Microcontinent Research Articles

Shear-wave splitting observations: Implications toward the variable anisotropic structure of the upper mantle below the Kachchh rift zone, Gujarat, India

The shear-wave splitting parameters (i.e. fast axis orientation (ψ) and delay time (δt)) are measured at twelve broadband stations in the Kachchh rift zone (KRZ), Gujarat, India, using SKS/SKKS core phases. The average splitting parameters (ψ, δt) vary from (49.41 ± 5.88, 1.63 ± 0.28) at Bhachau (i.e., 4 km SE of the 2001 Bhuj mainshock location) to (103.78 ± 12.82, 1.91 ± 0.35) at Mandvi near the coast. The present study reveals a mean ψ of (76.91o ± 4.57o) with a mean δt of 1.84 ± 0.30 s, for the KRZ. The observed large mean delay 1.84 ± 0.30 s for the region suggests a combined anisotropic contribution from both the lithospheric frozen anisotropy as well as the asthenospheric flow induced anisotropy. Inferred thicknesses of the anisotropic layers from the estimated δt vary from 160 km (in the central KRZ) to 269 km (in the surrounding unrifted zones (SURZ)). The measured deviation of ψ from the absolute plate motion (APM) direction of the Indian plate varies from 8.4o (in the central KRZ) to 62.8° (in the SURZ). The large δt and large deviation of ψ from the APM (>35°) are found to be associated with those stations (e.g. MND, KNM, VGH, NGR, VJP, TPM), which lie near to the geological boundaries in the SURZ. The main finding of the present modeling study depicts a thick highly anisotropic upper mantle underlying the KRZ, where mantle anisotropy parallel to the Kachchh rift axis direction is caused by both frozen lithospheric anisotropy and asthenospheric flow-induced anisotropy. The geographical and azimuthal variations of the measured splitting parameters detect three different anisotropic continental fragments, which might have preserved an ENE-WSW trending fossilized olivine fabric, which was formed before the assemblage of these continental fragments.

Revisiting to the Neoproterozoic tectonic evolution of the Tarim Block, NW China

The Tarim Block comprises two terranes, namely, the Northern Tarim terrane (NTT) and the Southern Tarim terrane (STT). The distinct geological features of the Pre- Cryogenian basement and igneous activity between these two terranes that share common Cryogenian cover sequences strongly suggest that they underwent independent pre-Cryogenian evolutions. However, owing to the likely position of the Tarim Block on the margin of Rodinia, the Neoproterozoic igneous rocks and sedimentary sequences of this block should provide important insights into the geodynamics of the evolution of this supercontinent. In this contribution, our new geochronological data show that the earliest glacial deposit in the Tarim Block, namely, the Beiyixi tillite, was deposited a little later than 720 Ma, which is consistent with the timing of the global Sturtian glaciation. The other two glacial deposits, namely, the Altungol–Tereenken and Hankachocugh tillites, were deposited at 685–635 Ma and later than 591 Ma, consistent with the global Marinoan and Gaskiers glaciations, respectively.Combining our field observations and new geochronological data with a comprehensive synthesis of the Precambrian basement rock units, metamorphic events, and igneous activities in both the NTT and STT, we construct a detailed late Mesoproterozoic to Cryogenian evolution for the Tarim Block, and discuss its relationship to Precambrian supercontinent evolution. We suggest that the STT might be a continental fragment that separated and drifted from the Congo Craton during the initial breakup of Rodinia and subsequently docked along the northern fringe of Australia a little later than ca. 800 Ma. In contrast, the NTT might be a continental fragment that detached and drifted from North China–India during the breakup of the Columbia supercontinent. During the process of breakup of Columbia and the assembly of Rodinia, the NTT amalgamated with the STT a little earlier than 800 Ma, owing to circum-Rodinia subduction between 1000 and 760 Ma. However, upwelling of the Rodinia plume could have affected subduction along the northern margin of the Tarim Block, as demonstrated by a voluminous ca. 770 Ma mafic dyke swarm. The Rodinia plume terminated the subduction along the northern margin of the Tarim Block at ca. 740 Ma, suggesting that the plume could have played a key role in the breakup of Rodinia. This conclusion is supported by Cryogenian–Cambrian sedimentary sequences with passive-continental-margin affinity in the Tarim Block, indicating the detachment and movement of this block from Rodinia. Both the characteristics of Cryogenian–Cambrian basins and coeval igneous activity argue strongly against long-term subduction along the northern margin of the Tarim Block.

