Displaced terranes
Displaced terranes, also known as exotic or suspect terranes, refer to regions of continental crust that have originated from distant geographical locations and have been transported to their current positions through tectonic processes. This concept helps geologists understand the growth of continents, which occurs through accretion—a process where smaller tectonic bodies, like island arcs or microcontinents, collide with and become part of larger continental landmasses. These terranes often exhibit unique geological features, such as distinct fossil records, mineral compositions, and structural patterns that differ from the surrounding continental rocks, indicating their foreign origin.
The displacement of terranes can involve significant distances, often exceeding 1,000 kilometers, and is supported by various geological evidence, including sedimentological, paleontological, and paleomagnetic studies. The study of displaced terranes has also led to the identification of complex geological structures known as collage tectonics, where multiple terranes are interspersed, forming intricate geological mosaics. Techniques such as biostratigraphy and radiometric dating are employed to analyze these terranes, providing insights into their history and the tectonic events that led to their current locations. Overall, the study of displaced terranes enhances our understanding of Earth's dynamic crust and the interactions that shape its surface over geological time.
Displaced terranes
The concept of displaced or exotic terranes explains how hitherto inexplicable regions of crust arrived at their present locations. It also provides a more detailed understanding of how continents grow by accretion of their crust through collision with smaller tectonic bodies.
Characteristics of Displaced Terranes
Geologists developed the concept of displaced terranes to explain how anomalous regions of continental crust may have originated. Such areas were discovered to possess indications that their origination sites differed from their present locations, often traveling more than 1,000 kilometers. Sedimentological, palaeontological, or palaeomagnetic sources may indicate the distance a terrane has traveled. Most terranes have one or more distinctive features, such as a fossil record, mineralogy, stratigraphy, or structural pattern that was foreign to the surrounding or adjacent continental rock units. As an additional clue that such regions may have been added to a continental landmass at a date later than the original formation of the landmass, they were usually found to have boundaries that displayed structural deformation, also suggesting subsequent emplacement. Thus, every indication pointed to starting points remote from their modern locations.
To complicate matters, geologists differentiate “terrain” from “terrane.” The former term describes an area of surface topography, while the latter is reserved for describing a region's subsurface. Whatever the precise terminology employed, the concept remains the same: Regions of questionable lineage are variously termed terranes of suspect, exotic, or displaced nature. Earth scientists working within the plate tectonic theoretical system consider such terranes as products of the collision of a continental lithospheric plate with lesser plate bodies or other entities such as island arcs. From various lines of physical evidence, researchers have concluded that such terranes have undergone accretion.
Accretion
In the Earth's crust, according to plate tectonic theory, all member units, called lithospheric plates, are in some type of interaction with one another. They are either spreading apart from some common center, as in the case of a mid-ocean ridge system (for example, in the mid-Atlantic Ocean); colliding with another unit with various end products and effects; or sliding alongside one another along transform faults, such as in the San Andreas fault zone system. Slow convection cells within the Earth are considered the propelling force behind the global tectonic system, driving the plates apart at one point in the heat exchange cycle, together at another, and alongside in still others.
In a scenario involving direct collision and not translational or separating motion, a plate will behave in various ways depending on its particular material composition and, thus, its relative density and thickness. If it is of similar density and thickness, it may ram up into a linear, folded mountain range, as in the case of the Himalayas. If it has a different relative density and thickness, it may be jammed below the forward end of the oncoming plate, termed the leading edge; in which case it experiences a process called subduction. It also may be jammed above the leading edge,;in which case it experiences a process termed obduction. Because of the general composition of its basement rocks, continental crust is somewhat lighter than oceanic crust. This difference is also described as one in the specific gravities of the two types of rock compositions. Thus, in a typical interaction between the two types, oceanic crust is subducted beneath continental material. That is not always the case, however, and exceptions apparently exist, the mechanics of which are still poorly understood.
Accretion is believed to occur when a leading edge of a continent either obducts heavier oceanic crust, adds other nonoceanic and non-continental material such as volcanic island arcs, or encounters smaller units of generally similar continental material termed microcontinents. In all these cases, the encountered terrane becomes incorporated into the forward-moving portion of the continental lithospheric plate. In this manner, for example, North America and other continents have added many thousands of square kilometers of area. Evidence indicates that in the case of North America, at least 25 percent of its surface landmass is constituted of exotic terranes. An extreme case is the Alaskan region, which is believed to be composed of about fifty distinct displaced terranes that make up almost one-half of its area.
