The Origin of the Continents

The origin of Earth's first continents and the onset of plate tectonics

Authors:

Hastie et al

Abstract:

The growth and recycling of continental crust has resulted in the chemical and thermal modification of Earth's mantle, hydrosphere, atmosphere, and biosphere for ∼4.0 b.y. However, knowledge of the protolith that gave rise to the first continents and whether the environment of formation was a subduction zone still remains unknown. Here, tonalite melts are formed in high P-T experiments in which primitive oceanic plateau starting material is used as an analogue for Eoarchean (3.6–4.0 Ga) oceanic crust generated at early spreading centers. The tonalites are produced at 1.6–2.2 GPa and 900–950 °C and are mixed with slab-derived aqueous fluids to generate melts that have compositions identical to that of Eoarchean continental crust. Our data support the idea that the first continents formed at ca. 4 Ga and subsequently, through the subduction and partial melting of ∼30–45-km-thick Eoarchean oceanic crust, modified Earth's mantle and Eoarchean environments and ecosystems.

Do Kimberlites Indicate the Start of Plate Tectonics?

Kimberlites and the start of plate tectonics

Authors:

Stern et al

Abstract:

We want to know when plate tectonics began and will consider any important Earth feature that shows significant temporal evolution. Kimberlites, the primary source of diamonds, are rare igneous features. We analyze their distribution throughout Earth history; most are young (∼95% are younger than 0.75 Ga), but rare examples are found as far back as the Archean (older than 2.5 Ga). Although there are differing explanations for this age asymmetry (lack of preservation, lack of exposure, fewer mantle plumes, or lack of old thick lithosphere in the Archean and Proterozoic), we suggest that kimberlite eruptions are a consequence of modern-style plate tectonics, in particular subduction of hydrated oceanic crust and sediments deep into the mantle. This recycling since the onset of modern-style plate tectonics ca. 1 Ga has massively increased mantle CO2 and H2O contents, leading to the rapid and explosive ascent of diamond-bearing kimberlite magmas. The age distribution of kimberlites, combined with other large-scale tectonic indicators that are prevalent only in the past ∼1 Ga (blueschists, glaucophane-bearing eclogites; coesite- or diamond-bearing ultrahigh-pressure metamorphic rocks; lawsonite-bearing metamorphic rocks; and jadeitites), indicates that plate tectonics, as observed today, has only operated for

The Recipe for Atmospheric Oxygen on Earth

Earth scientists from Rice University, Yale University and the University of Tokyo are offering a new answer to the long-standing question of how our planet acquired its oxygenated atmosphere.

Based on a new model that draws from research in diverse fields including petrology, geodynamics, volcanology and geochemistry, the team's findings were published online this week in Nature Geoscience. They suggest that the rise of oxygen in Earth's atmosphere was an inevitable consequence of the formation of continents in the presence of life and plate tectonics.

"It's really a very simple idea, but fully understanding it requires a good bit of background about how the Earth works," said study lead author Cin-Ty Lee, professor of Earth science at Rice. "The analogy I most often use is the leaky bathtub. The level of water in a bathtub is controlled by the rate of water flowing in through the faucet and the efficiency by which water leaks out through the drain. Plants and certain types of bacteria produce oxygen as a byproduct of photosynthesis. This oxygen production is balanced by the sink: reaction of oxygen with iron and sulfur in the Earth's crust and by back-reaction with organic carbon. For example, we breathe in oxygen and exhale carbon dioxide, essentially removing oxygen from the atmosphere. In short, the story of oxygen in our atmosphere comes down to understanding the sources and sinks, but the 3-billion-year narrative of how this actually unfolded is more complex."

Lee co-authored the study with Laurence Yeung and Adrian Lenardic, both of Rice, and with Yale's Ryan McKenzie and the University of Tokyo's Yusuke Yokoyama. The authors' explanations are based on a new model that suggests how atmospheric oxygen was added to Earth's atmosphere at two key times: one about 2 billion years ago and another about 600 million years ago.

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Ice Ages Really Triggered by Tectonic Activity?