Continental Interior and Edge Breakup at Convergent Margins Induced by Subduction Direction Reversal: A Numerical Modeling Study Applied to the South China Sea Margin

AbstractThe dynamics of continental breakup at convergent margins has been described as the results of backarc opening caused by slab rollback or drag force induced by subduction direction reversal. Although the rollback hypothesis has been intensively studied, our understanding of the consequence of subduction direction reversal remains limited. Using thermo‐mechanical modeling based on constraints from the South China Sea (SCS) region, we investigate how subduction direction reversal controls the breakup of convergent margins. The numerical results show that two distinct breakup modes, namely, continental interior and edge breakup (“edge” refers to continent above the plate boundary interface), may develop depending on the “maturity” of the convergent margin and the age of the oceanic lithosphere. For a slab age of ~15 to ~45 Ma, increasing the duration of subduction promotes the continental interior breakup mode, where a large block of the continental material is separated from the overriding plate. In contrast, the continental edge breakup mode develops when the subduction is a short‐duration event, and in this mode, a wide zone of less continuous continental fragments and tearing of the subducted slab occur. These two modes are consistent with the interior (relic late Mesozoic arc) and edge (relic forearc) rifting characteristics in the western and eastern SCS margin, suggesting that variation in the northwest‐directed subduction duration of the Proto‐SCS might be a reason for the differential breakup locus along the strike of the SCS margin. Besides, a two‐segment trench associated with the northwest‐directed subduction is implied in the present‐day SCS region.

Timing of Anatexis within the Berere HTHP Complex Belt of Maevatanana Area, North‐Central Madgascar, and its Geological Significance

AbstractThe Berere HTHP Complex belt in Maevatanana area of north–central Madagascar formed in the ∼ 2.5 Ga orogeny and underwent high temperature (up to 1050°C) and high pressure (up to 11.5 kbar) granulite facies metamorphism. Then a widespread anatexis took place and numerous widely distributed felsic leucosomes formed. The majority of these leucosomes are parallel to the schistosity of the complex or are present as stockworks, as thin layers, or as lenses at different scales in the host rocks. Here, we report new petrographic data, zircon LA‐ICP‐MS U‐Pb ages, and Lu–Hf isotopic data for felsic leucosomes within this complex. Anatexis, as identified by the petrological study of felsic leucosomes in the field and in thin sections, involved initial ternary feldspar exsolving to produce antiperthite and a quartz + plagioclase ± K‐feldspar + sericite mineral assemblage around feldspar grain boundaries. Dissolution is apparent along muscovite grain boundaries, and residual sericite is present around the margins of feldspar and quartz, all suggesting that anatexis was driven by reactions involving muscovite. Zircon U–Pb dating indicates that the felsic leucosomes within the complex formed at 2467–2369 Ma. The majority of samples have positive ∊Hf(t) values, although a few have negative values, suggesting their formation from magmas predominantly sourced from the depleted mantle, possibly with the involvement of minor amounts of crustal materials. Two‐stage Hf model ages and ∊Hf(t) values for these samples are consistent with those for gneisses of the basement, indicating that the felsic leucosomes were formed by the anatexis of gneisses and both of their protolith formed during the formation of continental crust in Meso‐Neoarchean (ca. 3.1–2.7 Ga). As such, the crystallization age of the felsic leucosome (∼2.4 Ga) represents the timing of regional anatexis and a change to post‐orogenic tectonism. And this anatexis is also corresponds to the thermal event in Dharwar craton in India which has a pronounced similar Precambrian geology with Madagascar, providing an important constraints on the correlation of the two continental fragments.

Denuding a Craton: Thermochronology Record of Phanerozoic Unroofing From the Pilbara Craton, Australia

AbstractCratons are ancient regions of relatively stable continental fragments considered to have attained long‐term tectonic and geomorphic stability. Low‐temperature thermochronology data, however, suggest that some cratons have experienced discrete Phanerozoic heating and cooling episodes. We report apatite fission track, and apatite and zircon (U‐Th)/He low‐temperature thermochronology data from the Archean Pilbara craton and adjacent Paleoproterozoic basement, NW Australia. Inverse thermal history simulations of this spatially extensive data set reveal that the region has experienced ~50–70°C cooling, which is interpreted as a response to the unroofing of erodible strata overlying basement. The timing of cooling onset is variable, mainly ~420–350 Ma in the southern and central Pilbara‐eastern Hamersley Basin and ~350–300 Ma in the northern Pilbara, while the westernmost Pilbara‐central Hamersley Basin does not record a significant Paleozoic cooling event. These differences are attributed to variations in sedimentary thickness and proximity to adjacent rift basins, which lack Archean age zircons in their Paleozoic strata. The onset of Paleozoic cooling coincides with the timing of the episodic intraplate late Ordovician‐Carboniferous Alice Springs Orogeny. This orogeny is thought to have resulted from far‐field plate margin stresses, which in turn caused the opening of the adjacent Canning Basin, to the north and east of the craton. We propose that basin development triggered a change of base level, resulting in denudation and the crustal cooling event reported here. Our results provide further evidence for the transmission of far‐field forces to cratons over hundreds of kilometers and support the view that cratons have experienced geomorphic changes during the Phanerozoic.

