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"Johnson, Tim E"
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Feral youth
Follows ten teens who are left alone in the wilderness amid a three-day survival test.
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Metamorphism and the evolution of plate tectonics
Earth’s mantle convection, which facilitates planetary heat loss, is manifested at the surface as present-day plate tectonics
1
. When plate tectonics emerged and how it has evolved through time are two of the most fundamental and challenging questions in Earth science
1
–
4
. Metamorphic rocks—rocks that have experienced solid-state mineral transformations due to changes in pressure (
P
) and temperature (
T
)—record periods of burial, heating, exhumation and cooling that reflect the tectonic environments in which they formed
5
,
6
. Changes in the global distribution of metamorphic (
P
,
T
) conditions in the continental crust through time might therefore reflect the secular evolution of Earth’s tectonic processes. On modern Earth, convergent plate margins are characterized by metamorphic rocks that show a bimodal distribution of apparent thermal gradients (temperature change with depth; parameterized here as metamorphic
T/P
) in the form of paired metamorphic belts
5
, which is attributed to metamorphism near (low
T/P
) and away from (high
T/P
) subduction zones
5
,
6
. Here we show that Earth’s modern plate tectonic regime has developed gradually with secular cooling of the mantle since the Neoarchaean era, 2.5 billion years ago. We evaluate the emergence of bimodal metamorphism (as a proxy for secular change in plate tectonics) using a statistical evaluation of the distributions of metamorphic
T/P
through time. We find that the distribution of metamorphic
T/P
has gradually become wider and more distinctly bimodal from the Neoarchaean era to the present day, and the average metamorphic
T/P
has decreased since the Palaeoproterozoic era. Our results contrast with studies that inferred an abrupt transition in tectonic style in the Neoproterozoic era (about 0.7 billion years ago
1
,
7
,
8
) or that suggested that modern plate tectonics has operated since the Palaeoproterozoic era (about two billion years ago
9
–
12
) at the latest.
Variability in Earth’s thermal gradients, recorded by metamorphic rocks through time, shows that Earth’s modern plate tectonics developed gradually since the Neoarchaean era, three billion years ago.
Journal Article
Critical role of water in the formation of continental crust
Continental arcs are the sites of production of continental crust, but the origin of these magmatic systems is not well understood. Although a number of processes have been suggested to be important, the role of water migrating from slab to surface during subduction has been underappreciated. Directly below the Moho, hot (approximately 1,100 °C), hydrous basaltic magmas fractionate as they cool to the regional geotherm at 750 to 800 °C, ultimately solidifying as mafic underplates. Cooling and fractionation cause water to exsolve and ascend, triggering fluid-fluxed melting of overlying mafic underplates and other crust. Melting of prior mafic underplates buffers temperatures and generates the voluminous, juvenile low-K magmas of Cordilleran batholiths. These granitoid magmas comprise a low-temperature slurry of melt and residue, and recrystallize into silicic mush during adiabatic ascent. Such hydrous mushes are intermittently infused by hotter, more mafic magmas, which hybridize and facilitate ascent and, potentially, eruption. Fluid-fluxed melting overcomes many of the general petrological and geochemical problems associated with models dominated by fractional crystallization. The role of water during repeated episodes of mafic underplating is critical to generate the juvenile granitoid infrastructure of the continents.Migration of water from the slab to the surface during subduction is highlighted as a key process in the formation of continental crust.
Journal Article
Delamination and recycling of Archaean crust caused by gravitational instabilities
The volume of Archaean crust preserved at Earth’s surface today is low. Thermodynamic calculations and geodynamic modelling show that the thick, primary crust that would have formed on a much hotter Archaean Earth was denser than the underlying mantle, and would have therefore been recycled back into the mantle as drips.
Mantle temperatures during the Archaean eon were higher than today. As a consequence, the primary crust formed at the time is thought to have been extensive, thick and magnesium rich, and underlain by a highly residual mantle
1
. However, the preserved volume of this crust today is low, implying that much of it was recycled back into the mantle
2
. Furthermore, Archaean crust exposed today is composed mostly of tonalite–trondhjemite–granodiorite, indicative of a hydrated, low-magnesium basalt source
3
, suggesting that they were not directly generated from a magnesium-rich primary crust. Here we present thermodynamic calculations that indicate that the stable mineral assemblages expected to form at the base of a 45-km-thick, fully hydrated and anhydrous magnesium-rich crust are denser than the underlying, complementary residual mantle. We use two-dimensional geodynamic models to show that the base of magmatically over-thickened magnesium-rich crust, whether fully hydrated or anhydrous, would have been gravitationally unstable at mantle temperatures greater than 1,500–1,550 °C. The dense crust would drip down into the mantle, generating a return flow of asthenospheric mantle that melts to create more primary crust. Continued melting of over-thickened and dripping magnesium-rich crust, combined with fractionation of primary magmas, may have produced the hydrated magnesium-poor basalts necessary to source tonalite–trondhjemite–granodiorite melts. The residues of these processes, with an ultramafic composition, must now reside in the mantle.