Orogenic belts, or deformed mountain belts, are sometimes listed as another example of suspect terrane accretion onto the edge of a continental, lithospheric superstructure. The superstructure itself is termed a continental craton or shield and is believed to have a great age in relation to microcontinents and other relatively transitory phenomena. The age disparity is along the order of billions of years for cratons as compared to less than 100 million years for a microcontinent—probably the smaller units, from creation to destruction or accretion, last an average of only tens of millions of years. Various past orogenies, or mountain-building episodes, are interpreted as evidence for microcontinent collision and accretion with regard to different cratons.
Southern
One example is a scenario devised to explain the evolution of the southern Appalachian orogenic belt in the United States. This area is structurally complex and, before the acceptance of plate tectonic interpretations, generally defied any hypothesis that satisfactorily explained what its geologic history might have been. According to the scenario involving accretionary tectonics, the Cambrian period (about 544 to 505 million years ago) of the early Paleozoic era witnessed, among other things, a tectonic rifting event that resulted in the proto-Atlantic Ocean and production of various microcontinents, among them one termed the Piedmont. As time passed, the Ordovician period (about 505 to 438 million years ago) saw the onset of crustal subduction and the closure of the marginal sea. The Ordovician period gave way to the Silurian period (about 438 to 408 million years ago), during which the Piedmont microcontinent collided with and accreted to North America, resulting in the Taconian orogeny.
Further subduction resulted in another collision during the Late Devonian period (about 380 to 360 million years ago), which involved North America and another microcontinent termed Avalonia. This accretionary episode resulted in the Acadian orogeny. Further subduction continued into the Late Paleozoic era (about 360 to 245 million years ago), resulting, at its close, in the Permian period collision of a large continent called Gondwanaland with North America and the generation of extensive overthrust faulting. This event is known as the Appalachian, or Alleghenian, orogeny. Subsequent to these events, the Mesozoic era (about 245 to 66 million years ago) witnessed still more large-scale changes, such as the rifting action that caused the present Atlantic Ocean to appear and widen, a phenomenon still in evidence. Because of the compounding of all these large-scale tectonic events, the geology of Appalachia is an intricate and confusing affair, with a number of exotic terranes of different ages in close proximity to one another.
Collage Tectonics
This patchwork of accreted terranes has been described as collage tectonics. Such a collage includes not only jammed-together microcontinents and volcanic arcs but also narrow units termed suture zones, considered relic lines of collision. The presence of suture zones is an additional line of evidence used to substantiate the existence of an accretionary event. Suture zones are easily identified because they display a narrow area of intensely metamorphosed and deformed basalts and ultramafic rocks. The rocks, such as ophiolites, and structures of such zones are interpreted as representing the remains of the last vestiges of unsubducted oceanic plate material that once separated a microcontinent and the continent proper. The vestigial, dense ocean rocks have been jammed up onto the lighter, less dense continental rock.
Other areas where collage tectonics is prominently expressed are the northern North American Cordillera and eastern Siberia regions. This area is largely composed of several named and as-yet-unnamed suspect terranes, including, as mentioned, a significant part of Alaska. One of the better-understood terranes in this region has been called Wrangellia, a narrow, sinuous terrane thousands of kilometers long that stretches from south-central Alaska to about the northern border of Washington.
Evidence indicates that the anomalous terrane of Wrangellia has been displaced from 35 to 65 degrees of latitude northward from an unknown equatorial area. Some geologists, relying on similarities among fossils, believe Wrangellia originated in the South Pacific near the latitude of present-day Indonesia. Wrangellian basement rocks are oceanic basalts overlain by a sequence of limestones of the Middle and Upper Triassic periods (about 220 to 208 million years ago). These limestones are stratigraphically inconsistent with the rocks of the other suspect terranes surrounding or adjacent to Wrangellia, not to mention the actual continental rocks to the east. The exotic, displaced terrane of Wrangellia and its neighboring members of the tectonic collage attest to the long accretionary history that North America, along with all other continents, has experienced probably throughout most of geologic time.
Study of Displaced Terranes
Geologists researching the problem of suspect or displaced terranes have recourse to many techniques and methods to analyze physical evidence. Firsthand, on-site fieldwork or recording sites are the preferred data acquisition methods. In the case of the terranes of North America, investigations into the geologic history of former or active continental margin areas where suspect terranes abound were initially economically motivated for mining or oil exploration purposes. Later, academic fieldwork increased the database upon which the theory of displaced terranes was developed. The database was greatly expanded by new techniques involving seismic and other higher-technology approaches motivated by public considerations such as earthquake risk and the increasing need for new mineral resources for expanding industries and populations.
Among the more prominent techniques employed in the study of displaced terranes are biostratigraphy and comparing fossil records of terranes to their host continents. Fossil floras and faunas, both microscopic and macroscopic, can be used to correlate rock layers and date them concerning one another. This technique, termed relative dating, is one of the original dating methods used in geology. Fossil correlation and comparing the temporal and geographic distribution of certain particularly useful, key fossil species, called index fossils, were, until radiometric or absolute dating, was the cornerstone of all geologic dating and still are of great utility.