The evolution of the continental crust and the onset of plate tectonics.

The Earth is the only known planet where plate tectonics is active, and different studies have concluded that plate tectonics commenced at times from the early Hadean to 700 Ma. Many arguments rely on proxies established on recent examples, such as paired metamorphic belts and magma geochemistry, and it can be difficult to establish the significance of such proxies in a hotter, older Earth. There is the question of scale, and how the results of different case studies are put in a wider global context. We explore approaches that indicate when plate tectonics became the dominant global regime, in part by evaluating when the effects of plate tectonics were established globally, rather than the first sign of its existence regionally. The geological record reflects when the continental crust became rigid enough to facilitate plate tectonics, through the onset of dyke swarms and large sedimentary basins, from relatively high-pressure metamorphism and evidence for crustal thickening. Paired metamorphic belts are a feature of destructive plate margins over the last 700 Myr, but it is difficult to establish whether metamorphic events are associated spatially as well as temporally in older terrains. From 3.8-2.7 Ga, suites of high Th/Nb (subduction-related on the modern Earth) and low Th/Nb (non-subduction-related) magmas were generated at similar times in different locations, and there is a striking link between the geochemistry and the regional tectonic style. Archaean cratons stabilised at different times in different areas from 3.1-2.5 Ga, and the composition of juvenile continental crust changed from mafic to more intermediate compositions. Xenon isotope data indicate that there was little recycling of volatiles before 3 Ga. Evidence for the juxtaposition of continental fragments back to ~2.8 Ga, each with disparate histories highlights that fragments of crust were moving around laterally on the Earth. The reduction in crustal growth at ~ 3 Ga is attributed to an increase in the rates at which differentiated continental crust was destroyed, and that coupled with the other changes at the end of the Archaean are taken to reflect the onset of plate tectonics as the dominant global regime.

Microcontinents and Continental Fragments Associated With Subduction Systems

AbstractMicrocontinents and continental fragments are small pieces of continental crust that are surrounded by oceanic lithosphere. Although classically associated with passive margin formation, here we present several preserved microcontinents and continental fragments associated with subduction systems. They are located in the Coral Sea, South China Sea, central Mediterranean and Scotia Sea regions, and a “proto‐microcontinent,” in the Gulf of California. Reviewing the tectonic history of each region and interpreting a variety of geophysical data allows us to identify parameters controlling the formation of microcontinents and continental fragments in subduction settings. All these tectonic blocks experienced long, complex tectonic histories with an important role for developing inherited structures. They tend to form in back‐arc locations and separate from their parent continent by oblique or rotational kinematics. The separated continental pieces and associated marginal basins are generally small and their formation is quick (<50 Myr). Microcontinents and continental fragments formed close to large continental masses tend to form faster than those created in systems bordered by large oceanic plates. A common triggering mechanism for their formation is difficult to identify, but seems to be linked with rapid changes of complex subduction dynamics. The young ages of all contemporary pieces found in situ suggest that microcontinents and continental fragments in these settings are short lived. Although presently the amount of in‐situ subduction‐related microcontinents is meager (an area of 0.56% and 0.28% of global, non‐cratonic, continental crustal area and crustal volume, respectively), through time microcontinents contributed to terrane amalgamation and larger continent formation.

Crustal growth and reworking of Archean crust within the Rhyacian domains of the southeastern Guiana Shield, Brazil: Evidence from zircon U–Pb–Hf and whole-rock Sm–Nd geochronology

