Accretionary orogens in space and time
Published:January 01, 2007
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Kent C. Condie, 2007. "Accretionary orogens in space and time", 4-D Framework of Continental Crust, Robert D. Hatcher, Jr., Marvin P. Carlson, John H. McBride, José R. Martínez Catalán
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Accretionary orogens form along continental margins where oceanic lithosphere is subducted. They are primary sites of juvenile continental crust production and have been active on Earth since the earliest Archean. Orogen lifetimes expressed as accretion intervals range from 50 to over 300 m.y. The short duration of Late Archean accretionary orogens (<70 m.y.) may reflect the short duration of a global mantle plume event at 2.7 and 2.5 Ga. Although there is no simple relationship between the onset or duration of accretionary orogens and the supercontinent cycle, many post-Archean orogens terminate with continent-continent collisions during supercontinent assembly.
Average terrane lifespan is typically 100–200 m.y. in post–1 Ga orogens, 50–100 m.y. in pre–1 Ga Proterozoic orogens, and 70–700 m.y. in Archean orogens. Accretionary orogens can be grouped into two end members: simple orogens containing chiefly juvenile terranes with lifespans of <100 m.y., and complex orogens with both juvenile accreted components and exotic microcratons, with terrane lifespans of ≥100 m.y. Terrane lifespan is controlled by (1) terrane tectonic setting, (2) complexity of precollisional terrane history, (3) availability of continental crust on Earth, and (4) plate history of ocean basins adjacent to accretionary orogens.
Average accretion rates in accretionary orogens are 70 to 150 km3/km/m.y. in Phanerozoic orogens and 100 to 200 km3/km/m.y. in Precambrian orogens. Some orogens at 2.7 Ga have unusually high accretion rates greater than 300 km3/km/m.y., which may reflect a global mantle plume event. Production rates of juvenile crust in accretionary orogens are typically 10%–30% lower than total accretion rates, but can be up to 50% lower in Phanerozoic orogens.
Accretionary orogens are major sites of continental growth and mineralization and include Archean granite-greenstone terranes, many Proterozoic orogens (e.g., the Birimian of West Africa, Svecofennian of Finland and Sweden, Yavapai in the southwestern United States, and the Arabian-Nubian shield), Neoproterozoic-Paleozoic orogens in central Asia and eastern North America, as well as Neoproterozoic to Holocene orogens of the circum-Pacific and Caribbean. Accretionary orogens form at sites of subduction of oceanic lithosphere and consist of accretionary wedges containing material accreted from the downgoing plate and eroded from the upper plate, island arcs, ophiolites, fragments of oceanic plateaus, microcratons, syn- to postaccretion granitic and metamorphic rocks, exhumed high-pressure metamorphic rocks, and syn- to postaccretion clastic sedimentary basins.
The major site of juvenile continental crust production is along convergent plate margins in accretionary orogens. Juvenile crust is produced in arc systems from magmas derived primarily from the mantle wedge, and it also may be added to continents by collision and accretion of oceanic arcs, oceanic plateaus, and oceanic crust (ophiolites).
Accretionary orogens have much in common, yet there are many differences, some of which appear to change with geologic time. Although much is known about a few accretionary orogens, such as the American Cordillera, Archean greenstone terranes of the Superior province, and terranes of the Appalachian orogen, little effort has gone into comparing and contrasting accretionary orogens. In this paper, accretionary orogens of various ages are compared and discussed in terms of terrane lifespan, the supercontinent cycle, and rates of accretion and juvenile crust production.
ACCRETIONARY INTERVALS AND TERRANE LIFESPAN
Two important characteristics of accretionary orogens are (1) the orogen accretion interval and (2) terrane lifespan. In this study, accretion interval is defined as the difference between the oldest and youngest age of terrane accretion to a continent in a given orogen. For orogens with long strike lengths, such as the Altaids in the Phanerozoic and the Yavapai-Penokean-Svecofennian in the Paleoproterozoic, accretion interval may be diachronous along the orogen. Some terranes are rifted from continental margins, which provides a time marker for the age of these terranes. However, rifting events have proved impractical to define terrane lifespan because (1) many, if not most, terranes did not begin life by rifting, and (2) the timing of rifting is often unknown. For this reason, terrane lifespan is defined as the time interval between the age of the oldest rocks in a terrane (excluding basement) and the time that a terrane collides with and is accreted to a continent. In the case of composite terranes (two or more terranes that collide before collision with a continent), lifespan is the time interval between the oldest terrane and the final collision of the composite terrane with a continent.
Orogen accretion intervals vary widely from ∼50 m.y. to over 300 m.y., with the shortest intervals (<100 m.y.) characteristic of the Archean (Fig. 1). Of the Archean orogens studied, only the Yilgarn has an accretion interval in excess of 70 m.y. Phanerozoic and Proterozoic orogens typically have accretion intervals between ∼100 and 300 m.y. The Cordilleran, Andean, and Japan orogens are still active, and hence their accretion intervals are minimum values only.
