Retreating Subduction Zone Research Articles

Stratigraphic overview of the Neogene Transylvanian Basin, Romania

Although the Transylvanian Basin is often referred as part of the Miocene Pannonian Basin System, we point out important tectonic and sedimentary differences reflected in the stratigraphy. The Neogene of the Transylvanian Basin can be split into two tectonic megasequences: Lower Miocene and Middle to Upper Miocene. The Early Miocene onset of the back arc extension in the Pannonian Basin was not observed in the Transylvanian Basin, where shallow to deep marine sediments were deposited in a flexural basin. Diverse types of facies developed in the continental (e.g., Sânmihai) and marine (e.g., Coruş, Chechiş, Hida) Lower Miocene formations. Although these are poorly constrained to the standard biostratigraphy, recent 87 Sr- 86 Sr data support their limited correlation with the Burdigalian. The Middle Miocene extension in the Transylvanian Basin is very weak compared to that the Pannonian Basin. As the high-resolution Middle Miocene regional biostratigraphy of the Badenian and Sarmatian was calibrated by magnetostratigraphy and radiometric dating of the volcanic tuffs up to the Serravallian/Tortonian boundary, the influence of the regional tectonics on the basin's evolution and stratigraphy can be better understood. The regional tectonics produced a progressive basin isolation starting with the late part of the Middle Miocene. The uplift of the Carpathians isolated the basin at the beginning of the Pannonian and initiated the fan delta, lacustrine, and fluvial sedimentation. The basin was rapidly uplifted and eroded, while post-extensional subsidence was still ongoing in the Pannonian Basin. Endemic fossil assemblages allow only regional-scale correlations for this interval. The uppermost part of the stratigraphic record is represented by the volcano-sedimentary formations generated by the late evolution of the Eastern Carpathians. The sedimentation and subsidence pattern of the Transylvanian Basin reflect the effect of retreating subduction zone in the Eastern Carpathians.

Subduction retreating caused the external breakup of Rodinia: Constraints from the Neoproterozoic igneous rocks in the western Yangtze Block, South China

The western Yangtze Block is an ideal place for investigating the Precambrian tectonic evolution of the South China Craton and the geodynamic mechanism responsible for the breakup of Rodinia. However, the Neoproterozoic tectonic evolution of the western Yangtze Block has not been fully elucidated; in particular, the driving mechanism has changed from an arc setting to a rift setting. Here, we present new zircon U–Pb geochronology and O–Hf isotopic compositions, whole-rock geochemistry, and Sr–Nd isotope data for Yanbian andesites and Shimian gabbros from the western Yangtze Block, South China. The ca. 850–840 Ma Yanbian andesites are high-Mg andesites with moderate SiO2 (56.75–60.21 wt%) and high MgO (4.24–6.44 wt%) and Mg# (52–66). These rocks show enrichments in large-ion lithophile elements (LILEs; e.g., Rb, Ba, Sr, and K) and light rare earth elements (LREEs) and depletions in high-field strength elements (HFSEs; e.g., Nb, Ta and Ti) and display decoupled Nd–Hf isotopes (εNd(t) = +3.9 to + 5.0, εHf(t) = +9.7 to + 13.1) and relatively high zircon δ18O values (5.82–7.72 ‰); these findings indicate that these rocks were derived from a mantle wedge enriched by slab fluids and sediment melts. The ca. 820–810 Ma Shimian calc-alkaline gabbros are characterized by enriched LREEs and LILEs (e.g., Rb, Sr, and K) and depleted HFSEs (e.g., Nb, Ta and Ti). These rocks show decoupled Nd–Hf isotopes (εNd(t) = +3.2 to + 4.2, εHf(t) = +3.1 to + 7.5) and relatively low zircon δ18O values (4.28–5.31 ‰), indicating that they were formed by partial melting of mantle wedge metasomatized by sediment and oceanic slab melts. Compiled geochronological data suggest that Neoproterozoic subduction-related magmatism along the western Yangtze Block formed a long-term active arc system. Slab retreat in this subduction zone led to a shift in the tectonic setting from a compressional setting to an extensional setting in the Yangtze Block during the middle to late Neoproterozoic. Moreover, the temporal link between retreating subduction zone and extensional tectonics in the Yangtze Block suggests that protracted subduction retreating resulted in external rifting in Rodinia.