The southeastern Guiana Shield, northern Amazonian Craton, is part of a Paleoproterozoic orogenic belt that was built up during the Transamazonian orogenic cycle (2.26–1.95 Ga). This cycle includes large segments of Rhyacian juvenile crust and some reworked Archean terranes. The geology in this region consists mainly of Paleoproterozoic granulitic-migmatitic-gneissic complexes, deformed and metamorphosed metavolcanic and metasedimentary rocks, and granitoids (granitic and TTG magmatism). Three tectonic domains are distinguished in the Brazilian territory of the southeastern Guiana Shield. They are known as the Amapá Block, Lourenço Domain, and Carecuru Domain. The Amapá Block is a Meso–Neoarchean continental block that was intensely reworked during the Transamazonian orogeny. The other two domains represent Rhyacian landmasses, the evolution of which involved several stages of subduction of oceanic lithosphere in magmatic arc environments. There are also relics of reworked Archean continental crust, the formation of which was followed by a collisional stage of tectonic accretion of the magmatic arcs. Whole-rock Sm–Nd and U–Pb zircon geochronology have confirmed the juvenile character of much of this Transamazonian orogenic belt. However, for the Lourenço and Carecuru domains, Nd isotopic signatures indicate the participation of Meso–Neoarchean crustal material in the sources of the magmatic rocks. Combined zircon U–Pb and Lu–Hf isotopic analyses by LA-ICP-MS were performed on eleven Rhyacian granitoids and orthogneisses from the Lourenço and Carecuru domains. The aim was to verify the extension of the influence of the Archean continental crust in the adjacent Paleoproterozoic domains. The main magmatic episodes were identified in the Lourenço Domain (~2.17–2.18, 2.14 and 2.12–2.09 Ga) and Carecuru Domain (2.14 Ga) by U–Pb zircon geochronology. The Lu–Hf isotope data point to the predominance of crustal reworking processes (ƐHf(2.2–2.1 Ga) < 0; 67% of zircon crystals) during the formation of Lourenço and Carecuru domains. Hf model ages were found to be mostly Archean (98.4%), even for zircon grains that have positive ƐHf(2.2–2.1 Ga) values. For the terrane at the border of the Lourenço and Carecuru domains with the Amapá Block, assimilation of Archean crust of different ages and proportions in a magmatic arc environment accounts for the Hf–Nd isotopic signatures and Hf model ages of Rhyacian magmatism. In the northwestern part of the Lourenço Domain, more than 100 km north of the Amapá Block, the Hf–Nd isotopic signatures and Hf model ages indicate the participation of Archean crustal material, either as continental fragments and/or through incorporation of continental sediments in island arc environments, similar to what has been recorded for some Birimian terranes of the West African Craton in Ghana.

U–Pb geochronology and REE geochemistry of zircons in mafic granulites from the Lützow-Holm complex, East Antarctica: Implications for the timing and P–T path of post-peak exhumation and Antarctica–Sri Lanka correlation

We report new petrological and geochronological data, together with zircon REE patterns, of mafic granulites from the Lützow-Holm Complex (LHC), East Antarctica, and investigate the pressure-temperature-time (P-T-t) evolution of granulite-facies metamorphism in order to unravel the timing and tectonics of Neoproterozoic collisional processes related to the amalgamation of Gondwana. The LHC is composed of three tectonic units: a ca. 2.5 Ga continental fragment (Shirase Microcontinent), a ca. 1.0 Ga magmatic arc (Prince Olav–Vijayan Block; PVB), and a latest Neoproterozoic suture zone (Lützow-Holm Bay Suture Zone; LHSZ) separating the two. Mafic granulites from Tenmondai Rock and Sudare Rock, which belong to the PVB and the LHSZ, respectively, contain orthopyroxene + plagioclase symplectites around garnets, probably formed during post-peak decompression. The application of phase equilibrium modeling and geothermobarometry indicates similar peak P-T ranges of 850–860 °C/7.8–8.4 kbar (Tenmondai Rock) and 800–810 °C/8.5–9.0 kbar (Sudare Rock), together with clockwise P-T paths for both localities. LA-ICP-MS zircon U-Pb dating of the studied granulites yielded Mid-Neoproterozoic magmatic ages of >808 Ma (Tenmondai Rock) and >783 Ma (Sudare Rock) from igneous cores, and Late Neoproterozoic to Cambrian metamorphic ages of ca. 630–481 Ma from overgrown rims and/or structureless grains. Zircon REE analyses indicate that younger 560–510 Ma metamorphic zircons from the localities exhibit heavy-REE-enriched patterns and negative Eu anomalies, suggesting that the consumption of garnet and growth of plagioclase took place during zircon growth. This process was probably related to the formation of post-peak decompressional textures (orthopyroxene + plagioclase symplectite around garnet) observed in the mafic granulites. The results of this study therefore indicate that post-peak exhumation of the LHC started at ca. 560 Ma. Available U-Pb and REE data from 560 to 510 Ma metamorphic zircons of the Highland Complex, Sri Lanka, show similar heavy-REE-enriched patterns and Eu negative anomalies, suggesting that post-peak exhumation of both the LHC and the Highland Complex took place almost simultaneously at ca. 560 Ma. The results of this study further support the proposed tectonic model in which the Highland Complex and the LHSZ are parallel collisional sutures formed during the Late Neoproterozoic-Cambrian Gondwana amalgamation.