Within a given orogen, terrane lifespan is extremely variable, ranging between 20% and 60% for one standard deviation of the mean (Fig. 1). The shortest lifespans occur in the Late Archean and Paleoproterozoic where mean lifespan is generally less than 100 m.y. The longest terrane lifespans are found in the Itsaq orogen in SW Greenland, with an average ∼760 m.y. for six terranes, and in the Pilbara craton in Western Australia, with two large terranes with lifespans of 530 and 280 m.y., respectively (Table 1). Average terrane lifespan in Phanerozoic orogens generally falls between 100 and 250 m.y. It is clear from Figure 1 that there is no relation between average terrane (or composite terrane) lifespan and orogen accretion interval.
These results suggest that two end-member scenarios exist for accretionary orogens, which we will refer to as simple and complex orogens. A simple orogen is one that is characterized by rapid accretion of juvenile oceanic terranes (arcs, oceanic crust, oceanic plateaus) where average terrane lifespan is <100 m.y. (Fig. 2). In contrast, a complex orogen comprises not only juvenile accreted components but also exotic microcratons, and average terrane lifespan is ≥100 m.y. Extensional accretionary orogens (such as the Lachlan orogen in Australia) are a special case of simple orogens, which are characterized by poorly developed foreland basins, little or no crystalline basement, and an extended history of turbidite deposition in backarc basins (Collins, 2002). Unlike most simple orogens, complex orogens may undergo extensive rifting and trans-current faulting during the accretion cycle, which creates new ter-ranes and moves them laterally along the orogen. Of course, both simple and complex scenarios may occur along strike in the same orogen, but one or the other type often dominates. There also seems to be a time dependence of orogen type: Simple orogens are most common in the Proterozoic, whereas complex orogens dominate during the Phanerozoic (Japan is an exception). The Archean has both types of orogens.
A SURVEY OF ACCRETIONARY OROGENS
One of the most well-known Phanerozoic accretionary orogens is the American Cordillera, and it is perhaps the type example of a complex orogen. Although some of the Cordilleran terranes appear to have traveled great distances before accretion to Laurentia, it is probable that the majority of these terranes formed along or near the west coast of Laurentia (Plafker et al., 1989; Plafker and Berg, 1994). Some appear to have been composite terranes at the time of accretion (Monger et al., 1982). Almost all tectonic settings are represented in Cordilleran terranes, with continental-margin arcs and backarcs dominating (Condie and Chomiak, 1996). Many terranes contain several rock packages separated by unconformities indicating complex histories involving several tectonic settings prior to final accretion to the continent. Terranes were accreted over a long time interval beginning ca. 250 Ma in central Nevada and continuing to 55 Ma when the Gravina terrane was accreted to Alaska (“Cordilleran” in Table 1). Some terranes, such as Alexander, Peninsular, and Wrangellia, are composite terranes, having undergone one or more collisions with constituent blocks before finally colliding with Laurentia (Monger et al., 1982; Plafker et al., 1989; Condie and Chomiak, 1996). Terrane lifespan is quite variable, ranging from ∼450 m.y. for Kandik River to 50 m.y. for Chugach, Gravina, and Pacific Rim terranes (Condie and Chomiak, 1996). The average terrane lifespan is ∼170 m.y. (Table 1). As long as the Cordilleran orogen opens into the Pacific basin, future terrane accretion is possible.
Although the Appalachian orogen is a classic example of a collisional orogen, prior to the terminal continent-continent collision in the Late Permian, this orogen was a complex accretionary orogen. Terrane collisions occurred between ca. 500 and 280 Ma, and the terminal collision with Baltica–West Africa occurred at ca. 250 Ma. Terranes are very diverse in character, ranging from microcratons (rifted fragments of continent with cratonic sedimentary successions) to both oceanic and continental-margin arcs, ophiolites, and continental rifts (Keppie, 1989). Some micro-cratons include older Precambrian basement with ages between 1 and 2 Ga (Murphy et al., 1999, 2004; Keppie et al., 2003). Many terranes were rifted from the northern margin of Gondwana, beginning in the Neoproterozoic and Cambrian, and fall into two categories (Murphy et al., 2004): (1) terranes that have 1.3–1.0 Ga basement and formed along the northern Gondwana margin by 650 Ma, and (2) terranes that formed along the West African margin and show recycling of 2–3 Ga crust. Later rifted from Gondwana, these terranes were caught up in Appalachian, Caledonide, and Variscan accretionary orogens and now reside in North America and western Europe. Appalachian terrane lifespan ranges from as little as 50 m.y. (for instance the Shelburne Falls terrane, a North American continental-margin arc; Karabinos et al., 1998) to more than 300 m.y. in some superterranes (Yucatan, Avalonia, and Carolina) (“Appalachians” in Table 1).