For hundreds of millions of years, Earth's climate has remained on a fairly even keel, with some dramatic exceptions: Around 80 million years ago, the planet's temperature plummeted, along with carbon dioxide levels in the atmosphere. The Earth eventually recovered, only to swing back into the present-day ice age 50 million years ago.

Now geologists at MIT have identified the likely cause of both ice ages, as well as a natural mechanism for carbon sequestration. Just prior to both periods, massive tectonic collisions took place near the Earth's equator -- a tropical zone where rocks undergo heavy weathering due to frequent rain and other environmental conditions. This weathering involves chemical reactions that absorb a large amount of carbon dioxide from the atmosphere. The dramatic drawdown of carbon dioxide cooled the atmosphere, the new study suggests, and set the planet up for two ice ages, 80 million and 50 million years ago.

"Everybody agrees that on geological timescales over hundreds of millions of years, tectonics control the climate, but we didn't know how to connect this," says Oliver Jagoutz, associate professor of Earth, Atmospheric and Planetary Sciences (EAPS) at MIT. "I think we're the first ones to really link large-scale tectonic events to climate change."

Jagoutz and his colleagues, EAPS Professor Leigh Royden, and Francis McDonald of Harvard University, have published their findings in the Proceedings of the National Academy of Sciences.

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The tectonics of Titan: Global structural mapping from Cassini RADAR

The tectonics of Titan: Global structural mapping from Cassini RADAR

Authors:

Yung-Chun Liu et al

Abstract:

The Cassini RADAR mapper has imaged elevated mountain ridge belts on Titan with a linear-to-arcuate morphology indicative of a tectonic origin. Systematic geomorphologic mapping of the ridges in Synthetic Aperture RADAR (SAR) images reveals that the orientation of ridges is globally E–W and the ridges are more common near the equator than the poles. Comparison with a global topographic map reveals the equatorial ridges are found to lie preferentially at higher-than-average elevations. We conclude the most reasonable formation scenario for Titan’s ridges is that contractional tectonism built the ridges and thickened the icy lithosphere near the equator, causing regional uplift. The combination of global and regional tectonic events, likely contractional in nature, followed by erosion, aeolian activity, and enhanced sedimentation at mid-to-high latitudes, would have led to regional infilling and perhaps covering of some mountain features, thus shaping Titan’s tectonic landforms and surface morphology into what we see today.

Circulation in Titan’s seas may be Driven by the Sun and Methane Precipitation

Sun-stirred Kraken Mare: Circulation in Titan’s seas induced by solar heating and methane precipitation

Authors:

Tokano et al

Abstract:

Density-driven circulation in Titan’s seas forced by solar heating and methane evaporation/precipitation is simulated by an ocean circulation model. If the sea is transparent to sunlight, solar heating can induce anti-clockwise gyres near the sea surface and clockwise gyres near the sea bottom. The gyres are in geostrophic balance between the radially symmetric pressure gradient force and Coriolis force. If instead the sea is turbid and most sunlight is absorbed near the sea surface, the sea gets stratified in warm seasons and the circulation remains weak. Precipitation causes compositional stratification of the sea to an extent that the sea surface temperature can be lower than the sea interior temperature without causing a convective overturning. Non-uniform precipitation can also generate a latitudinal gradient in the methane mole fraction and density, which drives a meridional overturning with equatorward currents near the sea surface and poleward currents near the sea bottom. However, gyres are more ubiquitous than meridional overturning.

How Titans Fluids Might Influence its Tectonic Evolution

Role of fluids in the tectonic evolution of Titan

Authors:

Yung-Chun Liu et al

Abstract:

Detailed analyses of slopes and arcuate planform morphologies of Titan’s equatorial mountain ridge belts are consistent with formation by contractional tectonism. However, contractional structures in ice require large stresses (4–10 MPa), the sources of which are not likely to exist on Titan. Cassini spacecraft imagery reveals a methane-based hydrological cycle on Titan that likely includes movement of fluids through the subsurface. These crustal liquids may enable contractional tectonic features to form as groundwater has for thrust belts on Earth. In this study, we show that liquid hydrocarbons in Titan’s near subsurface can lead to fluid overpressures that facilitate contractional deformation at smaller stresses (less than 1 MPa) by significantly reducing the shear strength of materials. Titan’s crustal conditions with enhanced pore fluid pressures favor the formation of thrust faults and related folds in a contractional stress field. Thus, surface and near-surface hydrocarbon fluids made stable by a thick atmosphere may play a key role in the tectonic evolution of Titan.