Tectonic characteristics of the Eratosthenes Seamount and its periphery: Implications for evolution of the eastern Mediterranean

A comprehensive tectonic analysis of the Eratosthenes Seamount (ESM) and its surrounding domains covered by our seismic data, contributes to better understanding of the evolution of the eastern Mediterranean. Tectonic origins of the ESM and the Zohr-like sub-Messinian salt highs between the ESM and the Nile delta are both continental fragments detached from the African-Arabian plate. These continental fragments form a NE-SW striking tectonic belt acting as the western margin of the Levant basin. After the Neotethyan rift, isolated carbonate platforms grew on these continental fragments and existed until the Miocene with alternations of shallow-water and deep-water carbonates caused by sea-level variation in the eastern Mediterranean. The subduction beneath the Cyprus arc beginning in the Early Miocene triggered onset of tectonic inversion of the deep-water area of the eastern Mediterranean. The ESM shielded the Zohr-like platforms from deforming by assimilating compressional stress from the Miocene subduction and the Messinian-present collision through its own folding, which led to tectonic evolutionary discrepancy between the Eratosthenes and the Zohr-like platforms since the Early Miocene. In addition, syn-sedimentary folding deformations related to the subduction were recognized in the southern Levant basin, the Herodotus eastern sub-basin, and the Cyprus arc. These deformations were all intense in the Early Miocene, attenuated gradually in the Middle and Late Miocene, and terminated at the very beginning of the Messinian. The weakening of these deformations could be associated with an increase in the subduction angle which also induced the development of the Mid-Late Miocene normal faults in the northern Levant basin. The initial African–Eurasian collision in the Messinian terminated these subduction-related deformations. Finally, we proposed a new evolutionary model of the deep-water area of the eastern Mediterranean, consisting of four stages, i.e. the Neotethyan rift stage from the Late Permian to the Early Jurassic, the post-rift tectonic stability stage from the Middle Jurassic Bajocian to the Oligocene, the subduction-related compression stage during the Miocene, and the collision-related compression-transpression stage from the Messinian to the present.

Tectonic history across the Iapetus suture zone in Ireland

Abstract The early Paleozoic rocks of eastern Ireland span the suture zone between the Laurentian and Ganderian continental margins of the Iapetus Ocean. The Grangegeeth Terrane comprises a Laurentian continental fragment and Ordovician volcanic arc that formed the southern margin of the late Ordovician Rathkenny Basin of Moffat Shale facies mudstone. Together, these were overstepped in Wenlock time by Laurentia-derived greywackes and became the southernmost tract of the Longford–Down Terrane accretionary prism as subduction brought them into the Laurentian-margin trench. South of the suture, Middle Ordovician failed rifting of a Ganderian volcanic arc terrane was followed by shortening during continuing subduction under the north-facing arc. In Newfoundland, a Ganderian volcanic arc migrated across the ocean in mid-Ordovician times to accrete to Laurentia. In Ireland, the late-accreted arc on the Laurentian margin was formed on a Laurentian microcontinent, and Ganderia had an active margin throughout Late Ordovician time. Silurian closure of Iapetus was between the leading edge of Ganderia and the Laurentian margin, unlike in the Canadian Appalachians, where separate Ordovician and Silurian sutures are recognized. Iapetus was narrow by Katian time but subduction-related magmatism continued into Wenlock times on both margins.

Rapid cold slab subduction of the Paleo-Tethys: Insights from lawsonite-bearing blueschist in the Changning–Menglian orogenic belt, southeastern Tibetan Plateau