Another large and long-lived accretionary orogen in the Phanerozoic is the Altaids in eastern Asia, which has features of both simple and complex orogens (Xiao et al., 2004). Recent work in Mongolia shows the existence of at least 44 accreted terranes in this region (Şengör and Natal'in, 1996; Badarch et al., 2002). Reconnaissance mapping and description of stratigraphic sections in each terrane indicate numerous tectonic settings, including microcratons, island arcs, continental-margin arcs, forearc/backarc regimes, accretionary complexes, and ophiolites. The arcs, ophiolites, and accretionary complexes appear to have accreted around a small Precambrian craton in the Hangay region of northern Mongolia. The oldest rocks in any of the Mongolian terranes (excluding basement) appear to be Neoproterozoic (“Mongolia” in Table 1). Like most other accretionary orogens, the timing and spatial distribution of accretion is complex and does not fall into a simple order outwards from the Precambrian craton. The final accretion ages are chiefly Cambro-Ordovician, Devonian, and Late Permian, with an average accretion age of ca. 380 Ma (Table 1). Although there is a scarcity of Nd isotopic data from this region, inferred tectonic settings indicate that the Mongolian terranes represent a mixture of juvenile and reworked crust. Based on 25 terranes for which there are data, terrane lifespan ranges from 80 to at least 490 m.y., with an average of ∼226 m.y. (Table 1).
The Altaids, NW China, and adjacent areas in Siberia comprise at least 11 large terranes accreted to Siberia chiefly in the late Paleozoic (“NW China” in Table 1) (Huang et al., 1987; Coleman, 1989). Most or all of these terranes are composite and include two or more tectonic settings. Rock associations and structure indicate the presence of microcratons, island and continental-margin arcs, and ophiolites. The Tarim and Siberia cratons, south and north of the NW China terranes, respectively, have distinct polar wander paths until the end of the Permian when they collided to form the intervening Altaids orogen. Ages of the oldest rocks in these terranes (excluding basement) are not well known but appear to fall near the Cambrian-Neoproterozoic boundary. Accretion ages for 11 of the terranes range from ca. 360 to 250 Ma, with most falling in the Late Permian. Terrane lifespan ranges only from ∼290 to 145 m.y., with an average of ∼212 m.y. (Table 1). Although the Altaids orogen came to an end with the collision of the Tarim–North China craton with Asia on the south in the Late Permian, terrane and microcraton accretion continued during the Mesozoic and Tertiary, adding China, Tibet, SE Asia and India. In fact, it is continuing today with the collision of Australia with SE Asia in Indonesia.
The Japanese islands have grown oceanward by ∼400 km since the Devonian, chiefly by growth of accretionary complexes (Isozaki, 1997; Maruyama, 1997). The Oki and Southern Kitakami belts in western Japan were originally part of the Yangtze craton and were rifted away ca. 750–700 Ma when Rodinia began to break up. Circa 500 Ma, a subduction zone developed along the eastern margin of these terranes, rifting subsided, and accretionary growth began along the Pacific margin. Unlike many Phanerozoic accretionary orogens, Japan comprises chiefly accreted juvenile oceanic terranes, and for this reason is classified as a simple orogen. Except for the fragments of oceanic crust accreted to the growing continental margin, most terranes in Japan are nearly autochthonous (Isozaki et al., 1990). They have grown chiefly by addition of oceanic crust and trench sediments scraped off into the accretionary prism. Hitachi-Takanuki and related terranes in northern Honshu, however, are allochthonous terranes that collided with Honshu during the collision of the Yangtze and North China cratons ca. 250 Ma. Accretion ages of Japanese terranes range from ca. 70 to 300 Ma, with an average accretion age of 180 Ma (“Japan” in Table 1). Terrane lifespan is highly variable, ranging from 20 m.y. or less to over 200 m.y., with an average of 118 m.y. Today, Japan is still an active, open-ended accretionary orogen.
Recent studies in the Andes indicate that as much as 40% of the growth in this orogen since the Neoproterozoic is due to terrane collision and accretion (Cordani et al., 2000; Vujovich et al., 2004). One of the first terranes identified with Laurentian affinities is the Precordillera terrane, rifted from SW Laurentia during the Cambrian and accreted to South America ca. 455 Ma (Thomas and Astini, 1996). Although terranes and terrane boundaries are not as yet well established, at least six major terranes compose the southern Andes in Argentina and Chile (Ramos, 2004), and at least two of these terranes (Precordillera and Cuyania) were derived from Laurentia. The six Andean terranes were accreted to the Rio de La Plata craton between 600 and 430 Ma and have an average terrane lifespan of ∼219 m.y. (“Southern Andes” in Table 1).
