Pondering the Precambrian #25

Proterozoic:

NeoProterozoic:

Cobalt deposits may be 150 million years younger than previously thought and due to an orogenic event.

There's evidence suggesting scavenging was the origin of predation during the Ediacaran.

There's evidence of shifting from continental collision to a subduction in Africa during the Ediacaran.

There were very diverse microfossils from the Ediacaran Doushantuo Formation.

The fossilization method of the Ediacaran gets examined for the earliest metazoans.

Fossils from the Ediacaran site at Nilpena, Australia are biological snapshots of benthic communities.

Ediacaran discoid fossils from the Anti-Atlas Mountains in Africa have been found.

In the early Ediacaran ocean, there seems to have been a global zinc signature.

Did the complex forms of the Ediacaran arise in the benthic depths of the ocean because there were stable temperatures?

In China, the carbonate caps suggest a oxygen recovery after the Marinoan Glaciation ended.

The oxygen isotopes from the Cryogenian meteoric water are similar to today's.

A group attempts to model the global temperatures from the Tonian to the present.

MesoProterozoic:

Fossils from Brazil suggest two different microbial habitats from the Stenian at the site.

There is evidence of oxygenated lakes 1.1 billion years ago during the Stenian.

There is evidence of oxygenation in the form of red beds that were created during an orogeny around the Stenian/Ectasian boundary.

PaleoProterozoic:

Statherian microfossils from China suggest eukaryotes were moderately diverse by that point in time.

Evidence of the Huronian Glaciation has been discovered in China.  This may have been the first time a Snowball Earth scenario took place.

Phosphorite deposits appear to have come from a microbial reef in the Siderian of Australia.

Archean:

Features from 2.7 billion year old stromatolite formations hint at the presence of oxygen.

The is evidence of a shift from vertical to horizontal plate tectonics in the Archean.

During the Mesoarchean the bottom waters of the ocean appear to have been anoxic.

There appears to have been a cryptic craton core that emerged in the Paleoarchean.

Aerobic photosynthesis might have arisen 3.5 billion years ago during the Paleoarchean.

Origin of Life:

Abiotic amino acids from the Earth's mantle have been observed for the first time.

Are liquid crystal matrices an analog to the early protocells?

Orogeny on Io: how do you Make Mountains on Jupiter's Moon?

Mountains aren't the first thing that hit you when you look at images of Jupiter's innermost moon, Io. But once you absorb the fact that the moon is slathered in sulfurous lava erupted from 400 active volcanoes, you might turn your attention to scattered bumps and lumps that turn out, on closer inspection, to be Io's version of mountains.There are about 100 of them, and they don't look anything like the low lying volcanoes.

They also don't look like mountains on our home world. While we favor majestic ranges stretching from horizon to horizon, the mountains on Io are isolated peaks of great height that jut up out of nowhere. From space, they look rather like the blocky chips in the fancier kind of chocolate chip cookie.

For planetary geophysicists like William McKinnon, professor of earth and planetary science in Arts & Sciences at Washington University in St. Louis, the mountains of Io are an intriguing puzzle. By what process consistent with everything that is known about Io could these bizarre mountains have formed?

Since Io buries the evidence of its tectonic processes under a continually refreshed coating of lava (adding 5 inches a decade), the scientists have turned increasingly to computer simulations to solve the problem. In the May 16 online advance issue of Nature Geoscience, McKinnon and Michael T. Bland, a research space scientist at the USGS Astrogeology Science Center in Flagstaff, Ariz., publish a computer model that is able to make numerical mountains that look much like the jutting rock slabs on Io.

link.

Tectonic Implications From Ural Russia for the Columbia Supercontinent

U–Pb zircon geochronology and geochemistry of Paleoproterozoic magmatic suite from East Sarmatian Orogen: tectonic implications on Columbia supercontinent

Authors:

Terentiev et al

Abstract:

Extensive Paleoproterozoic magmatism occurred in the East Sarmatian Orogen (ESO), located at the junction of the Sarmatian and Volga–Ural segments of the East European Craton, generating numerous felsic to mafic-ultramafic complexes. Here we report geochemical and zircon U-Pb age data from a suite of ultramafic–mafic and diorite–granodiorite intrusive rocks from this orogen. SHRIMP U–Pb dating of zircons yielded the following concordant ages for the magmatic suites: (1) 2085 ± 17 to 2093 ± 20 Ma (sub-volcanic high-Mg intrusions of SHMB-like melanorite–melaquartz-diorite–melagranodiorites and quartz-diorite–tonalite–granodiorites); (2) 2072.4 ± 8, 2073 ± 8.3, and 2068 ± 13 Ma (ultramafic–mafic–diorite layered intrusions); (3) 2053 ± 14 to 2058 ± 22 Ma (magnesian–ferroan quartz-diorite–tonalite–granodiorite intrusions) and (4) 2035 ± 18 Ma (ferroan norite–diorite intrusions). The trace and rare earth element geochemistry indicate arc-related features. The enrichment in LREE and LILE in the mafic-ultramafic and diorite-granodiorite suites is correlated with melting of metasomatised lithosphere mantle along with involvement of crustal components in their petrogenesis. The third magmatic event conforms to the age of the post-collision S- and A-granitoids of the region. Our data indicate two major magmatic events in the post-collisional setting, the former immediately after metamorphism with the formation of schistosity in the host rocks, and the latter two during the culmination of the orogenic cycle in the Voronezh Crystalline Massif.

The duration of the Paleoproterozoic orogeny during the collision of the Sarmatian and Volga–Ural segments of the East European Craton is estimated to be approximately 100 million years from ∼2140 to ∼2035 Ma. We propose that the layered orogenic plutons were derived from relatively deeper mantle source, whereas the sub-volcanic rock series are derivatives of the slightly shallow mantle source. The Paleoproterozoic collision in the region broadly coincides with timing of amalgamation of the global supercontinent Columbia.

The Race Between Orogeny and Glacial Erosion in the St Elias Mountains, Alaska During the Pleistocene Quaternary

Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska

Authors:

Gulick et al

Abstract:

Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8–1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2–0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50–80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale.

Pollen Evidence Suggests Late Eocene Paleogene Hoh Xil Basin (Tibet) was Below 2,000 m

A Late-Eocene palynological record from the Hoh Xil Basin, northern Tibetan Plateau, and its implications for stratigraphic age, paleoclimate and paleoelevation

Authors:

Miao et al

Abstract:

The Hoh Xil Basin, lying in the central Tibetan Plateau, is key to understanding the Cenozoic tectonics, paleoelevation and paleoclimate changes that have occurred in the Tibetan Plateau since the collision of the Indian and Asian tectonic plates. However, the stratigraphic age and paleoelevation indicated by the sediments of the Hoh Xil Basin remain hotly debated. Here we report on one palynological record from the TTH-C section, extracted from the Yaxicuo Group (the stratigraphic unit between the Fenghuoshan and Wudaoliang groups), and analyze its implications for stratigraphic age, paleoclimate and paleoelevation in the Hoh Xil Basin. The record shows that palynological taxa are mainly dominated by xerophytic Ephedripites, Nitrariadites (Nitrariapollis) and Chenopodipollis, with few ferns and conifers. Rich morphologies correspond well with those in the Xia Ganchaigou Formation (Fm) of the Qaidam Basin to the north. Palynological percentages are well correlated with the middle member of the Xia Ganchaigou Fm in the Qaidam Basin as well as the lower member of the Mahalagou Fm in the Xining Basin to the northeast. The ages of the middle member of the Xia Ganchaigou and lower member of the Mahalagou Fms from these two basins are both identical to the Bartonian Stage (~ 40-37 Ma) of the Late Eocene, according to their respective high-resolution magnetostratigraphic dating. This means that the age of the Yaxicuo Group at least covers the Bartonian Stage. Besides the Qaidam and Xining basins, the palynological assemblages of the TTH-C section are also similar to those of three other sites (the Jiuquan, Tu-ha and Hetao basins), indicating similarly arid climates dominated by a northwestern Chinese subtropical high, and a relatively low paleoelevation in the Hoh Xil Basin (mostly less than 2,000 m a.s.l.) in the Late Eocene.