The Contraction/Expansion History of Charon with implication for its Planetary Scale Tectonic Belt

The Contraction/Expansion History of Charon with implication for its Planetary Scale Tectonic Belt

Authors:

Malamud et al

Abstract:

The New-Horizons mission to the Kuiper Belt has recently revealed intriguing features on the surface of Charon, including a network of chasmata, cutting across or around a series of high topography features, conjoining to form a belt. It is proposed that this tectonic belt is a consequence of contraction/expansion episodes in the moon's evolution associated particularly with compaction, differentiation and geophysical reactions of the interior. The proposed scenario involves no need for solidification of a vast subsurface ocean and/or a warm initial state. This scenario is based on a new, detailed thermo-physical evolution model of Charon that includes multiple processes. According to the model, Charon experiences two contraction/expansion episodes in its history that may provide the proper environment for the formation of the tectonic belt. This outcome remains qualitatively the same even if we assume a different initial composition and mass. Two alternative explanations for the precise localization and orientation of Charon's tectonic belt are speculated.

Pondering the Precambrian #1

Archean:

A study used diamonds to suggest the start of plate tectonics was approximately 3.5 billion years ago during the PaleoArchean.

Carbon deposited in the PaleoArchean Australia appears to be biological in origin.

Another study claims plate tectonics started 3 billion years ago during the MesoArchean through the examination of granites and magnesium.

The Helen Iron Formation in Canada was formed during the NeoArchean, 2.75 billion years ago, in moderately deep water when underwater volcanism ceased for a time.

There is evidence of NeoArchean plate tectonics in India

PaleoProterozoic:

Oxygen levels during the PaleoProterozoic Great Oxygenation Event may have been higher than the following Proterozoic steady state.

MesoProterozoic:

Evidence of freshwater stromatolites have been found from the Stenian of Michigan in the Copper Harbor Conglomerate.

There is paleoclimatic evidence also from the Stenian of Michigan.  The paleosols appear to indicate it was a floodplain.

NeoProterozoic:

Evidence from China gives insight into marine plate subduction during the Tonian.

Evidence from the Doushantuo Formation in China suggest the Ediacaran there had a carbon anomaly and the phosphated embryos found predate the Gaskiers Glaciation.

Contrary Evidence From North China Craton That Plate Tectonics Didn't Start Until MesoProterozoic

Detrital zircon U-Pb, Lu-Hf, and O isotopes of the Wufoshan Group: Implications for episodic crustal growth and reworking of the southern North China craton

Authors:

Zhang et al

Abstract:

The Paleoproterozoic orogens of the North China Craton (NCC) provide important windows to evaluate the crustal evolution history during early Precambrian. Here we report results from SIMS zircon geochronology and oxygen isotopes, as well as LA-ICPMS zircon Hf isotopic analyses on detrital zircons from Mesoproterozoic-Neoproterozoic sandstones of the Wufoshan Group from the southern NCC to probe the Paleoarchean-Paleoproterozoic crustal evolution. Our data show episodic magmatism during the Paleoarchean-Neoarchean and Paleoproterozoic as inferred from the 207Pb/206Pb age range of 3514-2751 Ma and the well-defined 207Pb/206Pb weighted mean ages of 2669 ± 15 Ma, 2510 ± 8 Ma, 2402 ± 39 Ma, 2298 ± 36 Ma, 2179 ± 10 Ma, 2058 ± 16 Ma, 1940 ± 6 Ma, 1874 ± 7 Ma and 1819 ± 5 Ma. The similarity in U-Pb geochronology and Hf isotopes of the Archean detrital zircons from the Wufoshan Group to those from the Paleoproterozoic Songshan Group reveal that the 2669 Ma zircons record a major phase of Neoarchean crustal growth, and that 2510 Ma zircons correspond to the latest Neoarchean crustal reworking. The 18O-depleted isotopic features displayed by some detrital zircons are absent in those from Songshan Group, and together with the rarity of core-rim textures which were widespread in Archean zircons from the Songshan Group, we suggest that the source materials for the Mesoproterozoic-Neoproterozoic Wufoshan Group are different from the Archean cratonic source for the Paleoproterozoic Songshan Group. We propose that the zircons were probably sourced from the newly-formed Trans-North China Orogen (TNCO). Our Paleoproterozoic zircon data provide robust evidence for continuous and complex reworking of the Mesoarchean to early Neoarchean crust in the southern NCC during Early Paleoproterozoic at ca. 2402 Ma, 2298 Ma, and 2179 Ma. The Middle Paleoproterozoic ages (2058 Ma, 1940 Ma, and 1874 Ma) mark a partial cycle of oceanic crust subduction to continental collision during the formation of the TNCO. The extreme oxygen isotopic variation with the fairly restricted range in Hf isotopes demonstrates that the components from both high-temperature and the low-temperature rock-water interaction induced by the release of hydrothermal fluids from the subducted altered oceanic crust must have played a significant role during the growth of the zircons. The Paleoproterozoic thermal event at 1819 Ma records post-collisional uplifting, as evidenced by an apparent negative correlation in the ɛHf(t) vs. δ18O plot, possibly inherited from the mixing of the ancient crust and the juvenile input during the oceanic subduction and continental collision. Integrated with the information from granulite facies metamorphic rocks in the orogenic root of the TNCO, our data suggest that the episodic Paleoproterozoic magmatism was mainly induced by the tectonics of plate subduction and subsequent continental collision, leading to the final cratonization of the NCC. Following the Mesoproterozoic-Neoproterozoic sedimentation of the Paleoproterozoic zircons, except for the Pb loss during the subsequent thermal events, there was no major alteration in the Hf and oxygen isotopic compositions. Thus, the remarkably large variation of oxygen isotopes from the lower than normal mantle to the supracrustal value (δ18O = 2.28 to 10.09 ‰) in the Paleoproterozoic detrital zircons further demonstrates that these zircons came from the uplifted and exposed upper part of the TNCO. Our detrital zircon data provide convincing evidence for the repeated crustal reworking in Early Paleoproterozoic and the onset of modern-style plate tectonics in the NCC in Middle Paleoproterozoic.

More Evidence for Active Plate Tectonics and Subduction From NeoArchean/PaleoProterozoic North China

Petrology and geochemistry of the Guyang hornblendite complex in the Yinshan block, North China Craton: Implications for the melting of subduction-modified mantle

Abstract:

Ma et al

Abstract:

The hornblendite complexes hosted in the Guyang granite-greenstone terrane form part of the Neoarchean basement in the Western Block of the North China Craton. In this study, we focus on the largest one from this block, previously named the Guyang komatiite, and present results from lithological, geochronological and geochemical studies. The dominant lithology in the Guyang hornblendite complex is greenschist facies hornblendite, and can be divided into clinopyroxene hornblendite (∼75%) and olivine-orthopyroxene hornblendite (∼25%). The oldest calculated Re depletion model ages(TRD) of these hornblendites is of 2454 Ma, and the zircon U-Pb age of the wallrock is 2480 Ma, with single-stage depleted mantle Nd model ages TDM1(Nd) varying from 2.61 to 2.88 Ga. These data suggest that the Guyang hornblendite complexes formed during Neoarchean-Paleoproterozoic time. The hornblendites show low SiO2, high MgO contents and are enriched in Cr, Ni, and LREE with negative Ce anomalies and depleted in Ti, Nb and Ta. Their 187Os/188Os ratios range from 0.11145 to 0.11279, with γOs(2.5Ga) varying from −3.9 to +1.4. Geochemically, the olivine-orthopyroxene hornblendite shows little variation with typical cumulate feature. In contrast, the clinopyroxene hornblendite shows a large range of chemical variation. In the CaO vs. MgO and CaO/Al2O3 vs. MgO diagrams, the clinopyroxene hornblendite shows Opx fractionation trend. Combined with lithological information, we infer that the hornblendite magma was emplaced as a crystal mush, with orthopyroxene as cumulus phase. The Os and Nd isotope composition, negative Nb, Ta, Ce anomalies, and the relationships in Th/Yb-Nb/Yb diagram suggest that the Guyang hornblendites formed from a source mantle that was modified by subduction-related melts/fluids derived from a seawater-altered basaltic slab. Compared with typical Archean komatiites, the rocks of present study show significant differences in lithological and geochemical characteristics, and thus it cannot be named as komatiite. To explain these characteristics, we propose a geodynamic model involving ridge subduction and slab window mechanism to account for the formation of the Guyang hornblendite complex.