Late Neoproterozoic tectonic evolution of the northern Tarim block: New insights from integrated detrital zircon and rutile geochronology and trace element geochemistry

In the Neoproterozoic era, the breakup of the Rodinia supercontinent had a profound impact on different aspects of Earth; yet, the precise driving mechanism behind this event has been a source of ongoing controversy. The Tarim block, as a crucial component, presents an opportunity to gain valuable insights into the breakup process through its tectonic-sedimentary evolution history and dynamic background. However, the comprehension of basin evolution is limited due to the presence of an ambiguous prototype basin during the late Neoproterozoic and its intricate interconnections with surrounding blocks. In this contribution, we integrate detailed field observations, U-Pb dating of detrital zircon and rutile, and trace element analysis of detrital rutile to elucidate the source-to-sink relationships and the tectonic setting of Ediacaran sedimentary strata in the Aksu area (northern Tarim block, China). Detrital zircon age spectra exhibit consistent distributions with ages between ∼850 and ∼600 Ma and between ∼2100 and ∼1800 Ma. Rutile age groups however exhibit an abrupt transition between the lower member of the Sugatbrak Formation and the succeeding upper member to the Qigebrak Formation in the basin, with ages between ∼1800 and ∼900 Ma and nearly identical to those between ∼2100 and ∼1800 Ma. Calculated Zr-in-rutile temperatures and Cr-Nb compositions imply that the majority of the detrital rutiles were sourced from amphibolite facies metapelites, with an abrupt transition from amphibolitic schist to amphibolite-granulite facies for the ∼1800 Ma group grains. Provenance tracing shows that the detrital sediments of the lower Sugatbrak Formation are sourced from both the Central Tianshan and the Tarim blocks, and that the overlying strata have a single source in the Tarim block. A corresponding change from syn-rift detrital facies to a shallow-marine environment in on a passive continental margin is suggested in terms of depositional setting. In light of prior published evidence from petrology, geochronology and paleomagnetism, we propose that the Neoproterozoic northern Tarim margin was affected by a protracted subduction zone retreat. Associated back-arc spreading eventually was responsible for the opening of the southern Tianshan Ocean and rifting of the Central Tianshan block from the Tarim block in the late Ediacaran. This development ultimately prevented detrital material from the Central Tianshan block from reaching the northern Tarim depocenters as the latter evolved into a passive continental margin.

Middle Permian granitoids in the western section of the northern Qaidam Block, NW China: Petrogenesis and tectonic implications

Origin and tectonic setting of late Late Paleozoic to Early Mesozoic igneous rocks in the northern Qaidam Block (NQ) have long been a matter of debate. In this paper, we report new zircon U-Pb ages and Hf isotopic compositions and bulk rock major and trace elemental compositions for granitoids of the Yanchangbeishan area in the NQ. The granodiorite, tonalite, granodiorite, and syenogranite yielded zircon U-Pb ages of 271.9 Ma, 262.0 Ma, 272.4 Ma and 261.3 Ma, respectively, corresponding to εHf(t) values are 13.04–14.32, 8.14–10.77, 8.66–11.88, and 8.22–10.43, respectively. These granitoids are compositionally I-type granites and exhibit varying degrees of enrichment in Rb, Th, U, and K, and depletion in Nb, Ta, and Ti. According to field relationships, petrology, geochronology, and geochemistry, the granitoid types can be divided into three groups: (1) ca. 272 Ma granodiorites, which are formed through partial melting of metagreywackes and amphibolites derived from depleted mantle; (2) ca. 262 Ma tonalite, which represents a typical high-silica adakite derived from partial melting of subducted oceanic crust during the evolution of depleted mantle; and (3) ca. 261 Ma syenogranite, which is formed by the partial melting and high-degree fractional crystallization of mixed crustal materials. Considering our findings and the regional geological background, we propose a five-stage tectonic evolution model of subduction slab roll-back and subduction zone retreat for the late Late Paleozoic to Early Mesozoic of NQ.

Compositional change from high-Mg to low-Mg magmatism at ca. 150 Ma in the central Lhasa terrane, Tibet: Switching from advancing to retreating subduction of the Bangong Tethyan slab

Differentiating between advancing and retreating subduction zones and delineating the timing and processes for switching between these end-member types are critical factors in understanding orogenic evolution. In this study, we used temporal composition variations and spatiotemporal distribution of igneous rocks to constrain the types and directions of the subduction zone beneath the Lhasa terrane in southern Tibet during the mid-Mesozoic. Comprehensive geochronological and geochemical data from mid-Mesozoic magmatic rocks in five areas of the central Lhasa terrane document two compositionally distinct magmatic suites: older (ca. 172−150 Ma) high-Mg diorite-granodiorite and younger (ca. 150−130 Ma) low-Mg granodiorite-syenogranite. The high-Mg rocks with enriched isotopes were likely generated by the mixing of Lhasa terrane basement-derived magmas with mantle-derived magmas that incorporated extensive contributions from subducted sediments. The low-Mg rocks with relatively depleted isotopes are interpreted as derived from a mixed lower-crust source (ancient basement and juvenile crust) with minor mantle contributions followed by crystal fractionation. Both high-Mg and low-Mg rocks show arc signatures and formed the 1200 km Shiquan-Zhegu-Menba arc. A combination of the Mesozoic geological and geochemical data suggests that both the Neo-Tethyan and Bangong Tethyan slabs were likely subducting beneath the Lhasa terrane, creating two opposing subduction zones. The change from high-Mg to low-Mg magmatism within the Shiquan-Zhegu-Menba arc likely records a tectonic switch from advancing to retreating subduction of the south-dipping Bangong Tethyan slab. Our results provide a good example for identification of subduction polarities and types of ancient orogenic belts through spatiotemporal distributions and changes in geochemical compositions of igneous rocks.