Orosirian Paleoproterozoic Fe-rich Mafic Sills From the Zhongtiao Mountains

Geochronology and geochemistry of the Paleoproterozoic Fe-rich mafic sills from the Zhongtiao Mountains: Petrogenesis and tectonic implications

Authors:

Li et al

Abstract:

Despite the existence of the Trans-North China Orogen (TNCO) in the middle part of the North China Craton (NCC) is widely accepted, its formation timing and mechanism are still controversial. A suite of Paleoproterozoic Fe-rich mafic sills in the Zhongtiao Mountains (ZTM) in the southern TNCO has been investigated to constrain the formation of the TNCO. Our new zircon U–Pb dating on these sills has yielded a Paleoproterozoic age of 1897 ± 30 Ma, and our new geochemical data have suggested an arc-affinity with significant Nb, Ta and Ti depletions. Geochemical evidence further reveals that the mantle source of the ZTM Fe-rich mafic sills was mainly composed of spinel lherzolite, and may have been metasomatized by reduced subduction-related fluids. With these age and geochemical data, we propose a genetic model for the ZTM Fe-rich mafic sills: (1) Subduction-related fluids may have transported Fe from the subducted plate and the overlying mantle wedge to generate the Fe-enriched source at the upper part of mantle wedge; (2) Partial melting of the Fe-enriched source may have generated the primary magmas of the Fe-rich mafic sills, which has evolved under a low oxygen fugacity (fO2) condition and eventually formed the Fe-rich mafic sills. The ZTM Fe-rich mafic sills have moderate Ti/V values (28–36) and La/Nb values (2.3–3.3), suggesting that they may have formed in a back-arc setting. The existence of a back-arc basin at ∼1900 Ma in the ZTM indicates that the TNCO was formed during the ∼1850 Ma final collision between the Western- and Eastern blocks of the NCC.

Evidence of the Oblique Collision of Volgo-Sarmatia and Fennoscandia Continents During the Assembly of PaleoProterozoic Supercontinent Columbia

Trans-Baltic Palaeoproterozoic correlations towards the reconstruction of supercontinent Columbia/Nuna

Authors:

Bogdanova et al

Abstract:

A comparative study of the Palaeoproterozoic accretionary the central and southern parts Svecofennian orogen in the Baltic/Fennoscandian Shield and the platform area to the east and south of the Baltic Sea indicates that at least of these parts of the orogen are built up of several NW-SE trending, 100 to 300 km wide tectonic megadomains separated from each other and complicated by major zones of mostly dextral shearing. The generation of these zones occurred successively between 1.86 and 1.75 Ga, concomitantly with continuing crustal accretion consistently younging toward the southwest. Even considering the distorting presence of a number of microcontinents, this indicates the one-time existence and repeated episodic activity of a master subduction zone stepwise falling back toward the present south-southwest. At 1.82–1.80 Ga, the continent-continent oblique collision of megacontinent Volgo-Sarmatia with the preexisting Fennoscandia interrupted the accretionary growth of the crust in the Svecofennian orogen. In the west, the system of Svecofennian tectonic domains and shear zones is delimited by 1.70–1.55 Ga rock belts marking part of the Laurentia-Greenland-Baltica margin of Columbia. Altogether, the present U-Pb zircon datings and studies of key rocks and structures in the South Baltic region allow more detailed Trans-Baltic correlation and the creation of new integrated models of the structural and tectonic evolution of the Svecofennian orogen in particular and northern Europe in general. The new findings will be important also in the continuing study of supercontinent formation and supercontinent cycles, and the drifting of Palaeoproterozoic protocontinents during the assembly of Columbia/Nuna.

NeoArchean Skjoldungen Orogen Appears to be a Modern Continental Collision

Exhumation rates in the Archean from pressure–time paths: Example from the Skjoldungen Orogen (SE Greenland)

Authors:

Berger et al

Abstract:

The discussions on the orogenic evolution during Earth's history converge to the question of a different thermal structure in the Archean compared to the Phanerozoic and the applicability of the plate tectonic paradigm. However, geothermal structures are transient in orogens and are difficult to translate into large-scale tectonics and exhumation rates. Therefore, we propose depth–time data in the Archean Skjoldungen Orogen (SE Greenland, North Atlantic Craton) that allow for reconstruction of an exhumation rate independent of geothermal gradients. The resulting exhumation rate of ca. 0.4 km/Ma is similar to exhumation rates during erosion-controlled processes in modern orogens. These exhumation rates can only be established by erosion time constants similar to modern orogens. The occurrence of erosion-controlled exhumation is best explained by a stiff foreland promoting localized deformation in the orogen. Therefore, a switch from magmatic-dominated processes to localized deformation is proposed in the Skjoldungen Orogen area. This is supported by a change in magma composition and volume, from widespread granodiorite to localized alkaline intrusions. In addition, the involved metasedimentary rocks include detrital zircons of the only 50 Ma older foreland, which also correspond to erosion and tectonics as in modern orogens, i.e. flysh-type sediments. Relatively fast exhumation rates and the structural-magmatic evolution of the Neoarchean Skjoldungen Orogen thus indicate modern-style tectonic processes where stiff Mesoarchean continental crust forms a foreland to a collisional orogen instead of typical accretionary tectonics of weak island arc-like terranes in granite-greenstone terranes.