The best-known example of a Neoproterozoic accretionary orogen is the Arabian-Nubian orogen, which formed by terrane collisions chiefly between 760 and 630 Ma (Stoeser and Camp, 1985; Abdelsalam and Stern, 1996; Stern, 1994). Terranes are largely juvenile in character with island arcs and ophiolites dominating, and thus, the Arabian-Nubian orogen is generally classified as a simple orogen. Based on six terranes for which adequate data are available (“Arabian-Nubian” in Table 1), terrane lifespan ranges from over 200 m.y. to <50 m.y., with an average of ∼143 m.y.
Perhaps the largest accretionary orogen of all time is the Yavapai-Penokean-Svecofennian orogen, which formed along one margin of a Paleoproterozoic supercontinent. This orogen, with a strike length >10,000 km, may have extended from south-central Australia, along the southern and eastern coasts of Laurentia, across southern Baltica, and perhaps into western Amazonia (Karlstrom et al., 2001; Geraldes et al., 2001). In addition to the Yavapai, Penokean, and Svecofennian events, it includes events such as the Labradorian, Mazatzal, and Gothian. Terrane accretion in this orogen began ca. 1900 Ma and continued until the orogen shut down during Grenvillian continent-continent collisions ca. 1100 Ma. Chiefly between 1650 and 1350 Ma, large numbers of A-type granitic plutons were intruded into this orogen. Although any given segment of the orogen preserves a history of terrane collision and accretion of no more than 350 m.y., the orogen's total lifetime approaches 800 m.y. (“Svecofennian-Gothian” in Table 1). Accretion intervals for various segments of the orogen should be considered minima, because the younger parts of the orogen (1.5–1.2 Ga) either are poorly exposed or were rifted away during the breakup of Rodinia 700 Ma. To characterize this immense orogen, we will examine three sections across the orogen: Baltica, NE Canada, and SW United States. Results from Scandinavia and Russia indicate that the Baltic shield grew westward for over 300 m.y. (1910–1580 Ma; Table 1) by terrane accretion (Park, 1985; Skiold, 1988; Gorbatschev and Bogdanova, 1993; Ahall et al., 1995; Bogdanova et al., 2001). Accreted terranes are chiefly continental-margin arcs and associated back-arc basins, with a few occurrences of ophiolite and remnants of oceanic plateaus. The last terranes for which we have isotopic ages accreted to Baltica in SW Sweden and adjacent parts of Norway ca. 1580 Ma during the Gothian event (Table 1). The history of the Baltic segment of the orogen between 1580 and 1100 Ma (the terminal Grenvillian collision) remains largely unknown. Paleoproterozoic terrane lifespan in the Baltic shield ranges from 15 to 145 m.y., with an average lifespan of ∼63 m.y.
The Labradorian orogen in NE Canada is also part of the Yavapai-Penokean-Svecofennian orogen, and this part of the orogen was active for over 150 m.y., between ca. 1800 and 1650 Ma (“Labrador” in Table 1) (Gower, 1996; Ahall and Gower, 1997; Gower and Krogh, 2002). The Labradorian orogen includes chiefly backarc basins and other arc-related terranes accreted between 1800 and 1650 Ma, and it is largely a simple orogen. Little evidence is preserved for the time period between 1650 and ca. 1500 Ma. Perhaps the plate boundary changed from convergent to divergent (rifting) or transcurrent during this time interval. Between 1500 and 1100 Ma, tectonic settings in the Labra-dorian orogen ranged from backarc rifts to passive continental margins (Gower, 1996). The orogen terminated with the collision of Baltica and North America as part of the formation of Rodinia ca. 1100 Ma.
The Yavapai-Mazatzal (hereafter, Yavapai) orogen in the southwestern United States formed by terrane accretion chiefly between 1760 and 1650 Ma with an average accretion age of ca. 1704 Ma (“Yavapai” in Table 1) (Condie, 1992; Condie and Chomiak, 1996; Karlstrom et al., 2001). From outcrop, 11 terranes have been recognized in the Yavapai orogen, which include four island arc terranes and seven continental-margin arc and associated backarc basin terranes. Remnants of an accreted oceanic plateau have also been described from west-central Arizona (Condie et al., 2002). Terrane lifespans fall into two groups: four terranes in west-central Arizona with lifespans of 60–80 m.y. and the remaining seven terranes with very short lifespans of 20–40 m.y. (Table 1). Again this segment of the orogen is an example of a simple orogen. Although it may be possible to sort out the post–1650 Ma history of the Yavapai orogen with detrital zircon ages in younger sediments, at present we have little evidence (other than A-type granites) for the tectonic setting of this region between 1630 and 1300 Ma when sedimentary basins developed on older crust (Karlstrom et al., 2001).