Does the North China Craton Show Evidence of NeoArchean Microblock Amalgamation?

Neoarchean convergent margin tectonics associated with microblock amalgamation in the North China Craton: Evidence from the Yishui Complex

Authors:

Li et al

Abstract:

Archean tectonic history of the North China Craton (NCC) involved complex processes of amalgamation of microcontinents along multiple subduction zones prior to the consolidation of the major crustal blocks and their assembly into unified cratonic architecture. Here we report a suite of granitoids, diabase, metabasalts, volcanic tuff, banded iron formations and quartzite from the Yishui Complex along the southern margin of the Jiaoliao microblock within the Eastern Block of the NCC. The geochemical features of the magmatic suite are consistent with calc-alkaline magmatism in a convergent margin setting. In tectonic discrimination diagrams, the mafic suite shows variable IAB, MORB and OIB affinities typical of rocks formed in an arc-related subduction environment. Zircon grains in most of the rocks from Yishui Complex display core-rim texture with the cores showing magmatic crystallization and the narrow structureless rims corresponding to metamorphic overgrowth. The 207Pb/206Pb ages of magmatic zircons show 2504±19 Ma for the volcanic tuff, 2581±21 Ma for the granitoid, 2501±19 Ma for the metavolcanics, 2537±38 Ma for the pyroxenite, and 2506±13 Ma for the diabase. Metamorphism is constrained from the 2451±18 Ma and 2466±23 Ma age groups in the metavolcanics and (meta-) pyroxenites. Zircons from BIF show multiple population with the oldest showing a spot age of 2503 Ma, followed by a number of distinct groups of Paleoproterozoic zircons corresponding to later thermal events. The oldest population of magmatic zircons from the quartzite shows 207Pb/206Pb mean age of 2495±24 Ma. The dominantly positive εHf(t) values of the magmatic zircons from the Yishui suite are broadly consistent with a depleted mantle source with only minor input of crustal components. Their Hf crustal residence ages (TDMC) range from 2586 to 3181 Ma and Hf depleted mantle model ages(TDM) are in the range of 2548-2927 Ma. The data indicate that magma production involved Meso- to Neoarchean juvenile sources within a continental arc setting, suggesting the Jiaoliao microblock as one of the ancient continental nuclei in the NCC. We trace the continuity of a Neoarchean subduction system along the western and southern margins of the Jiaoliao microblock with convergence of the Qianhuai and Xuhuai microblocks towards the Jiaoliao microblock with subduction-accretion-collision during the Archean- Proterozoic transition

Evidence of Subduction During the NeoArchean

Petrogenesis of intermediate volcanic assemblages from the Shebandowan greenstone belt, Superior Province: Evidence for subduction during the Neoarchean

Author:

Lodge

Abstract:

Research on the petrogenesis of andesites and their implications for geodynamic setting are an important facet to understanding controversial tectonic processes during the Archean. The genesis of Archean intermediate volcanic rocks and their relationship to mantle- and crust-derived melts can either support subduction-dominated tectonic processes or plume-crust interactions. This study describes the lithogeochemistry and Nd-isotopic composition of intermediate volcanic assemblages in the Shebandowan greenstone belt of the Wawa-Abitibi terrane that were deposited prior to deformation and tectonic assembly of the Superior Province. The intermediate rocks of the Shebandowan greenstone belt are unique in that they are voluminous, are relatively weakly deformed and are low metamorphic grade, and contain significant amounts of magnesian andesites and adakites.