Multiple tectonic switches in the Eastern Tianshan, Central Asian Orogenic Belt: Implications for crustal growth and metallogenesis

Subduction zones are crucial sites for inputs of juvenile crust and the generation of porphyry copper deposits by arc magmatic processes that generally involve episodic trench advance and retreat. It is well documented that the generation of juvenile crust is associated with lithospheric extension during the retreat of subduction zones, whereas crustal reworking is associated with lithospheric compression in continental arcs during the advance of subduction zones. However, it remains unclear whether such models can be applied to island arc settings and how they relate to metallogenesis. To address this issue, we examine intermediate–felsic igneous rocks (∼460–300 Ma) of the Dananhu–Harlik island arc in the southern Central Asian Orogenic Belt. Our new and compiled data indicate three magmatic episodes in the Dananhu–Harlik island arc during the northward subduction of the Kangguer oceanic plate, with each involving a tectonic switch from compressional trench advance to extensional retreat. Crustal growth occurred mainly during the compressional stages of trench advance, which is in contrast with typical continental arcs. Tectonic transitions also controlled regional mineralization patterns, and the typical porphyry and related skarn-type deposits formed during subduction zone advance, whereas volcanogenic massive sulfide (VMS) deposits and overprinting mineralization were generated during subduction zone retreat. A combination of all data on tectonic settings and physical–chemical conditions (e.g., temperature, oxygen fugacity, and water content) of the pre-mineralization and ore-related intrusions led us to the conclusion that normal intra-crustal arc magmatic processes, involving sulfide pre-enrichment and reactivation, generated the typical Tuwu–Yandong porphyry Cu deposits in the Dananhu arc without the prerequisite of slab melting. Our study provides an excellent example of crustal growth in an island arc setting as well as metallogenesis during the repeated advance and retreat of a subduction zone.

Continental crust delamination in a retreating subduction zone: A case study in the southern Alxa (Inner Mongolia, China), Central Asian Orogenic Belt

Carboniferous-Permian igneous rocks in the southern Alxa (Inner Mongolia, China) of the southern Central Asian Orogenic Belt exhibit an oceanward migration trend, which provides an ideal area to reconstruct the process of crust-mantle material exchange within a retreating subduction zone. This study presents new geochronological and geochemical data for the Shouji plutons from the southern Alxa to address crust-mantle material exchange process. All these diorite, granodiorite, and K-feldspar granite samples from the Shouji batholith exhibit Early Permian zircon U-Pb ages (ca. 282−270 Ma). These diorite and granodiorite are characterized by high Sr contents as well as low Y, and Yb concentrations, coupled with distinctively high Sr/Y values. They exhibit fertile Nd-Hf isotopic compositions with negative whole-rock εNd(t) values, and zircon εHf(t) values, similar to those of continental crust-related adakites, which might originate from partial melting of lower crust, and followed by melt-mantle interactions. These K-feldspar granite samples display peraluminous and high-K calc-alkaline compositions, and negative zircon εHf(t) and whole-rock εNd(t) values, and originated from partial melting of Paleoproterozoic igneous rocks, with additions of mantle-derived melts in their sources, in low-temperature, low-pressure conditions. Previously published zircon U-Pb age data show that numerous mafic igneous rocks were mostly generated during the Early Permian, marking the onset of mafic volcanism in the southern Alxa. Moreover, compiled detrital zircon U-Pb age data for Late Carboniferous−Permian sedimentary succession imply a regional topographic uplift during the Early Permian. Both of these two lines of evidence suggest an Early Permian lower crustal delamination in the southern Alxa. These Shouji adakitic plutons were most likely to be generated during the lower crustal delamination. Furthermore, Sr-Nd isotope data of Carboniferous-Permian igneous rocks show that the continental crust was recycled back into the mantle during Early Permian within the southern Alxa by means of subduction erosion and lower crustal delamination.