Is There a Link Between Ural Orogeny Preadapt Archosaurs to Hypoxic Conditions?

Animal evolution and atmospheric pO2: is there a link between gradual animal adaptation to terrain elevation due to Ural orogeny and survival of subsequent hypoxic periods?

Author:

Kurbel

Abstract:

Considering evolution of terrestrial animals as something happening only on flat continental plains seems wrong. Many mountains have arisen and disappeared over the geologic time scale, so in all periods some areas of high altitude existed, with reduced oxygen pressure (pO2) and increased aridity. During orogeny, animal species of the raising terrain can slowly adapt to reduced oxygen levels.

This review proposes that animal evolution was often driven by atmospheric oxygen availability. Transitions of insect ancestors and amphibians out of water are here interpreted as events forced by the lack of oxygen in shallow and warm water during Devonian. Hyperoxia during early Carboniferous allowed giant insects to be predators of lowlands, forcing small amphibians to move to higher terrains, unsuitable to large insects due to reduced pO2. In arid mountainous habitats, ascended animals evolved in early reptiles with more efficient lungs and improved circulation. Animals with alveolar lungs became the mammalian ancestors, while those with respiratory duct lungs developed in archosaurs. In this interpretation, limb precursors of wings and pneumatised bones might have been adaptations for moving on steep slopes.

Ural mountains have risen to an estimated height of 3000 m between 318 and 251 Mya. The earliest archosaurs have been found on the European Ural side, estimated 275 Myr old. It is proposed that Ural orogeny slowly elevated several highland habitats within the modern Ural region to heights above 2500 m. Since this process took near 60 Myr, animals in these habitats fully to adapted to hypoxia.

The protracted P-Tr hypoxic extinction event killed many aquatic and terrestrial animals. Devastated lowland areas were repopulated by mammaliaformes that came down from mountainous areas. Archosaurs were better adapted to very low pO2, so they were forced to descend to the sea level later when the lack of oxygen became severe. During the Triassic period, when the relative content of O2 reduced to near 12%, archosaurs prevailed as only animals that could cope with profound hypoxia at the sea level. Their diverse descendants has become dominant terrestrial animals, until the K-Pg extinction due to meteor impact.

Evidence of Minas Accretionary Orogeny From Siderian PaleoProterozoic

A juvenile accretion episode (2.35-2.32 Ga) in the Mineiro belt and its role to the Minas accretionary orogeny: zircon U-Pb-Hf and geochemical evidences

Authors:

Teixeira et al

Abstract:

Zircon U-Pb-Hf and geochemical data provide new clues for a juvenile TTG segment in the Mineiro belt, outlined by published information about the nearby 2.35 Ga Lagoa Dourada suite. The Resende Costa Orthogneiss and coeval rocks yield U-Pb crystallization ages from 2351 ± 48 to 2317 ± 16 Ma and coherent Sm-Nd TDM ages between 2.3 and 2.5 Ga. These rocks show ɛNd(2.35Ga) and ɛSr(2.35Ga) values that suggest derivation from a short-lived, slightly depleted source akin to products of island arcs, whereas the geochemistry is consistent with a tholeiitic source with minor crustal assimilation. The positive to negative zircon ɛHf(2.35Ga) suggest the subordinate involvement of crustal components during the hypothesized c. 2.35 Ga slab subduction event. In a similar manner, metabasalts (lower unit) of the adjoining Congonhas-Itaverava belt show Nd-Sr isotopic and chemical constraints indicating derivation from an early Paleoproterozoic tholeiitic source subjected to crustal assimilation. A coeval metagraywacke from the upper unit yields U-Pb zircon detrital ages as young as 2349 + 14 Ma suggesting the maximum depositional age for the precursor basin (foreland setting?) whilst most of the grains indicate contribution from nearby Archean sources. The Resende Costa-Lagoa Dourada rocks characterize the precocious arc magmatism of the Minas accretionary orogeny. This composite orogeny created the Mineiro belt and the adjoining Mantiqueira and Juiz de Fora belts in the 2.35-2.00 Ga interval. Due to such a polycyclic framework the Resende Costa Orthogneiss and coeval rocks show multiple metamorphic overprints dated at 2140 and 2050 Ma. The new and compiled data and regional geologic correlations provide evidence for a growing Paleoproterozoic landmass in the South America continent.