Journal Article
Coexisting divergent and convergent plate boundary assemblages indicate plate tectonics in the Neoarchean
The coexistence of divergent (spreading ridge) and convergent (subduction zone) plate boundaries at which lithosphere is respectively generated and destroyed is the hallmark of plate tectonics. Here, we document temporally- and spatially-associated Neoarchean (2.55–2.51 Ga) rock assemblages with mid-ocean ridge and supra-subduction-zone origins from the Angou Complex, southern North China Craton. These assemblages record seafloor spreading and contemporaneous subduction initiation and mature arc magmatism, respectively, analogous to modern divergent and convergent plate boundary processes. Our results provide direct evidence for lateral plate motions in the late Neoarchean, and arguably the operation of plate tectonics, albeit with warmer than average Phanerozoic subduction geotherms. Further, we surmise that plate tectonic processes played an important role in shaping Earth’s surficial environments during the Neoarchean and Paleoproterozoic.
This study reports coexisting Neoarchean divergent and convergent plate boundary rock assemblages, providing new evidence for the operation of plate tectonics 2.55–2.51 billion years ago; and also suggests the subduction zone was warm then.
Journal Article
Oxygen isotopes trace the origins of Earth’s earliest continental crust
Much of the current volume of Earth’s continental crust had formed by the end of the Archaean eon
1
(2.5 billion years ago), through melting of hydrated basaltic rocks at depths of approximately 25–50 kilometres, forming sodic granites of the tonalite–trondhjemite–granodiorite (TTG) suite
2
–
6
. However, the geodynamic setting and processes involved are debated, with fundamental questions arising, such as how and from where the required water was added to deep-crustal TTG source regions
7
,
8
. In addition, there have been no reports of voluminous, homogeneous, basaltic sequences in preserved Archaean crust that are enriched enough in incompatible trace elements to be viable TTG sources
5
,
9
. Here we use variations in the oxygen isotope composition of zircon, coupled with whole-rock geochemistry, to identify two distinct groups of TTG. Strongly sodic TTGs represent the most-primitive magmas and contain zircon with oxygen isotope compositions that reflect source rocks that had been hydrated by primordial mantle-derived water. These primitive TTGs do not require a source highly enriched in incompatible trace elements, as ‘average’ TTG does. By contrast, less sodic ‘evolved’ TTGs require a source that is enriched in both water derived from the hydrosphere and also incompatible trace elements, which are linked to the introduction of hydrated magmas (sanukitoids) formed by melting of metasomatized mantle lithosphere. By concentrating on data from the Palaeoarchaean crust of the Pilbara Craton, we can discount a subduction setting
6
,
10
–
13
, and instead propose that hydrated and enriched near-surface basaltic rocks were introduced into the mantle through density-driven convective overturn of the crust. These results remove many of the paradoxical impediments to understanding early continental crust formation. Our work suggests that sufficient primordial water was already present in Earth’s early mafic crust to produce the primitive nuclei of the continents, with additional hydrated sources created through dynamic processes that are unique to the early Earth.
Oxygen isotopes and whole-rock geochemistry show that the water required to make Earth’s first continental crust was primordial and derived from the mantle, not surface water introduced by subduction.
Journal Article
No evidence for high-pressure melting of Earth’s crust in the Archean
Much of the present-day volume of Earth’s continental crust had formed by the end of the Archean Eon, 2.5 billion years ago, through the conversion of basaltic (mafic) crust into sodic granite of tonalite, trondhjemite and granodiorite (TTG) composition. Distinctive chemical signatures in a small proportion of these rocks, the so-called high-pressure TTG, are interpreted to indicate partial melting of hydrated crust at pressures above 1.5 GPa (>50 km depth), pressures typically not reached in post-Archean continental crust. These interpretations significantly influence views on early crustal evolution and the onset of plate tectonics. Here we show that high-pressure TTG did not form through melting of crust, but through fractionation of melts derived from metasomatically enriched lithospheric mantle. Although the remaining, and dominant, group of Archean TTG did form through melting of hydrated mafic crust, there is no evidence that this occurred at depths significantly greater than the ~40 km average thickness of modern continental crust.
Some of Earth’s earliest continental crust has been previously inferred to have formed from partial melting of hydrated mafic crust at pressures above 1.5 GPa (more than 50 km deep), pressures typically not reached in post-Archean continental crust. Here, the authors show that such high pressure signatures can result from melting of mantle sources rather than melting of crust, and they suggest there is a lack of evidence that Earth’s earliest crust melted at depths significantly below 40 km.