A combination of plume and subduction tectonics contributing to breakup of northern Rodinia: Constraints from the Neoproterozoic magmatism in the Dunhuang-Alxa Block, northwest China

Neoproterozoic igneous rocks are widely distributed in the Dunhuang-Alxa Block, northwest China, and record geodynamic processes caused by the assembly and break-up of Rodinia. In this study, we present new petrological, geochemical, and zircon U-Pb-Hf isotope data for the Langshan gabbros in the northeastern Alxa Block. Langshan gabbros (827−819 Ma) have depleted whole-rock Nd (+3.2 to +4.9) and zircon Hf (+4 to +21) isotopic compositions and weak enrichments in large-ion lithophile elements. These rocks were probably derived by partial melting of a depleted mantle wedge metasomatized by fluids released from a subducted slab in a back-arc basin environment. Geochemical and zircon Hf isotope data for early Neoproterozoic igneous rocks are consistent with the presence of a long-lived (ca. 930−810 Ma) retreating subduction zone along the northern margin of the Dunhuang-Alxa Block, indicating it was located at the northern margin of Rodinia. Some mantle plume-related magmatism, including A1-type silicic volcanic rocks in the Langshan Group and Jinchuan ultramafic-mafic intrusions, occurred in the interior of the Dunhuang-Alxa Block, which was similar to early Neoproterozoic plume-related magmatism (ca. 850−820 Ma) in the interior of northern Rodinia. The temporal link between subduction and extension suggests that this retreating subduction zone along the northern margin of Rodinia, in combination with a mantle plume beneath its interior, resulted in lithospheric extension and break-up of Rodinia during ca. 850−810 Ma.

Contrasting influence of sediments vs surface processes on retreating subduction zones dynamics

Surface processes and sediments have been suggested to play a crucial role in subduction initiation and to speed up subduction and plate tectonics. The exact roles of sediments and surface processes and the way they influence slab retreat dynamics has however not been constrained yet. Here, we analyse these roles by using 3D numerical thermo-mechanical modelling code coupled to surface processes model in order to investigate large scale dynamics of retreating subduction zones. We focus on two regional-scale geodynamic scenarios: a slab retreating in a homogeneous oceanic domain and a slab retreating along a continental margin before entering an oceanic domain. We study the influence of the thickness of the sediment cover on the oceanic crust and the intensity of surface processes. We show that the sediment thickness and the surface processes both affect the retreat dynamics in a distinct manner, and, when the slab retreats along a continental margin, also influence its trajectory. A thick sediment cover leads to the building of a large accretionary prism which acts as a lock on the system and slows down the slab retreat. When the slab retreats along a continental margin, the thicker the sediment cover, the stronger the slab deviation from that margin. Surface processes have a more complex role which depends on the amount of sediments available. In systems where little sediments are available, a higher efficiency of surface processes leads to a slow down of slab retreat by accumulating sediments in the accretionary wedge. In systems where a lot of sediments are available, a higher efficiency of surface processes slightly speeds up the retreat, by eroding the overriding plate and the subducting plate respectively at the front and the back of the accretionary wedge, as well as the prism itself when reaching a high enough elevation, thereby decreasing the mechanical locking caused by the accumulation of sediments.

The upper Cretaceous Ermioni VMS deposit, Argolis Peninsula, Peloponnese, Greece: Type, genesis, and geotectonic setting

The age, type and geotectonic setting of the Ermioni VMS are explored. Although obscured by post-ore deformation and metamorphism, the Ermioni VMS is genetically and spatially related to hydrothermally altered volcaniclastic rocks and arkoses at the footwall, and turbidites at the hangingwall. The Ermioni VMS deposits occur at Karakasi, Roro and Cambrorosso mine sites, and in close proximity to each other. At Karakasi, the identified hydrothermal alteration zones include a lower and innermost grunerite zone, succeded by an intermediate silica-chlorite zone which shows the largest spatial development and surrounded by a silica-epidote-chlorite zone at the flanks, whereas an albite-prehnite zone is developed at the top of the VMS system. The Ermioni VMS comprises nearly monomineralic pyrite with elevated Cu and Co content, and Cu/(Cu + Zn) and Co/Ni ratios typical for “Mafic-Pelitic”-type VMS deposit. Two ore stages are recognized, Stage I euhedral and coarse-grained pyrite (major Co carrier) with minor chalcopyrite, magnetite and pyrrhotite was followed by Stage II pyrite and minor chalcopyrite and sphalerite. Rubidium-Sr and Os-Re radiogenic pairs on pyrite from the Roro massive and Karakasi stringer ores define an upper Cretaceous age of VMS formation (65.58 ± 0.9 to 66.02 ± 0.1 Ma and 64.96 ± 0.9 to 65.12 ± 0.1 Ma, respectively). This age debunks the previously proposed classification as Cyprus-type. The stable isotope compositions of the hydrothermal fluids, including δ18OΗ2Ο, δDΗ2Ο, δ13CCO2, δ30SiNBS, point to mixing between subduction related hydrothermal fluids, seawater and seafloor turbidites, which accords well with the proposed classification. The δ34SH2S and δ57Fe values confirm a magmatic origin of sulfur and leaching of the footwall volcaniclastic rocks with contributions from seawater and turbidites. Radiogenic isotope and trace element geochemistry indicate that the Ermioni “Mafic-Pelitic” VMS was formed at a partly sediment-covered, upper Cretaceous back-arc spreading center above a retreating subduction zone setting within the “Adheres Melange unit”. The Ermioni VMS is the only known upper Cretaceous sulfide ore identified in Greece. The findings of this work may be used in expanding future research on the metallogenic potential of Greece.