Miocene Neogene Tarim Basin Drying due to Tibetan Plateau Uplift

Late Miocene episodic lakes in the arid Tarim Basin, western China

Authors:

Liu et al

Abstract:

The Tibetan Plateau uplift and Cenozoic global cooling are thought to induce enhanced aridification in the Asian interior. Although the onset of Asian desertification is proposed to have started in the earliest Miocene, prevailing desert environment in the Tarim Basin, currently providing much of the Asian eolian dust sources, is only a geologically recent phenomenon. Here we report episodic occurrences of lacustrine environments during the Late Miocene and investigate how the episodic lakes vanished in the basin. Our oxygen isotopic (δ18O) record demonstrates that before the prevailing desert environment, episodic changes frequently alternating between lacustrine and fluvial-eolian environments can be linked to orbital variations. Wetter lacustrine phases generally corresponded to periods of high eccentricity and possibly high obliquity, and vice versa, suggesting a temperature control on the regional moisture level on orbital timescales. Boron isotopic (δ11B) and δ18O records, together with other geochemical indicators, consistently show that the episodic lakes finally dried up at ∼4.9 million years ago (Ma), permanently and irreversibly. Although the episodic occurrences of lakes appear to be linked to orbitally induced global climatic changes, the plateau (Tibetan, Pamir, and Tianshan) uplift was primarily responsible for the final vanishing of the episodic lakes in the Tarim Basin, occurring at a relatively warm, stable climate period.

Gondwana's Ediacaran NeoProterozoic Afro-Brazilian "Himalayas" Found

Scientists have found evidence for a huge mountain range that sustained an explosion of life on Earth 600 million years ago.

The mountain range was similar in scale to the Himalayas and spanned at least 2,500 kilometres of modern west Africa and northeast Brazil, which at that time were part of the supercontinent Gondwana.

"Just like the Himalayas, this range was eroded intensely because it was so huge. As the sediments washed into the oceans they provided the perfect nutrients for life to flourish," said Professor Daniela Rubatto of the Research School of Earth Sciences at The Australian National University (ANU).

"Scientists have speculated that such a large mountain range must have been feeding the oceans because of the way life thrived and ocean chemistry changed at this time, and finally we have found it."

The discovery is earliest evidence of Himalayan-scale mountains on Earth.

"Although the mountains have long since washed away, rocks from their roots told the story of the ancient mountain range's grandeur," said co-researcher Professor Joerg Hermann.

"The range was formed by two continents colliding. During this collision, rocks from the crust were pushed around 100 kilometres deep into the mantle, where the high temperatures and pressures formed new minerals."

link.

Profiling a Paleogene Orogen in Western North America

Profile of a paleo-orogen: High topography across the present-day Basin and Range from 40 to 23 Ma

Authors:

Cassel et al

Abstract:

Records of past topography connect Earth's deep interior to the surface, reflecting the distribution of heat and mass, past crustal structure, and plate interactions. Many tectonic reconstructions of the North American Cordillera suggest the presence of an Altiplano-like plateau in the location of the modern Basin and Range, with conflicting timing and mechanisms for the onset of surface-lowering extension and orogen collapse. Here we show, through a paleotopographic profile, that from the Eocene to the Oligocene a high, broad orogen stretched across Nevada, with a distinct crest that divided a continuous westward-draining slope extending to central California from an internally drained eastern Nevada plateau. This paleo-orogen maintained demonstrably higher-than-modern elevations, reaching 3500 m in the late Oligocene. Despite the long-term high gravitational potential energy of the crust supporting this topography, surface-lowering extension did not occur until the transition to a transform margin changed the external kinematic framework of the system. Maximum surface lowering was spatially decoupled from brittle upper crustal extension, requiring a large component of mid-crustal flow.