The major and trace element geochemistry of the intermediate assemblages share many of the geochemical characteristics of modern volcanic and continental arcs. These features include enriched Th/Nb and Th/La ratios, steeply dipping rare earth patterns, and pronounced negative Ti and Nb anomalies on primitive mantle-normalized diagrams. Additionally, these rocks also contain distinct positive Pb and Cs anomalies without evidence of major mobility of these elements during secondary processes. Neodymium isotopic analyses indicate the interaction with older crust and show good correlations with evolved Nd-isotopic values with other crustal contamination proxies such as Th/Ce and Ti/Sc ratios. Modelling mixing and assimilation-fractional crystallization interactions between plume-derived and crustal melts reveal that plume-crust interactions cannot explain the compositional array obtained from the intermediate rocks in the Shebandowan greenstone belt.

A compilation of U-Pb geochronology for the Shebandowan greenstone belt reveals that tholeiitic/komatiite dominated strata are slightly older than the intermediate-dominated strata. Given that these domains are structurally separated, it is very likely that the Shebandowan greenstone belt was formed in two different geodynamic settings. These different geodynamic settings have important implications for the metallogeny of the belt and explain the relative enrichment of Au and Ni-Cu mineralization in the Shebandowan greenstone belt relative to other ca. 2720 Ma assemblages in the Wawa-Abitibi terrane.

Plate Tectonics Were Initiated by Mantle Plumes

Plate tectonics on the Earth triggered by plume-induced subduction initiation

Authors:

Gerya et al

Abstract:

Scientific theories of how subduction and plate tectonics began on Earth—and what the tectonic structure of Earth was before this—remain enigmatic and contentious. Understanding viable scenarios for the onset of subduction and plate tectonics is hampered by the fact that subduction initiation processes must have been markedly different before the onset of global plate tectonics because most present-day subduction initiation mechanisms require acting plate forces and existing zones of lithospheric weakness, which are both consequences of plate tectonics. However, plume-induced subduction initiation could have started the first subduction zone without the help of plate tectonics. Here, we test this mechanism using high-resolution three-dimensional numerical thermomechanical modelling. We demonstrate that three key physical factors combine to trigger self-sustained subduction: (1) a strong, negatively buoyant oceanic lithosphere; (2) focused magmatic weakening and thinning of lithosphere above the plume; and (3) lubrication of the slab interface by hydrated crust. We also show that plume-induced subduction could only have been feasible in the hotter early Earth for old oceanic plates. In contrast, younger plates favoured episodic lithospheric drips rather than self-sustained subduction and global plate tectonics.

A Plate Tectonic Scenario for the Iapetus and Rheic Oceans

A plate tectonic scenario for the Iapetus and Rheic oceans

Author:

Domeier

Abstract:

The tectonics, dynamics and biogeographic landscape of the early Paleozoic were dominated by the opening and expansion of one large ocean—the Rheic—and the diminution to terminal closure of another—Iapetus. An understanding of the evolution of these oceans is thus central to an understanding of the early Paleozoic, but their chronicle also presents a rich temporal profile of the Wilson cycle, illustrating continental-scale rifting, microcontinent formation, ocean basin development, arc accretion and continent-continent collision. Nevertheless, contemporary paleogeographic models of the Iapetus and Rheic oceans remain mostly schematic or spatiotemporally disjointed, which limits their utility and hinders their testing. Moreover, many of the important kinematic and dynamic aspects of the evolution of these oceans are impossible to unambiguously resolve from a conceptual perspective and the existing models unsurprisingly present a host of contradictory scenarios. With the specific aim to resolve some of the uncertainties in the evolution of this early Paleozoic domain, and a broader aim to instigate the application of quantitative kinematic models to the early Paleozoic, I present a new plate tectonic model for the Iapetus and Rheic oceans. The model has realistic tectonic plates, which include oceanic lithosphere, and are defined by explicit and rigorously managed plate boundaries, the nature and kinematics of which are derived from geological evidence and plate tectonic principles. Accompanying the presentation and discussion of the plate model, an extensive review of the underlying geological and paleogeographic data is also presented.