Sedimentology and provenance of newly identified Upper Cretaceous trench basin strata, Dênggar, southern Tibet: Implications for development of the Eurasian margin prior to India–Asia collision

AbstractTrench basins preserved along the southern margin of the Lhasa Terrane, Tibet, are sedimentologic records of convergent margin processes preceding Cenozoic India–Asia collision. We present new sedimentologic, petrographic and geochronologic data from the Rongmawa Formation and surrounding strata near Dênggar, Tibet, to determine depositional environment, provenance and age. Depositional ages range from ca. 92 to 87 Ma and lithofacies are consistent with deposition by low‐ and high‐density turbidity currents and suspension settling of pelagic detritus in a deep‐marine, trench basin setting. Sandstone modal analyses and U–Pb geochronology indicate that trench basin detritus in this region was derived from the Lhasa Terrane. We interpret that the Cretaceous subduction trench received detritus from an axial sediment dispersal system that transported sediment from headwaters in the central‐southern Lhasa terrane near Lhasa City directly to the trench and then flowed westwards parallel to the trench. The preservation of trench basin strata deposited during Late Cretaceous time compared with the lack of trench deposits prior to ca. 92 Ma and after ca. 80 Ma suggests the margin experienced a period of significant accretion during this interval. In addition, deposition of trench basin strata occurred during Late Cretaceous adakitic magmatism and high‐temperature metamorphism, which are hypothesized to be explained by subduction of an oceanic ridge or subduction zone retreat and related upper plate extension along the southern margin of the Lhasa terrane. Subduction of an oceanic ridge may provide a mechanism to potentially erode forearc basin strata and promote increased sediment delivery directly to the trench.

Early Paleozoic accretionary orogens along the Western Gondwana margin

Early Paleozoic accretionary orogens dominated the Western Gondwana margin and were characterized by nearly continuous subduction associated with crustal extension and back-arc basin development. The southwestern margin is represented by Famatinian and Pampean basement realms exposed in South America, both related to the protracted Paleozoic evolution of the Terra Australis Orogen, whereas the northwestern margin is mainly recorded in Cadomian domains of Europe and adjacent regions. However, no clear relationships between these regions were so far established. Based on a compilation and reevaluation of geological, paleomagnetic, petrological, geochronological and isotopic evidence, this contribution focuses on crustal-scale tectonic and geodynamic processes occurring in Western Gondwana accretionary orogens, aiming at disentangling their common Early Paleozoic evolution. Data show that accretionary orogens were dominated by high-temperature/low-pressure metamorphism and relatively high geothermal gradients, resulting from the development of extended/hyperextended margins and bulk transtensional deformation. In this sense, retreating-mode accretionary orogens characterized the Early Paleozoic Gondwana margin, though short-lived pulses of compression/transpression also occurred. The existence of retreating subduction zones favoured mantle-derived magmatism and mixing with relatively young (meta)sedimentary sources in a thin continental crust. Crustal reworking of previous forearc sequences due to trenchward arc migration thus took place through assimilation and anatexis in the arc/back-arc regions. Therefore, retreating-mode accretionary orogens were the locus of Early Paleozoic crustal growth in Western Gondwana, intimately associated with major flare-up events, such as those related to the Cadomian and Famatian arcs. Slab roll back, probably resulting from decreasing convergence rates and plate velocities after Gondwana assembly, was a key factor for orogen-scale geodynamic processes. Coupled with synchronous oblique subduction and crustal-scale dextral deformation, slab roll back might trigger toroidal mantle flow, thus accounting for bulk dextral transtension, back-arc extension/transtension and a large-scale anticlockwise rotation of Gondwana mainland.

