 | Supercontinent tectonics and biogeochemical cycle: A matter of life and death |
from http://www.sciencedirect.com/science/article/pii/S167498711000006X
Whereas supercontinent cycle describes the periodic assembly and disruption of land masses, the Wilson cycle refers to the opening and closing of large oceans. Although both are considered as complimentary, the processes associated with the assembly, dispersal and reorganization of supercontinents are more complex. Hoffman (1991) proposed the concept of inside-out process as a mechanism of assembling crustal fragments after the breakup of an earlier supercontinent. If the supercontinent was rifted on one side to bear a passive margin such as in the case of the Atlantic Ocean, then the opposite side becomes a consuming boundary along the continents, such as in the case of the Pacific margin. With time, the Pacific Ocean would shrink and finally close by the passive collision of two continents to generate a large continental mass. In the case of Atlantic, initially both continental margins are passive, but later turn to active margins; the ocean shrinks in size and may totally disappear. On the other hand, the Indian Ocean has two different types of margins simultaneously in operation, active and passive, transporting the northern continental margin of Gondwana by the Atlantic-type process and amalgamating the rifted continents to the southern margin of Asia by the Pacific-type process. Therefore, the Indian Ocean-type process illustrates simultaneous continental breakup and continental amalgamation exemplifying the inside-in mechanism (Murphy and Nance, 2005). Similar simultaneous rifting and accretion on different margins were also proposed in the case of the Paleoproterozoic supercontinent Columbia by [Rogers and Santosh, 2002] and [Rogers and Santosh, 2004]. The Tethyan process started at least by the Permian to Triassic time and operated simultaneously with the Pacific process to the east. Subsequently, double-sided subduction started to define the frontier of the future supercontinent (Maruyama et al., 2007). The Tethyan region is an example for the inside-in reassembly of supercontinents ( [Hoffman, 1991], [Murphy and Nance, 2003], [Murphy and Nance, 2005] and [Murphy et al., 2009]), whereas the Pacific region is related to the inside-out configuration. Thus, the process of completion of a supercontinent would involve a combination of both introversion and extroversion. Such a combination operated in the case of Rodinia assembly, and is also predicted in the amalgamation of the future supercontinent Amasia (see [Maruyama et al., 2007] and [Santosh et al., 2009]).
In another recent work, Murphy et al. (2009) discussed the two geodynamically distinct tracts of oceanic lithosphere generated during the breakup of supercontinents: a relatively young interior ocean floor that develops between the dispersing continents, and a relatively old exterior ocean floor, which surrounded the supercontinent before breakup. The geologic and Sm/Nd isotopic record synthesized in their study suggests that supercontinents may form by two end-member mechanisms: introversion, in which interior ocean floor is preferentially subducted, and extroversion, in which exterior ocean floor is preferentially subducted. Murphy et al. (2009) speculated that superplumes, perhaps driven by slab avalanche events, could occasionally overwhelm topdown geodynamics, imposing a geoid high over a pre-existing geoid low and causing the dispersing continents to reverse their directions to produce an introverted supercontinent. http://www.sciencedirect.com/science/article/pii/S167498711000006X
In a slightly different model, Silver and Behn (2008) proposed two modes of ocean closure during supercontinent formation which they termed as P-type (Pacific-type) and A-type (Atlantic-type) (Fig. 1). In A-type closure, the subduction begins at a passive margin of the internal ocean and the internal ocean begins to close. In P-type closure, the internal ocean continues to grow and becomes an enlarged ocean basin. When a supercontinent is assembled through A-type closure, the internal ocean closes, shutting down subduction zones in the internal ocean, while subduction and sea-floor spreading in the external ocean continues. In P-type closure, the external ocean closes, shutting down all subduction. Similar to the case of introversion and extroversion, the P-type and A-type closure represent two end-member cases, and the actual mode of closure may likely involve a combination of the two processes. http://www.sciencedirect.com/science/article/pii/S167498711000006X
The complexity of introversion and extroversion and the P-type and A-type processes associated with supercontinent cycles discussed above also reflects upon the destruction and creation of varied environments which impact on the extinction or survival and diversification of life. Future studies should integrate these conceptual models in unraveling the planets life history, which would lead to a better understanding of the geological control on environmental and biological processes.