The study of anomalous, extinct marine faunas of microorganisms such as the fusulinids has been a key factor in establishing the concept of suspect terranes. Permian period fusulinids occur anomalously in a particular zone of limestones that can be traced from Alaska to California. The next nearest fossil matches to these faunas occur in Japan and southeastern Asia. Fossil assemblages such as the fusulinids and others have helped emphasize the exotic nature of the suspect terranes.
After World War II, radiometric techniques were used to refine the dating of fossils and the strata or sediments in which they were found. Radiogenic isotopes are different nuclear species of an element that are unstable, and that slowly change from one element to another, producing radiation at a fixed, long-term rate. Using the natural decay rate of certain of these isotopes occurring in rocks and fossils as a kind of natural, internal clock, geologists have refined the results of relative dating. They have also been able to cross-check the results of both types of dating. Radiometric dating has proved to be of inestimable worth in the dating of most rocks and fossils and, consequently, in the analysis of accreted terranes.
Evidence from paleomagnetism also grew in importance for the study of suspect terranes and the acceptance of the theory of plate tectonics in general. Paleomagnetism is based on the principle that certain minerals, such as iron oxides, are responsive to the Earth's natural magnetic field. Many igneous rocks, such as basalts, possess these minerals and once the molten mineral cools down to a certain point (for example, following an eruption where they are extruded or blasted free), the oxides orient themselves regarding the global field and remain in this orientation indefinitely unless, once again, further heated above the critical temperature. Thus, such minerals are natural indicators of past magnetic orientations and the parent rock's respective, past, relative geographic locations. Such fossil magnetism, referred to as remanent magnetism, is one of the primary proofs of seafloor spreading and continental drift. This phenomenon, like radiometric dating, paleontology, and stratigraphy, has proved useful for tracing the possible paths and former locations of suspect terranes.
Principal Terms
accretion: the process of growth of a larger crustal unit, such as a continent, by collision with smaller tectonic terranes, such as volcanic arcs or microcontinents
collage tectonics: a complex patchwork of different types of terranes thought to represent a region in which accretion has joined together suspect terranes
craton: a large, geologically old, relatively stable core of a continental lithospheric plate, sometimes termed a continental shield
lithospheric plate: one of several crustal plates of various sizes that constitute the Earth's outer crust; their borders are outlined by major zones of earthquake activity
microcontinent: an independent lithospheric plate that is smaller than a continent but possesses continental-type crust; examples include Cuba and Japan
obduction: a tectonic collisional process, opposite in effect to subduction, in which heavier oceanic crust is thrust up over lighter continental crust
plate tectonics: the branch of geology that describes many crustal phenomena, such as volcanism, in terms of movements and interactions of large and small crustal units called lithospheric plates
subduction: the process by which one lithospheric plate, usually a continental one, collides with another, typically oceanic, plate and overrides it, causing it to dive below the continent
suture zone: a narrowly definable region of the Earth's crust thought to represent a place where two lithospheric plates have collided and subsequently been joined together
terrane: any sizable, discrete region of the earth's surface crust that is the product of tectonic forces; examples include island or volcanic terranes
Bibliography
Bartholomew, Mervin J., and Richard, P. Tollo. “Northern Ancestry for the Goochland Terrane as a Displaced Fragment of Laurentia.” Geology 32 (2004): 669-672.
Besse, J., and V. Courtillot. “Apparent and True Polar Wander and the Geometry of the Geomagnetic Field over the Last 200 Myr.” Journal of Geophysical Research 107 (2002): 2300.
Grotzinger, John, and Tom Jordan. Understanding Earth. 8th ed. W. H. Freeman, 2020.
Packham, Chris, and Andrew Cohen. Earth: Over 4 Billion Years in the Making. HarperCollins, 2023.
Plummer, Charles C., et al. Physical Geology. 17th ed., McGraw-Hill, 2022.
Prothero, D. R., and Robert H. Dott. Evolution of the Earth. 8th ed. McGraw-Hill, 2009.
"Alaskan Geology." United States Geological Survey, pubs.usgs.gov/gip/0168/gip168.pdf. Accessed 20 July 2024.
Van der Voo, Rob. Paleomagnetism of the Atlantic, Tethys and Iapetus Oceans. Cambridge University Press, 2004.
Wyld, Sandra J., and James E. Wright. “New Evidence for Cretaceous Strike-Slip Faulting in the United States Cordillera and Implications for Terrane-Displacement, Deformation Patterns, and Plutonism.” American Journal of Science 301 (2001): 150-181.