Geochronologic and Hf-isotope framework of Proterozoic rocks from central New Mexico, USA: Formation of the Mazatzal crustal province in an extended continental margin arc

The growth of southern Laurentia has been attributed to the accretion of juvenile arc terranes during the successive 1.74–1.68 Ga Yavapai and 1.65–1.60 Ga Mazatzal orogenies. However, in light of the increasing importance of the ca. 1.49–1.40 Ga Mesoproterozoic Picuris orogeny, the tectonic setting in which the Mazatzal crustal province and its distinctive quartzite-rhyolite successions were generated needs additional examination. The Sandia-Manzano-Los Pinos uplift in central New Mexico is an ideal place to characterize the tectonic history of the Mazatzal crustal province. A comprehensive geochronologic and Hf-isotopic dataset for Proterozoic rocks of the Sandia-Manzano-Los Pinos uplift is presented. Plutonic and metavolcanic rocks in the Sandia-Manzano-Los Pinos uplift were emplaced in three pulses at 1668–1655 Ma, 1587 Ma, and 1459–1453 Ma. Hf-isotope data from the Paleoproterozoic plutonic rocks are juvenile, with both leucogranite and arc-related granodiorite yielding εHf(t) values ranging from +6 to +12, compared to the coeval depleted mantle value of +10 at ca. 1.65 Ga. Inherited zircon in Paleoproterozoic rocks suggest that crust older than 1.7 Ga was involved in their genesis. Hf-isotope data from Mesoproterozoic plutonic rocks in the Sandia-Manzano-Los Pinos uplift are consistent with derivation from 1.7 to 1.6 Ga lithosphere. Detrital zircon indicate that metasedimentary rocks of the lower Manzano group were derived primarily from local sources that have U-Pb-Hf isotope compositions similar to the plutonic rocks which intrude and volcanic rocks that underlie the Manzano group. The detrital zircon provenance of the Manzano group broadens up-section from unimodal populations with age peaks at ca. 1.65 Ga to include 1.7–3.0 Ga detrital zircon derived from older Laurentian sources like the Yavapai and Mojave provinces. We offer a new model for the formation of the Mazatzal crustal province of New Mexico as a continental margin arc built on top of the previously assembled Yavapai province. The Manzano group quartzite-rhyolite succession was formed by lithospheric extension above a north-dipping, southward retreating subduction zone. The Manzano group was then subjected to ca. 1.65 Ga syn-magmatic tectonism and later intracratonic contractional tectonism, likely during the 1.46–1.40 Ga Picuris orogeny.

Palaeomagnetism of the Dundas–Fossey Trough, Tasmania: Oroclinal rotation and Late Cretaceous overprinting

The Dundas–Fossey Trough, a curved belt of Early Palaeozoic rocks in western and northern Tasmania, is hypothesised to have developed as an orocline driven by asthenospheric flow behind a rapidly retreating subduction zone on the East Gondwana margin. Rocks of the Wurawina Supergroup from the trough were sampled for a palaeomagnetic test of this hypothesis. Rock magnetic analysis identifies both magnetite and hematite remanence carriers. Hematite carries a normal-polarity overprint which was acquired at relatively low temperature. Thermal demagnetisation over the magnetite unblocking range reveals a linear trend that defines component A, which displays both normal and reversed polarity. Component A passes a modified version of the fold test, indicating that it is pre-deformational. The declination of component A correlates linearly with regional strike, thus passing the orocline test. A palaeomagnetic pole for the Cambrian Owen Group, after correction for oroclinal rotation, is displaced from the Gondwana polar wander path, suggesting that the western limb of the trough may have been rotated anticlockwise by about 40° prior to the development of the orocline, presumably during known Early to Middle Ordovician deformation. Adjustment of the palaeomagnetic declination data to account for this earlier phase of rotation improves the fit parameters of the orocline test. The palaeomagnetic results are consistent with the orocline model for the Dundas–Fossey Trough, and support the inference that the trough extends beneath later cover in southeastern Tasmania. Overprint component B yields a mean pole in broad agreement with other overprints previously observed both in Tasmania and the eastern seaboard of the Australian mainland, and like them appears to have been acquired during the Late Cretaceous normal polarity superchron. We speculate that magnetic overprinting in Tasmania is related to enhanced oxidation by meteoric water driven by regional uplift.

Oceanic crust recycling controlled by weakening at slab edges

Retreating subduction zones such as the Lesser Antilles, Gibraltar and Scotia have been migrating towards the Atlantic Ocean by cutting their way through the oceanic crust. This spontaneously retreating subduction is enabled by the development of faults at the edges of the slab, but the physical mechanisms controlling fault propagation and direction remain unknown. Here, using 3D numerical subduction models we show that oceanic lithosphere recycling is mainly controlled by the intensity of strain-induced weakening of fractures forming at the edges of the slab. Intense strain-induced weakening causes predominantly brittle fault propagation and slab narrowing until detachment. Without weakening, preponderantly ductile slab edge propagation occurs, which causes slab widening. This rheological control is not affected by the proximity of non-weakened passive continental margins. Natural examples suggest that slab edges follow convergent paths that could be controlled by fractures weakening due to deep water penetration into the oceanic lithosphere.