Although most investigators agree that Archean granite-greenstone provinces are accretionary orogens that have grown by plate-tectonic processes (Langford and Morin, 1976; Smithies and Champion, 2000), some have challenged this paradigm (Hamilton, 1998; Bleeker, 2002). However, it would seem to me that rather than abandon plate tectonics, which so well explains so much observational data in the Archean, it is better to look for relatively minor changes in tectonic regime to explain some of the differences we see in the Archean (Condie and Benn, 2006). No other Archean crustal province has received such intense study as the Superior province in Canada. This province seems to be the type example of progressive growth of an Archean craton by largely juvenile terrane collisions (Kimura et al., 1993; Mueller et al., 1996; Corfu and Stott, 1998; Percival et al., 2004). Rock associations and geochemistry suggest that collision and accretion of both arcs and oceanic plateaus contributed to the growth of the Superior province (Kimura et al., 1993; Tomlinson and Condie, 2001; Polat and Kerrich, 2001). Growth is concentrated around the southern and northern margins of a 3.0–2.8 Ga nucleus, with most collisions and accretion concentrated between 2700 and 2640 Ma (“Superior” in Table 1). Unlike most Phanerozoic accretionary orogens, terrane lifespan is relatively short, averaging ∼77 m.y.
The Yilgarn province in Western Australia formed at about the same time as the Superior province and is another example of an Archean accretionary orogen. Although the exact number of terranes is disputed, there are at least eight terranes, which appear to have collided and accreted over ∼110 m.y. at 2700 Ma (Myers, 1993; Griffin et al., 2004) (“Yilgarn” in Table 1). Although many Yilgarn terranes are remnants of oceanic arcs and backarcs, some, such as the Murchison and Narryer terranes, are microcratons containing rocks up to 3.1 Ga. Thus the Yilgarn, unlike the Superior, is an example of a complex orogen. Terrane lifespan is extremely variable, ranging from as low as 50 m.y. for Kalgoorlie and Eastern Goldfields to over 300 m.y. for the Murchison and Narryer microcratons.
In contrast to the Yilgarn craton, the Gawler craton in southern Australia is a relatively short-lived simple accretionary orogen (60 m.y.) (“Gawler craton” in Table 1). The oldest rocks are metasedimentary ca. 2640 Ma, and most igneous activity is concentrated ca. 2550 Ma (Betts et al., 2002; Swain et al., 2004). Both arc- and plume-like volcanic successions have been described from Gawler terranes. Terrane lifespan ranges from ∼70 to over 150 m.y.
The Zimbabwe craton is another Late Archean simple accretionary orogen that formed at ca. 2700 Ma. Isotopic ages are consistent with westward crustal growth between 2710 and 2645 Ma (Wilson et al., 1995; Jelsma et al., 1996) (“Zimbabwe” in Table 1). Terranes appear to be chiefly continental-margin arcs with a few remnants of oceanic plateaus. Average terrane lifespan for the Zimbabwe craton is 95 m.y.
There are a few remains of Early Archean accretionary terranes, one of the oldest of which is the Faeringehavn terrane at 3860 Ma in SW Greenland. This terrane amalgamated with five other pre–3000 Ma terranes at ca. 2720 Ma to form the Itsaq orogen (Friend et al., 1988; Nutman et al., 2000, 2002, 2004). Perhaps the most unusual feature of the Itsaq orogen is the large range in terrane lifespan, from 190 m.y. to over 1000 m.y. (“Itsaq” in Table 1). These terranes have undergone long, complex histories before finally becoming part of a Late Archean accretionary orogen. If the six preserved terranes are representative, the relatively short accretionary interval of this orogen (40 m.y.) is also striking.
At least five terranes are recognized in the Barberton region of the Kaapvaal craton in southern Africa, four of which comprise arc-like or oceanic plateau volcanic successions that range in age from 3.3 to 3.6 Ga (Lowe, 1999) (“Barberton” in Table 1). The Ancient Gneiss terrane comprises tonalitic gneisses and other deformed granitoids ca. 3640 Ma. All of these terranes collided and amalgamated in a relatively short time span of 50 m.y. at ca. 3250 Ma. As with the Itsaq orogen, the range in terrane lifespan in the Barberton orogen is significant, varying from over 400 m.y. to less than 100 m.y.
Two terranes have been recognized in the Pilbara craton in Western Australia, each of which had a long complex history before colliding ca. 3000 Ma (Van Kranendonk et al., 2004; Hickman, 2004) (“Pilbara” in Table 1). The East Pilbara has rocks as old as 3530 Ma and a lifespan of ∼530 m.y., whereas the West Pilbara has rocks only as old as 3280 Ma and a lifespan of 280 m.y. Because there are only two Pilbara terranes for which we have precise geochronology, accretion interval cannot be estimated for the craton, although it was likely <50 m.y.