Lessons From Venus: Impact Origin of Archean Cratons

Impact origin of Archean cratons

Author:

Hansen

Abstract:

Archean cratons consist of crustal granite-greenstone terrains (GGTs) coupled to roots of strong, buoyant cratonic lithospheric mantle (CLM). Although this association is unique to the Archean and formed from ca. 4.0 to 2.5 Ga, the origins of terrestrial cratons are debated. I propose that crustal plateaus, quasi-circular craton-like features (∼1400−2400 km diameter, 0.5−4 km high), on Earth’s sister planet Venus might serve as analogs for Archean cratons. Crustal plateaus, which are isostatically supported by a compositionally controlled low-density root, host a distinctive surface called ribbon-tessera terrain. Ribbon-tessera also occurs as arcuate-shaped inliers in the Venus lowlands, widely interpreted as remnants of rootless crustal plateaus. Within each crustal plateau, surface ribbon-tessera terrain comprises a vast igneous province analogous to terrestrial GGTs, and the plateau root is analogous to CLM. Crustal plateaus and ribbon-tessera terrain collectively represent Venus’ oldest preserved features and surfaces, and they formed during an ancient period of globally thin lithosphere. To explain the linked features of crustal plateaus, a bolide impact hypothesis has been proposed in which a large bolide pierces ancient thin lithosphere, leading to massive partial melting in the sublithospheric mantle. In this model, melt escapes to the surface, forming an enormous lava pond, which evolves to form ribbon-tessera terrain; mantle melt residue forms a strong, resilient buoyant root, leading to plateau support and long-term stability of an individual crustal plateau. Building on the similarity of GGT−CLM and Venus crustal plateaus, I propose an exogenic hypothesis for Archean craton formation in which a large bolide pierces thin Archean lithosphere, causing localized high-temperature, high-fraction partial melting in the sublithospheric mantle; melt rises, forming an igneous province that evolves as a GGT, and melt residue develops a complementary CLM. By this mechanism, Archean cratons may have formed in a spatially and temporally punctuated fashion at a time when large bolides showered Archean Earth.

Short PR.

A Unified Nomenclature for Enceladus' Tectonic Surface Structures

A unified nomenclature for tectonic structures on the surface of Enceladus

Authors:

Nahm et al

Abstract:

Enceladus has experienced widespread and diverse tectonic deformation. The diversity and nonuniform distribution of structures visible on the surface attests to a complicated and perhaps long-lived tectonic history. Currently, no fundamental classification scheme based on morphology exists for the tectonic structures on Enceladus, which limits analysis and discussion of tectonic structures and regional and global tectonic histories. Characterization and classification of structures on Enceladus is an important first step in understanding its tectonic history. Here, we propose a global morphologic (i.e., nongenetic) classification scheme for tectonic structures on Enceladus. Five classes of tectonic structures on Enceladus have been identified based on images and limited topographic data: troughs, scarps, chasmata, ridges, and bands. Morphological variation of structures exists within these classes, reinforcing the complicated tectonic history of this small body.

Plate Tectonics Shut Down on Venus Before Great Volcanic Resurfacing

The history of tectonism on Venus: A stratigraphic analysis

Authors:

Ivanov et al

Abstract:

The surface of Venus displays several tectonized terrains in which the morphologic characteristics of the original materials are almost completely erased by superposed tectonic structures whose large dimensions (»100 km) suggest formation related to mantle convection. The characteristics of these tectonized terrains are in contrast to volcanic units in which tectonic structures are less significant or absent and thus do not obscure the volcanic character of the units. We describe the temporal distribution of tectonized terrains, their stratigraphic relationships with volcanic units, and how these outline the major episodes in the geological evolution of Venus. Five major tectonized units make up ~20% of the planet: 1) tessera (t, 7.3%), 2) densely lineated plains (pdl, 1.6%), 3) ridged plains/Ridge belts (pr/rb, 2.4%), 4) groove belts (gb, 8.1%), and 5) rift zones (rz, 5.0%). Clear relationships of relative age are often seen among the tectonic and volcanic units at the global scale and define three contrasting regimes of volcanic and tectonic resurfacing. The majority of tectonized terrains (t through gb) are the products of tectonic resurfacing and are embayed by the vast volcanic plains and, thus, are older. There are no units with either mildly- or non-tectonized surfaces that interleave the tectonic terrains, which would be expected if the tectonic resurfacing operated only during specific repetitive phases in discrete regions. These tectonized terrains (t through gb) thus define a tectonically dominated regime of resurfacing that occurred at a global-scale near the beginning of the observable geological history of Venus. This ancient tectonic regime began with formation of tessera and was followed by formation of pdl and pr/rb. Groove belts formed near the end of this regime. Branches of groove belts compose the tectonic components of many coronae, suggesting that these features are genetically related (e.g., mutual development of mantle diapirs and zones of extension) and that coronae may have punctuated the final stages of the ancient tectonic regime. This regime was followed by emplacement of the vast volcanic plains, such as shield and regional plains, the surfaces of which are extensively deformed by the global network of wrinkle ridges. Emplacement of the plains defines the second, volcanically dominated regime, representing a time when surface tectonic deformation related to the mantle convection waned. Rift zones are the stratigraphically youngest manifestations of regional-scale tectonic deformation on Venus. Rifts are spatially and temporarily associated with the youngest lava flows and often cut the crest areas of large, but isolated, dome-shaped rises. Structures of rift zones always cut the surface of the vast plains, which means that rifts are separated in time from the ancient tectonic regime, post-date the regional plains, and represent a new phase of tectonism that was contemporaneous with the late volcanism of lobate plains. Rift zones and lobate plains define the third, network rifting-volcanism regime, of resurfacing that was related to late stages of evolution of the dome-shaped rises.

Explaining the Plume Differences Between Europa and Enceladus

Linking Europa's plume activity to tides, tectonics, and liquid water

Authors:

Rhoden et al

Abstract:

Much of the geologic activity preserved on Europa's icy surface has been attributed to tidal deformation, mainly due to Europa's eccentric orbit. Although the surface is geologically young (30 - 80 Myr), there is little information as to whether tidally-driven surface processes are ongoing. However, a recent detection of water vapor near Europa's south pole suggests that it may be geologically active. Initial observations indicated that Europa's plume eruptions are time-variable and may be linked to its tidal cycle. Saturn's moon, Enceladus, which shares many similar traits with Europa, displays tidally-modulated plume eruptions, which bolstered this interpretation. However, additional observations of Europa at the same time in its orbit failed to yield a plume detection, casting doubt on the tidal control hypothesis. The purpose of this study is to analyze the timing of plume eruptions within the context of Europa's tidal cycle to determine whether such a link exists and examine the inferred similarities and differences between plume activity on Europa and Enceladus.

The Link Between Tectonics & Climate in Miocene Neogene Asian Interior's Aridification

Late Miocene stepwise aridification in the Asian interior and the interplay between tectonics and climate

Authors:

Sun et al

Abstract:

The mid-latitudinal central Asian continent is characterized by large sand deserts and Gobi (stony desert). In this context, it is of interest to study the timing and forcing mechanisms of aridification in the region. Here we present multiple geochemical climatic proxies from late Cenozoic strata in the Tarim Basin of northwestern China, a region sensitive to climatic change. The results yield long-term climatic records covering a time interval of 13.3 to 2.5 Ma. We find that a general trend towards a dry climate was superimposed by two stepwise aridification events, the first lesser aridity phase occurred at ~ 7–5.3 Ma and the second extreme aridity episode was initiated at ~ 5.3 Ma. Based on the correlation between climatic change and regional tectonic events, we propose a mechanism to explain the climatic variations. The general long-term drying trend since the mid-Miocene was a response to global climatic cooling, while the stepwise aridification since the latest Miocene was controlled mainly by regional tectonic uplift.