Geology, geochronology and geochemistry of the Miocene Sulutas volcanic complex, Konya-Central Anatolia: genesis of orogenic and anorogenic rock associations in an extensional geodynamic setting

ABSTRACT Subduction-related environments are complex geological sites where various magmas occur in close spatial and temporal association. To better understand geological processes led to the generation of compositionally distinct magmas, we need to know the origin and evolution of magmas formed in these settings. Our purpose here is to determine the genesis of the Sulutas Volcanic Complex (SVC), located near Konya (Central Anatolia, Turkey) and to understand its relation to the subduction-related processes. Based on a detailed field study of the SVC, we present here Ar-Ar geochronology, mineral chemistry, whole-rock major, trace element and Sr-Nd-Pb isotope data. The SVC can be subdivided into three distinct chemical groups. The oldest group (~16 Ma) characterized by bimodal association is composed of Na-alkaline basaltic rocks and adakitic dacite. The Na-alkaline basaltic rocks were derived from an OIB-like enriched asthenospheric mantle source, but the adakitic dacites from a mafic lower crust. Geochemistry of potassic lamprophyres (~13.5 to 12.5 Ma), the second group, can be best explained by melting of a depleted lithospheric mantle source that was metasomatized by slab-derived sediment melts. This metasomatic component generated phlogopite-rich veins in their source. The youngest group (~13 to 11 Ma), high-K calc-alkaline rocks (HKCA) is composed of basalt, andesite and mafic enclave-bearing dacite. Parental magmas to the HKCA are consistent with the derivation from a lithospheric mantle source metasomatized by fluid-like subduction components. Geochemical variations in the HKCA can be explained by magmatic processes (e.g. assimilation-fractional crystallization for basalt and dacite, fractional crystallization for andesite) from a common parental magma similar to the mafic enclave. The SVC composed of orogenic and anorogenic rocks occurred in a graben-like extensional basin in the Central Anatolia, and this extensional tectonics is related to the retreating subduction zone along the Cyprus arc within the convergence system of the African-Eurasian plates during Miocene.

Differentiating advancing and retreating subduction zones through regional zircon Hf isotope mapping: A case study from the Eastern Tianshan, NW China

Advancing and retreating subduction zones at convergent margins, primary sites of continental crust growth and loss, are the cornerstones of tectonic evolution of accretionary orogens. Yet, distinguishing the tectonic mode (advancing, retreating, or switching) of subduction zones in fossil orogens is not always straightforward due to the scarcity of strong evidence. As an alternative approach, we utilized zircon Hf isotope mapping of arc magmatism (previously-published data) from different tectonic units in the Eastern Tianshan and the northern Tarim, revealing that (a) an early Paleozoic (≥ca. 450 Ma) southward subduction of the Junggar oceanic plate (subduction zone advance) resulted in compression and crustal thickening in the northern Tarim (upper plate), as evidenced by increasingly more enriched zircon Hf isotopic compositions, and (b) a slab rollback/retreat of the Junggar oceanic plate probably since ca. 450–440 Ma (tectonic switching from subduction zone advance to retreat), as manifested by progressive increases in zircon εHf(t) values, induced crustal extension and opening of the South Tianshan back-arc basin in the northern Tarim, separating the Central Tianshan continental arc as a micro-continent sliver. This case study highlights the great potential of regional zircon Hf isotope mapping, besides traditional investigations, in helping reveal the enigmatic tectonic mode of subduction zones in ancient orogens.

Petrogenesis and tectonic implication of the Late Mesozoic volcanic rocks in East Mongolia

This paper presents new geochemical and geochronological data of the Late Mesozoic volcanic rocks in East Mongolia. These volcanic rocks belong to high‐K calc‐alkaline and shoshonitic series and exhibit features of bimodal rocks. The mafic rocks have elevated incompatible trace element concentrations and significantly negative Nb, Ta, and Ti anomalies, with 87Sr/86Sr(i) and ƐNd(t) values of 0.70502–0.70572 and −1.72459 to +1.720736, respectively, which suggest that the mafic magma was derived from a lithospheric mantle source that might have been metasomatized by subduction‐derived fluids, experienced fractional crystallization, and was contaminated by crustal materials. The felsic rocks have similar rare earth element patterns to the mafic rocks but show much more significantly negative Eu and Sr anomalies. The felsic rocks have higher 87Sr/86Sr(i) values of 0.706496–0.71104 than the mafic rocks but similar ƐNd(t) values (−0.28003 to +2.928506) to the mafic ones. These data indicate that the felsic rocks originated from partial melting of a crustal source that is dominated by juvenile mafic rocks. Our new K‐Ar dating, together with previous data, shows that the Late Mesozoic volcanism in East Mongolia took place during the Late Jurassic‐Early Cretaceous between ca. 155 and 99 Ma. The model of back‐arc extension, possibly induced by slab rollback of the westward subducted Pacific Plate and the subduction zone retreat, can explain the geodynamic setting and the eastward younging trend of the Late Mesozoic volcanism in East Mongolia and in adjacent NE China.