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is considered as the main suture of the Paleozoic Paleo-Tethys that separates Gondwana-derived continental fragments from Eurasia-derived ones. Understanding the evolutionary history of this orogenic belt is of critical importance in the reconstruction of the tectonic history of Paleo-Tethys. The CMOB preserves well-exposed blueschist facies rocks, albeit their tectonometamorphic history and protolith signatures remain poorly constrained. Here we present, for the first time, results from a detailed investigation of lawsonite-bearing blueschist rocks, including epidote-magnesioriebeckite schist and garnet-ferroglaucophane schist from the CMOB and involving petrological, mineralogical, thermodynamic modeling, whole-rock geochemical, and geochronological studies. The epidote-magnesioriebeckite schist samples and garnet-ferroglaucophane schist samples display OIB- and E-MORB-like geochemical affinities, respectively, and have whole-rock εNd (t) values in the range of −5.4–+4.4, suggesting that their protoliths were mainly oceanic crust with limited degree of crustal assimilation. Magmatic zircon grains from the garnet-ferroglaucophane schist samples yield protolith ages of 253–250 Ma. Combined with previous data, our data represent the youngest ages of rock formation in the Paleo-Tethyan Ocean reported to date. The epidote-magnesioriebeckite schist samples show a peak assemblage of magnesioriebeckite + lawsonite + augite-aegirine + phengite + titanite + allanite, with peak P–T conditions of 12.4–16.8 kbar and 350–406 °C. In contrast, the garnet-ferroglaucophane schist samples preserve a peak assemblage of garnet + ferroglaucophane + omphacite + lawsonite + phengite + titanite/ilmenite/rutile ± allanite and yield peak P–T conditions of 19.5–22.6 kbar and 490–510 °C. The epidote-magnesioriebeckite schists and garnet-ferroglaucophane schists record a nearly complete clockwise P–T loop characterized by a steep prograde P–T path with a low thermal gradient of ~5–8 °C/km followed by cooling or overprinting by isothermal decompression. Reconstruction of the metamorphic P–T evolution, together an evaluation of previous age data, allows us to propose rapid subduction of cold oceanic lithosphere to depths of 50–75 km at a rate of ~6.3–9.3 km/Myr during Early-Middle Triassic (248–240 Ma), followed by exhumation in the Late Triassic (231–214 Ma). The short time lag (<10 Ma) between the protolith generation and the high-pressure metamorphic peak further supports a rapid subduction process. Our results suggest that the young oceanic slab was dragged down by the thicker, long-lived, and cold downgoing Paleo-Tethyan Ocean plate, where it experienced blueschist-facies metamorphism. The co-occurrence of lawsonite-bearing epidote-magnesioriebeckite schists and garnet-ferroglaucophane schists and eclogites further points to a cold thermal structure at the convergent plate interface, leading to the interpretation that the CMOB represents a typical oceanic subduction-accretion belt. The results presented in this study provide important insights into the geodynamic evolution of the Paleo-Tethys.

Archean–Paleoproterozoic tectonothermal events in the central Tarim Block: Constraints from granitic gneisses revealed by deep drilling wells

The Tarim Block is one of the oldest continental fragments in China. However, its Precambrian framework and basement are insufficiently studied because it is mainly covered by desert in its center. We report new zircon U-Pb-Hf isotopic, zircon rare earth elements (REEs), hornblende, and biotite Ar-Ar dating, and whole-rock geochemical data for granitic gneisses collected from three drilling wells in the central area of the Tarim Basin. Zircons from three granitic gneisses yielded different crystallization ages of 2.8, 2.4 and 2.2 Ga, but a similar metamorphic age of 2.0–1.9 Ga. Two samples yielded Ar-Ar plateau ages of 1.85–1.79 Ga. Combined with previous studies, our data provide Archean–Paleoproterozoic tectonic implications for the Tarim Block: (1) The varied εHf (t) values of the zircon cores suggest that the magmatism of 2.8, 2.4 and 2.2 Ga in the central Tarim Block involved synchronous crustal growth and reworking, respectively; (2) The 2.0–1.9 Ga metamorphic rocks, widely distributed in the Tarim Block, are most likely associated with the assembly of the Tarim Block to the Columbia supercontinent; (3) The Ar-Ar ages of 1.85 –1.79 Ga might be related to a post-orogenic extension.

Conjugate margins — An oversimplification of the complex southern North Atlantic rift and spreading system?

The prevalence of conjugate margin terminology and studies in the scientific literature is testimony to the contribution that this concept and approach has made to the study of passive margins, and more broadly extensional tectonics. However, when applied to the complex rift, transform, and spreading system of the southern North Atlantic (i.e., the passive margins of Newfoundland, Labrador, Ireland, Iberia, and southern Greenland), it becomes obvious that at these passive continental margin settings, additional geologic phenomena complicate this convenient description. These aspects include (1) the preservation of relatively undeformed continental fragments, (2) formation of transform systems and oblique rifts, (3) triple junctions (with rift and spreading axes), (4) multiple failed rift axes, (5) postbreakup processes such as magmatism, (6) localized subduction, and (7) ambiguity in identification of oceanic isochrons. Comparison of two different published reconstructions of the region indicates the ambiguity in conducting conjugate margin studies. This demonstrates the need for a more pragmatic approach to the study of continental passive margin settings where a greater emphasis is placed on the inclusion of these possibly complicating features in palinspastic reconstructions, plate tectonics, and evolutionary models.