ACCRETIONARY OROGENS AND THE SUPERCONTINENT CYCLE
Although it would seem that initiation and cessation of construction of accretionary orogens should be tied to the super-continent cycle, there are no simple correlations that apply to all orogens, as shown in Figure 3. Most accretionary orogens begin as passive continental margins along which subduction and/or transcurrent faulting is later initiated. Passive margins are remnants of the breakup phase of a supercontinent. For instance, when Rodinia fragmented some 700 Ma, it left passive margins along what are now the east and west coasts of Laurentia and the west coast of South America and around Siberia. As the Iapetus Ocean opened 600 Ma, passive margins formed on the continents facing the Iapetus basin (eastern North and South America, western Africa, and western Europe). Although some of these margins evolved into convergent boundaries that became sites of accretionary orogens, the time intervals between rifting of the supercontinent and onset of subduction and terrane accretion are extremely variable. For instance, it took 400–450 m.y. after rifting before the first terranes were accreted in the Cordilleran and Japan orogens, whereas terranes began to accrete in the Andes and Appalachians after only 120–150 m.y. (Table 1). The Yavapai-Penokean-Svecofennian orogen followed the breakup of one or more Archean supercontinents by 100–200 m.y., and its onset coincides with the formation of a new supercontinent at 1.9 Ga. Unlike most Phanerozoic accretionary orogens, the onset of activity in Late Archean accretionary orogens coincides with the formation of one or more supercontinents (Fig. 3). It is also noteworthy that both of the major periods of Late Archean crustal formation (2.7 and 2.5 Ga) and the onset of the Yavapai-Penokean-Svecofennian orogen correlate with possible global mantle plume events (Fig. 3). It is possible that the earliest Archean accretionary orogens (e.g., Pilbara and Barberton) formed along the margins of oceanic plateaus, which rapidly evolved into continental crust.
Most accretionary orogens terminate with continent-continent collision. For instance, the Appalachians and the Altaids turned off during major collisions with large continents (Baltica–West Africa with Laurentia in the case of the Appalachians; Tarim–North China with Asia in the case of Altaids) ca. 250 Ma, during the completion of Pangea (Fig. 3). Likewise, the Yavapai-Penokean-Svecofennian orogen ended with the Grenvillian collisions ca. 1100 Ma. Beginning life at ca. 1900 Ma, this orogen was active for at least 800 m.y., perhaps the longest-lived accretionary orogen in Earth history. The Arabian-Nubian orogen is unique among the post-Archean orogens: Its total lifespan correlates with the breakup of the supercontinent Rodinia (Fig. 3).
The termination of Late Archean accretionary orogens is problematic, and the relatively short duration of these orogens (Fig. 1) appears to be unique to the Archean, a feature related perhaps to a global mantle plume event (Condie, 1998). As mentioned above, the Archean orogens correlate with formation of supercontinents and with mantle plume events. For those orogens that formed at or near 2.7 or 2.5 Ga, possible global mantle plume events may have been responsible for both their beginning and ending. A major problem that needs to be addressed is what happened on Archean cratons between the cessation of major accretion at 2.7 and 2.5 Ga and supercontinent breakup, some 200–300 m.y. later?
WHAT CONTROLS TERRANE LIFESPAN?
Several factors contribute to terrane lifespan. First is the tectonic setting or settings of a terrane. Some terranes, such as continental-margin arcs, form very close to a convergent boundary in or near an accretionary orogen, and hence may be short-lived before colliding with and suturing to a continent. As an example, most Yavapai terranes are remnants of continental-margin arcs or associated backarc basins (Condie, 1992), and they have among the shortest of all terrane lifespans (many <50 m.y.) (Fig. 1; Table 1). Not all continental-margin arcs have short lifespans, however, as exemplified by the Koyukuk terrane in the Cordillera with a lifespan of 140 m.y.
Another factor affecting terrane lifespan is the complexity of terrane history. Many terranes, as exemplified by most of those in the Appalachian and Cordilleran orogens have long, complex multitectonic setting histories, and many are superterranes (Plafker et al., 1989; Keppie, 1989; Condie and Chomiak, 1996; Hatcher, 2002; Murphy et al., 2004). Although many of these terranes have a long lifespan, periods of volcanism and sedimentation are separated by significant unconformities (Plafker and Berg, 1994; Plafker et al., 1989; McClelland et al., 1992; Murphy et al., 1999, 2004). For instance, Wrangellia, Chugach, and Peninsular terranes have major age gaps separating supracrustal successions. Although more difficult to recognize because of deformation and lack of fossil control, unconformities also are undoubtedly present in Precambrian terranes. In some cases, however, the total amount of time represented by unconformities in Precambrian terranes is considerably less than in Cordilleran and Appalachian terranes (for instance in the Yavapai orogen; Condie, 1992). In Cordilleran and Appalachian terranes, tectonic settings often differ between unconformity-bounded or fault-bounded rock packages. For instance, in the Alexander terrane in western Canada, the tectonic setting changed from an island arc in the late Paleozoic to a rift in the Triassic and back to an arc in the Cretaceous (Gehrels and Saleeby, 1987). In most terranes in the Yavapai-Penokean-Svecofennian orogen, however, tectonic setting appears to have remained the same throughout the short terrane lifespan (Condie, 1986, 1992). Some terranes, such as Avalonia in the Appalachians, began as a rifted Pan-African continental margin upon which several arcs were built prior to accretion to Laurentia (Murphy et al., 1999). Both Avalonia and Alexander have lifespans in excess of 300 m.y. (Table 1).