Thermally Extreme Orogeny & Evolution of the Australian Musgrave Region From the Statherian PaleoProterozoic to the Ectasian MesoProterozoic

The burning heart - the Proterozoic geology and geological evolution of the west Musgrave Region, central Australia

Authors:

Howard et al

Abstract:

he Musgrave Province is one of the most geodynamically significant of Australia’s Proterozoic orogenic belts, lying at the intersection of the continent’s three cratonic elements – the West, North and South Australian Cratons. While remoteness and cultural sensitivity have slowed geological research into this region, recent collaborative programs in Western Australia (the west Musgrave Province) have done much to address this. This Focus Review provides a synthesis of this, and previous, work investigating the Mesoproterozoic to Neoproterozoic geological evolution of the province. The Musgrave Province is a Mesoproterozoic to Neoproterozoic belt dominated by granites formed and deformed during several major events. A cryptic juvenile basement is exposed mainly in the east Musgrave Province as c. 1600 – 1550 Ma orthogneiss and in the west Musgrave Province as isolated outcrops of granulite-facies metagranites of the c. 1575 Ma Warlawurru Supersuite. Zircon Hf-isotopic data suggest an earlier major juvenile crust-forming event at c. 1950–1900 Ma. There is, however, no evidence that the province evolved over Archean crust. The c. 1600 – 1550 Ma period probably involved evolution within a primitive arc setting, perhaps developed on c. 1950–1900 Ma oceanic or oceanic-arc crust. Voluminous calc-alkaline plutonism was accompanied by clastic and volcaniclastic basin formation during the 1345–1293 Ma Mount West Orogeny. This stage traced the evolution of a continental arc reflecting the final amalgamation of the combined North and West Australian Craton with the South Australian Craton. The intervening c. 1400 Ma primitive crust - the Madura Province – on which the proto-Musgrave Province had evolved, was consumed during amalgamation. The thickened crust resulting from this accretion was drastically thinned at the beginning of the c. 1220–1150 Ma Musgrave Orogeny as this central part of the new combined craton entered an extraordinary period of high heat flow characterised by c. 100 m.y. of ultrahigh-temperature metamorphism and high-temperature, anhydrous, alkali-calcic magmatism sourced from MASH chambers developed at the base of the thinned crust. The ridged cratonic architecture and a massive accumulation of high radiogenic heat producing granites within the mid crust perpetuated a thin crustal regime. Voluminous magmatism was again triggered during the c. 1090–1040 Ma Giles Event with the evolution of the magmatism-dominated, Ngaanyatjarra Rift. This event was likely initiated through renewed movement along translithospheric faults that intersected the thermally perturbed Musgrave Province, pinned at a cratonic junction. Mantle-derived bimodal magmatism extended more or less continuously for 50 m.y., producing one of the world’s largest layered mafic intrusions and supervolcano-sized additions of juvenile felsic crust, in the form of alkali-calcic to alkali, A-type, rhyolite deposits. Together, the Albany – Fraser Orogen, which developed over the southern margin of the West Australian Craton, and the Musgrave Province mark the preserved edge of the North and West Australian Craton. These two belts show remarkable chronological links between c. 1345 and 1150 Ma but contrasting histories before and after that period. Their period of shared evolution reflects collision and accretion of the South Australian Craton, but their tectonic setting and basement geology throughout that event were very different.

Evidence of Ridge Subduction From Ediacaran NeoProterozoic Namibia

Fingerprints of late Neoproterozoic ridge subduction in the Pan–African Damara belt, Namibia

Authors:

Meneghnini et al

Abstract:

Subduction of mid-ocean ridges is a common feature in recent convergent margins, but is rarely documented in Proterozoic to Paleozoic orogenic belts. Here we describe evidence for ridge-trench interaction in the deeply eroded late Neoproterozoic Damara orogenic belt, central Namibia. The earliest interaction is indicated by primary intrusive contacts between amphibolite facies mid-ocean ridge metabasalts and trench metasediments. U-Pb zircon ages of 550–540 Ma from syntectonic granites in the forearc indicate the timing of partial melting and mafic underplating of the prism in response to ridge subduction. The thermal peak in the Damara belt, associated widespread granitic and alkalic plutonism, and hydrothermal activity coincide with the waning stages of tectonism at 530–520 Ma and are interpreted to indicate slab window widening and slab delamination. We suggest that the proposed two-stage thermal evolution of the Damara belt, comprising latest Neoproterozoic ridge subduction and early Cambrian slab delamination, represents a fingerprint of ridge subduction in ancient orogens.