Origin of Lower Paleozoic S-type magmatism in a northern terrane of Gondwana (Central Iran): Geochemical and isotopic approach

In the center of Iran, Central-East Iranian Microcontinent (CEIM) was a part of Gondwana supercontinent in pre-Palaeozoic during Pan-African Orogeny. It is a zone of several tectonomagmatic and metamorphic episodes from Neoproterozoic to earliest Palaeozoic. In the north of CEIM, Airekan granite is a relic of Paleozoic magmatism in northern Gondwanaland. It is potentially a significance pluton that preserved the magmatic/metamorphic evolution of the active continental margin of the vanished Ocean of Proto-Tethys. This pluton is characterized by SiO 2 > 70 wt%, A/CNK>1, Rb >~160 ppm, Y 5. The δ 18 O value of quartz (average ~11.86 ‰; n=8), calculated δ 18 O value of whole rock (average ~10.75 ‰), absence of hornblende, presence of biotite, muscovite and inherited zircon, higher content of orthoclase, and microgranular granitic enclaves are all consistent with it being a continental collision-related peraluminous S-type granite. Th+U versus 206 Pb/ 238 U ratios of zircons correlate with decreasing crystallization temperatures related to the Cambrian-Ordovician magmatic events preserved in the inherited and magmatic zircons, toward their Devonian metamorphic overgrowth occurred via Caledonian Orogeny. It is probably formed by mica-dehydration melting at ~ 690-820 °C/ 10-15 kbar, and it is geochronologically and geochemically comparable with other Gondwanan collision-related granitic plutons (along north of Africa, Turkey, Iran to Himalaya).

Luminescence dating of Late Pleistocene faults as evidence of uplift and active tectonics in Sardinia, W Mediterranean

AbstractPrecise dating of the activity of Late Pleistocene to Holocene neo‐tectonic structures is crucial to quantify the rate of deformation in low‐seismicity regions. Sardinia is a relatively stable continental fragment set in the middle of the tectonically active Western Mediterranean belt. This paper provides evidences of significant uplift of northwest Sardinia that support an ongoing tectonic activity since the Marine Isotopic Stage 7 (MIS 7; ca. 220 ka). In particular, it documents for the first time Late Pleistocene to Holocene tectonics based on luminescence dating of travertine sealing a major NNE‐SSW fault.

Late Paleozoic back-arc basin in the Indochina block: Constraints from the mafic rocks in the Nan and Luang Prabang tectonic zones, Southeast Asia

Southeast Asia was mainly created by the late Paleozoic assemblage of the Eastern Cimmeria with other continental fragments. However, the tectonic nature and geodynamic setting of the key tectonic zones in the Indochina block are still in dispute, hindering the understanding of the Eastern Paleotethyan evolution of Southeast Asia. This study presents a set of new geochronological, elemental and Sr-Nd isotopic data for the late Paleozoic mafic rocks in the Nan and Luang Prabang tectonic zones. The mafic rocks from the Nan and Luang Prabang zones give zircon U-Pb ages of 279–294 Ma and 265–294 Ma, respectively, with xenocryst ages of 302–2524 Ma. They have SiO2 = 45.99–57.47 wt%, Al2O3 = 12.74–21.06 wt%, FeOt = 8.34–15.03 wt% and MgO = 2.79–14.74 wt% with mg-number of 34–74, and fall into the fields of basalt/basaltic andesite and its equivalents. These rocks are characterized by enrichment in LILEs and LREEs, depletion in HFSEs, low initial 87Sr/86Sr ratios (0.7032–0.7057) and high εNd(t) values (+4.8 to +7.4). They might be derived from a mantle wedge source modified by slab-derived components, and formed in a continental back-arc basin setting. Our analysis indicates the development of the Carboniferous-Permian mafic igneous event in the Nan and Luang Prabang tectonic zones, and that the Nan tectonic zone linked northerly with the Luang Prabang zone during the late Paleozoic, tectonically comparable to the Banpo-Nanlinshan back-arc basin in SW Yunnan, SW China. The back-arc basin finally closed in the early-middle Triassic and the subsequent assemblage continued till the latest Triassic in response to the eastward subduction of the Paleotethyan slab.

Permian–Triassic magmatism in response to Palaeotethys subduction and pre-Late Triassic arrival of northeast Gondwana-derived continental fragments at the southern Eurasian margin: Detrital zircon evidence from Triassic sandstones of Central Iran