Another factor that may be important in controlling terrane lifespan is the amount of continental crust on the surface of Earth. For instance, the long terrane lifespans of Itsaq and Pilbara terranes may reflect the scarcity of continental crust to which they could accrete. And finally, the plate history in the ocean basin adjacent to an accretionary orogen is important in controlling terrane lifespan. For instance, in the case of Japan, from the Mesozoic onwards, ocean floor has converged on the Japanese islands, forming terranes composed of oceanic crust and trench sediment that are scraped off during subduction (Isozaki, 1997). Although occasional oceanic plateaus and islands were also accreted, no microcratons appear to have collided with Japan.
ACCRETION RATES IN ACCRETIONARY OROGENS
It is probable that accretionary orogens are the sites of most continental growth (Taylor and McLennan, 1985; Reymer and Schubert, 1986). Such growth occurs directly by addition of new crust in continental-margin arcs or by collision and accretion of juvenile terranes such as oceanic arcs, ophiolites, and oceanic plateaus (Fig. 2). In addition, repositioning of continental crust can occur by rifting blocks from older cratons and reaccretion of these at other locations in the same orogen or in different orogens. Such repositioning, of course, does not necessarily involve production of new crust, but in some instances new crust can form on microcratons by arc magmatism, before or after amalgamation with a continent.
Nd isotopes are useful in identifying juvenile crustal components in accretionary orogens. A plot of ϵNd in various rocks (excluding basement) from accretionary orogens ranging in age from ca. 3000 Ma to 100 Ma reveals a wide range of ϵNd values from positive to negative (Fig. 4). Most of the values, however, are positive (76%), indicating a large input of juvenile crust into accretionary orogens. Most accretionary orogens contain a mixture of terranes with juvenile and recycled components, with the ratio of these components changing with time and along strike in an orogen. The northern Appalachian and Variscan orogens are unusual in having many rocks with negative ϵNd(t), reflecting significant contamination of both igneous and sedimentary components with older crust (Murphy et al., 2004). Probably 50% of the terranes accreted in these orogens contain recycled older crust. In contrast, in the southern Appalachians (Tugaloo and Carolina terranes) and in the American Cordillera, most terranes have positive ϵNd(t), indicating a strong juvenile crustal input (Patchett and Gehrels, 1998; Bream et al., 2004). Remnants of oceanic arcs, ophiolites and oceanic plateaus are widespread in the Cordillera. In the Altaids in Mongolia, granitoids emplaced outside the Precambrian basement have juvenile ϵNd(t) values, whereas those emplaced within the Precambrian basement have negative ϵNd(t), suggesting contamination by the basement (Jahn et al., 2004). This is also true for the Paleoproterozoic terranes that were accreted or formed close to Archean cratons, such as in the Penokean of Laurentia and the Svecofennian of the Baltic shield (Bennett and DePaolo, 1987; Andersen and Sundvoll, 1995). Even at 2700 Ma, when a large volume of juvenile crust formed, some negative ϵNd data indicate reworking of older crust (Fig. 4). In the western Superior province in Canada only ∼50% of the crust was produced at 2700 Ma; the remainder formed at earlier times (Henry et al., 2000).
The distribution of ϵNd(t) in Figure 4 also clearly shows the episodic nature of juvenile crust production (Condie, 1998). Strong peaks, for instance, are evident at 100–200, 550–600, 1800–1900, 2100, 2500, and 2700 Ma, with notable gaps around 1000 Ma and 2400–2200 Ma. It is still not clear if these gaps reflect times of low crustal production or lack of preservation.
It is possible to estimate both the total accretion rate and the rate of juvenile crust production in accretionary orogens. Orogen widths are measured from geologic maps, and rates are calculated using accretion intervals (Table 1), average crustal thickness, and Nd isotopic data to estimate juvenile input. We assume an original terrane crustal thickness of 40 km, 10 km of which is now removed by erosion. In estimating the total amount of accreted material, the “thin-skinned effect” has been taken into account where seismic data are available, as for instance in the American Cordillera (Monger and Price, 2002; Snyder et al., 2002) and in the northern Appalachians (Clowes et al., 1992). In these orogens, seismic reflection data and Os isotopic compositions of mantle xenoliths (Peslier et al., 2000) suggest that terranes were thrust over Precambrian crust during accretion, and thus, the crust added cannot be assumed to extend to the Moho. In most instances, only 30%–70% of the crustal volume is newly accreted crust, and in our calculations we assume only 50% represents newly accreted material in the Cordilleran and Appalachian orogens. However, since more juvenile crust may be present in the Appalachians in offshore areas, the calculated juvenile crust production rate for the Appalachians should be considered a minimum. In other orogens, where there is no seismic evidence for or against older crust at depth, we assume 80%–100% for the fraction of accreted crust, using Nd isotope data in younger granitoids as a guide.