Tibetan Plataeu Zanda Basin was Significantly Lower in the Early Plioecene Neogene

Early Pliocene paleo-altimetry of the Zanda Basin indicated by a sporopollen record

Authors:

Wu et al

Abstract:

Whether the uplift of the Tibetan Plateau (TP) led to the origin and development of the Indian monsoon is still in dispute. A helpful indicator would be greater knowledge about the chronological development of paleo-altimetry. The Zanda Basin, located on the SW margin of the TP, forms one of the most extensive Cenozoic sedimentary basins in Tibet. The modern climate of this area is controlled by the Indian Monsoon circulation system. It is thus an appropriate region for the study of paleo-altimetry, in addition to research into the relation between the TP and the Indian Monsoon. Sporopollen analyses of early Pliocene lacustrine strata in the Zanda Basin show that gymnosperm pollen is dominant (av. 77%) and mainly composed of Picea (av. 27.2%) and Abies (av. 24.2%); fern spores occupy a lesser proportion (av. 17.6%), while angiosperm pollen including temperate, sub-tropical broadleaved trees, shrubs and herbs, accounts for only a small proportion of the assemblage. This sporopollen assemblage suggests that sub-alpine dark coniferous forests were distributed around the basin and grow in a cool and wet climate, rather than in a modern alpine steppe environment. These characteristics indicate that the Zanda Basin's altitude throughout the early Pliocene was most probably lower than it is at present.

Swedish Knaften-Barsele arc Formed, Accreted onto Karelian Continent During Orosirian PaleoProterozoic

Hafnium isotope evidence for early-Proterozoic volcanic arc reworking in the Skellefte district (northern Sweden) and implications for the Svecofennian orogen

Authors:

Guitreau et al

Abstract:

The Skellefte district is a seemingly juvenile and heavily mineralized crustal domain in northern Sweden that formed between 1.90 and 1.87 Ga. It is commonly interpreted as a volcanic arc deposited on a basement (known variously as the Bothnian or the Knaften-Barsele group) that could be represented by older rocks (1.96-1.94 Ga) found in the vicinity. In order to understand the potential genetic relationship between the arc and the basement, Hf and Pb isotopes in magmatic zircons from key lithologies were measured by solution multi-collector inductively-coupled plasma mass spectrometry. It is shown that both geological groups display similar Hf isotope compositions, which translate into decreasing ɛHf with time. Overall, the data are compatible with reworking of the Knaften-Barsele arc to produce the Skellefte rocks over a short time interval from 1.90 to 1.87 Ga in a context of crustal extension with ongoing subduction. When the data presented here are integrated with general models of tectonic evolution of the Svecofennian orogen, they fit a scenario in which the juvenile Knaften-Barsele arc formed between 1.96 and 1.94 Ga and became accreted onto the Karelian continent located further north at about 1.92-1.91 Ga. Systematic north to south variations in Pb, Nd, and Hf isotope compositions throughout the Svecofennides, interpreted as resulting from an increase in Archean crust involvement towards the south, indicate a genetic link between the Proterozoic crustal domains of Sweden and Finland.

Orogenic Continental Collisions in Carboniferous Spain

SHRIMP U-Pb zircon dating of the Valencia del Ventoso plutonic complex, Ossa-Morena Zone, SW Iberia: Early Carboniferous intra-orogenic extension-related ‘calc-alkaline’ magmatism

Authors:

Cambesses et al

Abstract:

Mantle- and crustal-derived orogenic magmas provide insights into sources tapped during terrane accretion and supercontinent amalgamation. During Pangea formation volumetrically minor, but common, mafic-intermediate ‘calc-alkaline’ stocks intruded coeval granitoid plutons throughout the European Variscan. The Iberian Peninsula preserves the westernmost segment of this upper Paleozoic orogeny that resulted from collision of Laurentia and Gondwana. Whether the widespread calc-alkaline mafic–intermediate stocks may be used as petrogenetic tectonomagmatic markers of ancient subduction zones is an important question. Early Carboniferous calc-alkaline plutonic rocks from the Ossa-Morena Zone (OMZ), a northern Gondwana continental block, have traditionally been related to subduction. However, recent studies suggest that the tectonomagmatic regime at that time was continental collision. In this context, a link is made between Early Carboniferous intraplating of ultramafic and mafic sills into the mid-crust, the IBERSEIS Reflective Body (IRB), and the Variscan magmatism in the Olivenza-Monesterio antiform of the OMZ. Here we show that the OMZ Valencia del Ventoso plutonic complex comprising alkaline, calc-alkaline, metaluminous and peraluminous compositions formed during the Visean-Bashkirian, c. 334-320 Ma SHRIMP U-Pb zircon. The calc-alkaline character is, apparently, a result of intra-orogenic extension-related interaction of mantle-derived alkaline mafic and crustal-derived peraluminous felsic magma during terrane accretion. The new age is some c. 15–30 million years younger than previously defined and so coeval with IRB formation. All evidence points towards active subduction having ceased prior to the latest Devonian–earliest Carboniferous. Thus the younger age places the Olivenza-Monesterio antiform calc-alkaline magmatism after collision of the OMZ and the South Portuguese Zone, to the south. During Pangea formation other, comparable, Early Carboniferous extensional events have been described in adjacent terranes in northern Gondwana and Avalonia-Laurentia. Whether orogenic mafic magmatism provokes or results from crustal melting apparently depends on the process controlling, and the timing, of the mantle melting event.

Did Orogeny Cause the Eocene-Oligocene Paleogene Climate Change?

Orogeny forced terrestrial climate variation during the late Eocene–early Oligocene in Europe

Authors:

Kocsis et al

Abstract

Terrestrial climatic data reflect variable and often conflicting responses to the global cooling event at the Eocene-Oligocene transition (ca. 34 Ma). Stable isotopic compositions of the tooth enamel of large, water-dependent, herbivorous terrestrial mammals are investigated here to better understand the European continental climate during the late Eocene–early Oligocene. High 18OPO4 and δ13C values reflect a semiarid climate and ecosystem in the late Eocene. In the west-southwest region of Europe, these conditions prevailed until at least 33 Ma, after which it became more humid. A similar change was recorded north of the Alpine thrust, but it occurred 2 m.y. earlier. The north and west-southwest regions show a significant offset in δ18OPO4 composition between 35 and 31 Ma, indicating the influence of different air trajectories with different moisture sources (Atlantic versus Tethys). This also marks the presence of an orographic height in central Europe from the latest Eocene. After 31 Ma, a large drop in δ18OPO4 is registered, explained by altitude-induced fractionation on meteoric water isotopic composition. The related paleoaltitude change is estimated to be 1200 m, and the uplift could have taken place along the Alpine-Dinaridic orogenic system.

Rhyacian Paleoproterozoic Mountain Building Before the Columbia Supercontinent

The Paleoproterozoic Campinorte Arc: tectonic evolution of a central Brazil pre-Columbia orogeny

Authors:

de Oliveira Cordeiro et al

Abstract:

The Campinorte Arc is a poorly exposed 2.19 to 2.07 Ga Paleoproterozoic terrane in contact with the Neoproterozoic Goiás Magmatic Arc by the Rio dos Bois Fault in the northern Brasília Belt, Central Brazil. The Campinorte Arc is divided into Pau de Mel Suite metatonalites to metamonzogranites and the Campinorte volcano-sedimentary Sequence. Pau de Mel Suite whole rock geochemistry indicates at least three separate coeval parental magmas compatible with arc signatures. U-Pb geochronology data of paragranulite and mafic granulite exposed in the region as part of the Campinorte Arc provide additional information on this Paleoproterozoic orogenic cycle. The formation and preservation of these granulites was due to tectonic switching of the back arc basin and consequent lithospheric thinning from 2.14 to 2.09 Ga with metamorphic peak from 2.11 to 2.08 Ga. A Pau de Mel Suite granodiorite sample dated at c.a. 2.08 Ga indicates post-peak magmatism. The arc was thereafter rapidly contracted preserving Paleoproterozoic high metamorphic grade mineral assemblages. Additionally, the Campinorte Arc and the neighbouring Crixás/Guarinos/Faina greenstone belts may have shared the same source of sediments as attested by geographic proximity, coeval maximum sedimentation, felsic volcanism and the occurrence of similar rock types. Gravimetric and seismic data also support a common basin hypothesis. The formation of the Campinorte Arc is contemporaneous to other northern Brasília Belt basement terranes and, along with similar arcs within and at the São Francisco Craton edges, indicate a continental crust formation event that eventually led to the assemblage of Columbia.