The closure of Palaeotethys that led to the collision of the Cimmerian blocks with the southern Eurasian margin causing the Eo-Cimmerian orogeny during the Early Mesozoic is still controversially discussed. The Triassic Nakhlak Group in Central Iran is a key sedimentary succession for better understanding the closure of Palaeotethys and the Eo-Cimmerian orogeny in the Middle East. The Nakhlak Group is composed of the Alam (Olenekian to Middle Anisian), Baqoroq (?Upper Anisian to Middle Ladinian) and Ashin (Upper Ladinian to ?Carnian) formations, which consist mainly of volcaniclastic sandstones, mixed siliciclastic conglomerates, and marine carbonates. Here we present for the first time detrital zircon UPb ages from the Nakhlak Group to unravel its provenance and constrain its palaeotectonic position within the Palaeotethyan realm. Most detrital zircons from the Nakhlak Group are euhedral and subhedral with Permian–Triassic ages (ca. 280–240 Ma) suggesting sediment supply from Permian–Triassic magmatic rocks of the Silk Road Arc. Minor zircon populations show pre-Permian Palaeozoic ages, with age peaks at ca. 320 Ma and 480 Ma, which are probably derived from the basement on which the magmatic arc developed. Neoproterozoic–latest Mesoproterozoic (ca. 550–1100 Ma) and Palaeoproterozoic (ca. 1800–2200 Ma) zircon grains are anhedral (rounded). The latter are prominent in the upper Baqoroq Formation (Middle Ladinian) suggesting recycling of older sedimentary rocks. Sandstone petrography points toward an additional metamorphic provenance for this formation. This short-lived provenance change can be explained by tectonic uplift in the source area that led to erosion of metamorphosed rocks with a northeast Gondwanan affinity. It clearly indicates that northeast Gondwana-derived continental fragments likely belonging to the Cimmerian blocks already arrived at the southern Eurasian margin in pre-Late Triassic time. Current palaeotectonic models of the closure of Palaeotethys and the Eo-Cimmerian orogeny in the Middle East during the Triassic may need to be revised.

The crustal structure of the southern Davie Ridge offshore northern Mozambique – A wide-angle seismic and potential field study

Some of the oldest surviving oceanic basins in the world, the Mozambique and West Somali basins, were created during the breakup of Gondwana, starting around 180 Ma. Between the two basins, relative movements of West Gondwana and East Gondwana, including Madagascar, created a shear zone, the Davie Fracture Zone (DFZ) with a topographic elevation (Davie Ridge - DR) marking its centre. The crustal composition of the DFZ and DR is a subject of speculation and debate.In this study, we present seismic refraction data across the prominent topography of the southern DR. Ray tracing of the wide-angle data as well as additional seismic amplitude modelling and 2.5D density modelling constrain its crustal structure and architecture.The data indicate that in the Mozambique Channel the DR consists of fragments of continental crust with a thickness of 10 to 12 km. An oceanic crust indenter extends northward from the Mozambique Basin into the area between the DR and the East African margin at 16.5°S. Northeast of the DR, at 41.8°W/14.5°S, the Somali Basin is probably floored by 6 km thick oceanic crust. Hence, the continental DR separates oceanic crust of the Somali and Mozambique basins.The transitional crustal area at the central Mozambican margin is underlain by high velocity lower crust (HVLC). The HVLC has velocities up to 7.3 km/s and extents along the margin, vanishing northward between 16.5° and 14.5°S. At the Madagascan side of the DR, at 16.5°S, the highly intruded stretched continental crust is 9 km thick and possibly underlain with a smaller HVLC of 2.9 km thickness and an E-W extent of 120 km. The oceanic crust at 14.5°S represents the oldest part the Somali Basin, which formed after the initial NW-SE rifting between East and West Gondwana.

Evolution of subduction dip angles and seismic stress patterns during arc-continent collision: Modeling Mindoro Island

Here we dynamically model the temporal development of arc-continent collision, with particular attention to the evolution of slab dip angles and stress fields during approach to collisional locking (suturing). Our modeling is based on a simplified representation of Mindoro Island and the southern Manila Trench, which provide a natural laboratory in that convergence is ongoing to the north but collision is complete in the south, so that distance of 2D slices along the arc may serve as a reasonable proxy for time. We consider in detail the effects of the negative petrological buoyancy imparted to the slab upon encountering the thermally uplifted “410-km” (wadsleyite-forming) phase transition, as well as the effects of initial separation distance between the incoming Palawan continental fragment and the overriding Philippine Mobile Belt assemblage of arc-continent terranes. Despite simplifications in representing this tectonically complex region, our model reproduces important seismic observations, including the progressive steepening of slab dip and the growth of down-dip extensional stresses during the progress of collision. It also reveals that a significant contribution (of order 100 MPa) to the maximum attainable down-dip extensional stress arises from the negative petrological buoyancy of the uplifted “410-km” transition, and it illuminates how the initial separation of arc-continent fragments controls the temporal offset between collisional locking and the onset of negative petrological buoyancy. Finally, associated calculations of maximum shear stress may offer a partial explanation for the aseismic nature of tomographically proposed slab extensions below the Wadati-Benioff zone.