Journal Article
Changes in orogenic style and surface environment recorded in Paleoproterozoic foreland successions
The Earth’s interior and surficial systems underwent dramatic changes during the Paleoproterozoic, but the interaction between them remains poorly understood. Rocks deposited in orogenic foreland basins retain a record of the near surface to deep crustal processes that operate during subduction to collision and provide information on the interaction between plate tectonics and surface responses through time. Here, we document the depositional-to-deformational life cycle of a Paleoproterozoic foreland succession from the North China Craton. The succession was deposited in a foreland basin following ca. 2.50–2.47 Ga Altaid-style arc–microcontinent collision, and then converted to a fold-and-thrust belt at ca. 2.0–1.8 Ga due to Himalayan-style continent–continent collision. These two periods correspond to the assembly of supercratons in the late Archean and of the Paleoproterozoic supercontinent Columbia, respectively, which suggests that similar basins may have been common at the periphery of other cratons. The multiple stages of orogenesis and accompanying tectonic denudation and silicate weathering, as recorded by orogenic foreland basins, likely contributed to substantial changes in the hydrosphere, atmosphere, and biosphere known to have occurred during the Paleoproterozoic.
Two different styles of orogenesis during the Neoarchean and Paleoproterozoic are recorded in the depositional-to-deformational evolution of the orogenic foreland of the North China Craton, and would have differently changed the surface environment.
Journal Article
Evolution of the Neoproterozoic Kareim Basin, north Arabian – Nubian shield
The transition to continental collision c. 650 Ma induced the bimodal hypsometry of the Arabian-Nubian Shield (ANS), and triggered the formation of voluminous post-amalgamation basins. The intermontane Kareim Basin is a voluminous post-amalgamation depocenter within the ANS. It comprises a thick siliciclastic fill (~ 7 km thick) that accumulated over tens of millions of years during late Neoproterozoic East African orogeny related to the amalgamation of West and East Gondwana. The basin fill consists of four main facies associations (FA1 to FA4) associated with 11 siliciclastic lithofacies and one volcaniclastic lithofacies, which are interpreted as alluvial fan to lacustrine deposits that accumulated under humid to semi-arid conditions. A conglomerate-dominated lithofacies characterizes proximal alluvial fan deposits (FA1), whereas mid to distal alluvial fan strata are represented by braided stream sandstone-dominated lithofacies and conglomerate (FA2–3). Distal fan deposits are composed mainly of sandstone and fine-grained lacustrine sediments (FA4). Tectonically-induced unconformities separate three depositional stages in the Kareim Basin. The lower stage comprises three sandstone-dominant cycles, locally separated by unconformities. The middle stage of the basin represents a stage of syn-depositional tectonic inversion, consistent with the presence of recycled basal boulders derived from the lower stage, and a divergence in the dominant paleo-current directions. Furthermore, thrust faults, tilted and overturned older strata, and the first occurrence of material derived locally from Pan-African volcanic rocks (the Dokhan Volcanic Suite) and basement gneiss domes are additional clues for the syn-depositional tectonic inversion. The upper stage comprises conglomerate-dominant cycles, and represents the transition to post-collisional extension and rapid subsidence. Detrital zircon U–Pb ages constrain the syn-depositional inversion of the basin to later than c. 635 Ma, likely coinciding with the onset of collision between West with East Gondwana.
Journal Article
A Paleoarchaean impact crater in the Pilbara Craton, Western Australia
The role of meteorite impacts in the origin, modification, and destruction of crust during the first two billion years of Earth history (4.5–2.5 billion years ago; Ga) is disputed. Whereas some argue for a relatively minor contribution overall, others have proposed that individual giant impactors (>10–50 km diameter) can initiate subduction zones and deep mantle plumes, arguably triggering a chain of events that formed cratons, the ancient nuclei of the continents. The uncertainty is compounded by the seeming absence of impact structures older than 2.23 Ga, such that the evidence for the terrestrial impact flux in the Hadean and Archaean eons is circumstantial. Here, we report the discovery of shatter cones in a complex, dominantly metasedimentary layer, the Antarctic Creek Member (ACM), in the centre of the East Pilbara Terrane, Western Australia, which provide unequivocal evidence for a hypervelocity meteorite impact. The shocked rocks of the crater floor are overlain by (unshocked) carbonate breccias and pillow lavas, stratigraphically constraining the age of the impact to 3.47 Ga and confirming discovery of the only Archaean crater known thus far.
Shatter cones in rocks in the Pilbara craton provide unequivocal evidence for oldest known impact crater on Earth, which struck 3.5 billion years ago.
Journal Article