Results (Fig. 5) compare favorably with those published by Reymer and Schubert (1986) for four accretionary orogens. Both total accretion rate and juvenile crust production rate are higher in the Precambrian than afterwards, averaging between 100 and 200 km3/km/m.y. (Labrador and Barberton being the exceptions) (Fig. 5). In contrast, Phanerozoic rates are typically between 70 and 150 km3/km/m.y., NW China being the exception. It is noteworthy that crustal production (or at least preservation) does not parallel the cooling of Earth's mantle, which if it did, should show very high crustal production rates in the Early Archean. Accretion rates are highest in the Yilgarn and Superior provinces in the Late Archean where they exceed 300 km3/km/m.y. (Fig. 5). Juvenile crust production rates also are high in the Superior province, which may reflect a global mantle plume event at this time (Condie, 1998). It is important to note that the Yilgarn, Zimbabwe, and Itsaq orogens, which have the same accretion age as the Superior province (2700 Ma), do not show this high rate of juvenile crust production.
The lowest juvenile crust production rates occur in some Phanerozoic orogens, such as Japan and the southern Andes, and in the Archean Barberton orogen, all of which have rates <80 km3/km/m.y. (Fig. 5). The unusually low accretion and juvenile crust production rates in Japan may reflect, in part, the near absence of accreted continental-margin arcs and microcratons. A large number of older microcratons occur in the Andes. In other cases, such as Barberton, the calculated accretion interval may not apply to the entire craton, since the Kaapvaal craton continued to grow until ca. 2700 Ma (which increases the accretion interval from 50 to 280 m.y. and the volume of new crust by an order of magnitude) (Moser et al., 2001).
And finally, it is noteworthy that all juvenile crust production rates in accretionary orogens of all ages are greater than 20–40 km3/km/m.y., the growth rate of young oceanic arcs (Reymer and Schubert, 1986). This strongly supports the conclusion of Reymer and Schubert (1986) that accretion of oceanic arcs alone cannot account for production of juvenile crust in accretionary orogens. It is probable that a large contribution to juvenile crust production occurs in continental-margin arcs. An unknown, but possibly significant, volume of juvenile crust may also be added by accretion of oceanic plateaus (Condie, 1997).
Accretionary orogens are primary sites of juvenile continental crust production and have been active on Earth since the earliest Archean.
Orogen accretion intervals (lifetimes) range from ∼50 m.y. to over 300 m.y., with Archean intervals less than 100 m.y.
Average terrane lifespan in accretionary orogens is typically 100–200 m.y. in post–1 Ga orogens, 50–100 m.y. in pre–1 Ga Proterozoic orogens, and 70–700 m.y. in Archean orogens.
Accretionary orogens can be classified into two end members: A simple orogen is one that is characterized by rapid accretion of juvenile oceanic terranes with average terrane lifespan <100 m.y. A complex orogen comprises not only juvenile accreted components but also exotic microcratons and has an average terrane lifespan ≥100 m.y. Complex orogens may undergo extensive rifting and transcurrent faulting, which creates new terranes and moves them laterally along the orogen.
There is no simple relationship between onset and cessation of accretionary orogens and the supercontinent cycle, and many post-Archean orogens terminate with continent-continent collisions during supercontinent assembly. The short duration of Late Archean accretionary orogens (<70 m.y.) may reflect the short duration of global mantle plume events at 2.7 and 2.5 Ga.
Terrane lifespan is controlled by (1) terrane tectonic setting, (2) complexity of precollisional terrane history, (3) availability of continental crust on Earth, and (4) plate history of ocean basins adjacent to accretionary orogens.
Average accretion rates in accretionary orogens are 70–150 km3/km/m.y. in Phanerozoic orogens and 100–200 km3/km/m.y. in Precambrian orogens. Some orogens at 2.7 Ga have unusually high accretion rates greater than 300 km3/km/m.y., possibly related to a global mantle plume event.
Production rates of juvenile crust in accretionary orogens are typically 10%–30% lower than total accretion rates, but can be up to 50% lower in Phanerozoic orogens.
This paper was substantially improved from reviews by Bob Hatcher and Marvin Carlson. Hatcher also provided the author with age estimates of several terranes in the southern Appalachians as compiled in Table 1.
Figures & Tables
4-D Framework of Continental Crust
- accreting plate boundary
- continental crust
- cyclic processes
- Far East
- island arcs
- mantle plumes
- North America
- North American Cordillera
- oceanic lithosphere
- orogenic belts
- plate collision
- plate convergence
- plate tectonics
- rift zones