Subduction zone decoupling/retreat modeling explains south Tibet (Xigaze) and other supra-subduction zone ophiolites and their UHP mineral phases

The plate tectonic setting in which proto-ophiolite ‘oceanic’ lithosphere is created remains controversial with a number of environments suggested. Recent opinions tend to coalesce around supra-subduction zone (SSZ) forearc extension, with a popular conceptual model in which the proto-ophiolite forms during foundering of oceanic lithosphere at the time of spontaneous or induced onset of subduction. This mechanism is favored in intra-oceanic settings where the subducting lithosphere is old and the upper plate is young and thin. We investigate an alternative mechanism; namely, decoupling of the subducting oceanic lithosphere in the forearc of an active continental margin, followed by subduction zone (trench) retreat and creation of a forearc oceanic rift basin, containing proto-ophiolite lithosphere, between the continental margin and the retreating subduction zone. A template of 2D numerical model experiments examines the trade-off between strength of viscous coupling in the lithospheric subduction channel and net slab pull of the subducting lithosphere. Three tectonic styles are observed: 1) C, continuous subduction without forearc decoupling; 2) R, forearc decoupling followed by rapid subduction zone retreat; 3) B, breakoff of subducting lithosphere followed by re-initiation of subduction and in some cases, forearc decoupling (B-R). In one case (BA-B-R; where BA denotes backarc) subduction zone retreat follows backarc rifting. Subduction zone decoupling is analyzed using frictional-plastic yield theory and the Stefan solution for the separation of plates containing a viscous fluid. The numerical model results are used to explain the formation of Xigaze group ophiolites, southern Tibet, which formed in the Lhasa terrane forearc, likely following earlier subduction and not necessarily during subduction initiation. Either there was normal coupled subduction before subduction zone decoupling, or precursor slab breakoff, subduction re-initiation and then decoupling. Rapid deep upper-mantle circulation in the models during subduction zone retreat can exhume and emplace material in the forearc proto-ophiolite from as deep as the mantle transition zone, thereby explaining diamonds and other 10–15 GPa UHP phases in Tibetan ophiolites.

Juvenile crustal recycling in an accretionary orogen: Insights from contrasting Early Permian granites from central Inner Mongolia, North China

Coeval high-K calc-alkaline to alkaline granites constitute important components of post-collisional to post-orogenic igneous suites in most orogenic belts of various ages on Earth and their genesis harbors a key to ascertaining critical geodynamic controls on continental crustal formation and differentiation. This zircon U–Pb dating and geochemical study documents three contrasting Early Permian granites from Erenhot of central Inner Mongolia, eastern Central Asian Orogenic Belt (CAOB) and reveals concurrent high-K calc-alkaline to alkaline granite association derived from successive partial melting of distinct protoliths. The ca. 280Ma Gancihuduge (GCG) pluton shows a calc-alkaline I-type character, with initial 87Sr/86Sr ratios of 0.7035 to 0.7039, εNd(t) of +1.87 to +4.70, zircon εHf(t) of +8.0 to +13.2 and δ18O from 7.4 to 8.7‰. The ca. 276Ma Cailiwusu (CLS) pluton is magnesian and peraluminous, with initial 87Sr/86Sr ratios of 0.7036 to 0.7040, εNd(t) of +1.9 to +2.4, zircon εHf(t) of +6.5 to +12.1 and δ18O from 9.7 to 10.9‰. These features are consistent with partial melts of mixed sources composed of newly underplated meta-basaltic to -andesitic protoliths and variable supracrustal components, with distinctively higher proportion of the latter in the CLS pluton. By contrast, the ca. 279Ma Kunduleng (KDL) suite exhibits an A-type magmatic affinity, with typical enrichment in alkalis, Ga, Zr, Nb and Y, εNd(t) of +2.39 to +3.55, zircon εHf(t) from +8.3 to +12.3 and δ18O values from 6.8 to 7.5‰. These features suggest that they stem from high-temperature fusion of dehydrated K-rich mafic to intermediate protoliths. Besides presenting a snapshot into a stratified crustal architecture in δ18O, these contrasting granites could not only serve as a temporal marker for monitoring post-collisional extension in the aftermath of a retreating subduction zone, but also present spatial magmatic proxy for tracing crustal formation and differentiation within back-arc basin environments in the CAOB.