2.2. Supercontinent breakup
Models on the fragmentation of supercontinents can be broadly divided into plume-absent and plume-related processes. The plume-absent models consider that the thermal insulation effect of supercontinents (Anderson, 1982) leads to a temperature increase beneath the large landmass, which results in continental rifting and breakup. The concept of a thermal blanket (Fig. 2) envisages the breakup of supercontinents through radiogenic heating (e.g., Gurnis, 1988). Granites contain higher K, U and Th compared to mantle peridotite. Recent studies recognize that substantial volume of crust of granitic composition is subducted during the assembly of supercontinents through arc subduction, sediment trapped subduction, and subduction erosion. This TTG (tonalite-trondhjemite-granite) material is dragged down and is thought to accumulate in the mantle transition zone (Senshu et al., 2009). It is possible that the radiogenic elements in the subducted TTG crust heat up the overlying mantle with time to initiate continental rifting and dispersion leading to the opening of oceans. Komabayashi et al. (2009) performed phase assemblage analysis in the mid-oceanic ridge basalt (MORB)-anorthosite-TTG system down to core-mantle boundary (CMB) conditions. Their results show that all these materials can be subducted even to the CMB, thus leading to the during the assembly of supercontinents through arc subduction, sediment trapped subduction, and subduction erosion. This TTG (tonalite-trondhjemite-granite) material is dragged down and is thought to accumulate in the mantle transition zone (Senshu et al., 2009). It is possible that the radiogenic elements in the subducted TTG crust heat up the overlying mantle with time to initiate continental rifting and dispersion leading to the opening of oceans. Komabayashi et al. (2009) performed phase assemblage analysis in the mid-oceanic ridge basalt (MORB)-anorthosite-TTG system down to core-mantle boundary (CMB) conditions. Their results show that all these materials can be subducted even to the CMB, thus leading to the development of compositional stratification in the D layer
Coltice et al. (2007) tested the hypothesis that the assembly of supercontinents would force larger length scales and, therefore, drive the underlying mantle to higher temperatures. They investigated the intrinsic temperature difference between supercontinent and dispersed continents. The position of the continents was fixed and an equilibrium temperature field was computed by stacking the temperature fields over several billion years to remove the time-dependent features and in order to obtain a statistical steady state. Their results show that subcontinental temperature correlates inversely with the number of continents, and their convection modeling an internally heated led them to conclude that the assembly of continents into supercontinents would naturally lead to mantle global warming without the involvement of hot active plumes.
Supercontinents effectively self-destruct in response to the buildup of heat and resultant magmatism, since these effects significantly weaken the lithosphere and make it more susceptible to breakup in response to regional tectonics. Vaughan and Storey (2007) presented a conceptual model where, the subduction relation to the formation of supercontinents focuses on narrow areas of the 660 km mantle discontinuity, triggering superplume events which eventually lead to the fragmentation of the continental mass above. This supercontinent-triggered superplume mechanism for continental breakup was examined in the light of the Mesozoic fragmentation of Pangea-Gondwana. Vaughan and Storey (2007) summarized evidence for a superplume event that occurred between 227 and 183 Ma ago during the Late Triassic- Early Jurassic break, which is comparable in scale with those in the Late Proterozoic (ca. 800 Ma) and during Cretaceous time (ca. 12080 Ma).
During the amalgamation of continental fragments, the subducted oceanic lithosphere of intervening oceans either moves down to the deep mantle or gets horizontally flattened as stagnant slabs in the mantle transition zone ( [Fukao et al., 1992], [Fukao et al., 2001], [Fukao et al., 2009], [van der Hilst et al., 1997], [Grand, 2002], [Zhao, 2004] and [Zhao, 2009]). Blobs of these stagnant slabs sink down into the deep mantle and accumulate as slab graveyards at the core-mantle boundary. Zhao (2004) synthesized a P-wave tomographic image for the Western Pacific region, along a transect covering Beijing to Tokyo, where ca. 1200 km-long stagnant slabs are seen floating in the mantle transition zone between 410 and 660 km, also termed as the 660 km phase boundary. The image shows the presence of a high P-wave velocity anomaly close to the bottom of the mantle and immediately above the core-mantle boundary (CMB) which has been interpreted as a slab graveyard ( [Maruyama et al., 2007] and [Maruyama et al., 2009]). The recycled oceanic lithosphere at the core-mantle boundary contributes potential fuel for generating superplumes which rise from the core-mantle interface to the uppermost mantle (Fig. 4), penetrating the mantle transition zone and eventually giving rise to hot spots (e.g., Maruyama et al., 2007). Highly resolved seismic tomography models clarify the configuration and amplitude of the velocity anomaly of two superplumes beneath the southern Africasouthern Atlantic Ocean and the southern Pacific.