the material was deeply buried and then rapidly exhumed. Furthermore, terrane boundaries can be identified using isotopic signatures, such as a sharp change in the ratio of strontium isotopes (e.g., the Sr-87/Sr-86 ratio) across the suture zone. This change reflects the difference between the ancient continental crust and the younger oceanic crust of the terrane, allowing researchers to reconstruct the fragment’s history.
Accreted Terranes and Continental Growth
The accretion of exotic terranes is recognized as a primary mechanism by which continents have grown laterally throughout geologic history, a process often described as continental amalgamation or collage tectonics. This steady outward expansion has increased the size of continental landmasses, particularly along active plate margins. The North American Cordillera, which forms the mountain chains of western North America, illustrates this process.
Vast sections of Alaska, British Columbia, and the western United States are composed of numerous individual accreted terranes added during the Mesozoic and Cenozoic eras. The addition of these buoyant fragments expanded the continent westward by hundreds of kilometers, creating the complex geological “collage” seen today. These boundary zones and associated magmatic activity are often linked to the concentration of valuable mineral resources, including deposits of copper, gold, and silver, mobilized by heat and fluid circulation during and after the accretion events.