Interrelation of the stagnant slab, Ontong Java Plateau, and intraplate volcanism as inferred from seismic tomography

We investigated the seismological structure beneath the equatorial Melanesian region, where is tectonically unique because an immense oceanic plateau, a volcanic chain and subduction zones meet. We conducted a multi-frequency P-wave tomography using data collected from an approximately 2-year-long seismic experiment around the Ontong Java Plateau (OJP). High-velocity anomalies were revealed beneath the center of the OJP at a depth of ~ 150 km, the middle-eastern edge of the OJP at depths of 200–300 km, and in the mantle transition zone beneath and around the OJP; low-velocity anomalies were observed along the Caroline volcanic island chain above 450 km depth. These anomalies are considered to be associated with the thick lithosphere of the OJP, remnant dipping Pacific slab, stagnant Pacific slab, and a sheet-like upwelling. The broad stagnant slab was formed due to rapid trench retreat from 48 to 25 Ma until when the OJP with thick lithosphere collided with a subduction boundary of the Pacific and Australian plates. This collision triggered slab breakoff beneath the arc where the dipping slab remained. The stagnant Pacific slab in the mantle transition zone restricted the plume upwelling from the lower mantle causing sheet-like deformed upwelling in the upper mantle.

Unconformity development in retroarc foreland basins: Implications for the geodynamics of Andean-type margins

Unconformities in foreland basins may be generated by tectonic processes that operate in the basin, adjacent fold-thrust belt, and broader convergent margin. Foreland basin unconformities represent shifts from high accommodation to nondepositional or erosional conditions in which the interruption of subsidence precludes net sediment accumulation. This study explores the genesis of long duration (>1–20 Myr) unconformities and condensed stratigraphic sections by considering modern and ancient examples from the Andes. These cases highlight potential geodynamic mechanisms of accommodation reduction and hiatus development in Andean-type retroarc foreland settings, including: (a) shortening-induced uplift in the frontal thrust belt and proximal foreland; (b) growth and advance of a broad, low-relief flexural forebulge; (c) uplift of intraforeland basement blocks; (d) tectonic quiescence with regional isostatic rebound; (e) cessation of thrust loading and flexural subsidence during oblique convergence; (f) diminished accommodation or sediment supply due to changes in sea level, climate, erosion, or transport; (g) basinwide uplift during flat-slab subduction; and (h) dynamic uplift associated with slab window formation, slab breakoff, elevated intraplate (in-plane) stress, or related mantle process. These contrasting mechanisms can be distinguished on the basis of the spatial distribution, structural context, stratigraphic position, paleoenvironmental conditions, and duration of unconformities and condensed sections.Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts

Delamination in Tibet: Deriving constraints from the density of eclogite

Tibet, which is characterized by collisional orogens, has undergone the process of delamination or convective removal. The lower crust and mantle lithosphere appear to have been removed through delamination during orogenic development. Numerical and analog experiments demonstrate that the metamorphic eclogitized oceanic subduction slab or lower crust may promote gravitational instability due to its increased density. The eclogitized oceanic subduction slab or crustal root is believed to be denser than the underlying mantle and tends to sink. However, the density of eclogite under high-pressure and high-temperature conditions and density differences from the surrounding mantle is not preciously constrained. Here, we offer new insights into the derivation of eclogite density with a single experiment to constrain delamination in Tibet. Using in situ synchrotron X-ray diffraction combined with diamond anvil cell, experiments focused on minerals (garnet, omphacite, and epidote) of eclogite are conducted under simultaneous high-pressure and high-temperature conditions, which avoids systematic errors. Fitting the pressure-temperature-volume data with the third-order Birch-Murnaghan equation of state, the thermal equation of state (EoS) parameters, including the bulk modulus (KT0), its pressure derivative (KT0′), the temperature derivative ((KT/T)P), and the thermal expansion coefficient (α0), are derived. The densities of rock-forming minerals and eclogite are modeled along with the geotherms of two types of delamination. The delamination processes of subduction slab breakoff and the removal of the eclogitized lower crust in Tibet are discussed. The Tibetan eclogite which containing 40–60 vol. % garnet and 37–64 % degrees of eclogitization can promote the delamination of slab break-off in Tibet. Our results indicate that eclogite is a major controlling factor in the initiation of delamination. A high abundance of garnet, a high Fe-content, and a high degree of eclogitization are more conducive to instigating the delamination.

Numerical models of Cretaceous continental collision and slab breakoff dynamics in western Canada

ABSTRACT The North American Cordillera is generally interpreted as a result of the long-lived, east-dipping subduction at the western margin of the North American plate. However, the east-dipping subduction seems problematic for explaining some of the geological features in the Cordillera such as large volume back-arc magmatism. Recent studies suggested that westward subduction of a now-consumed oceanic plate during the Cretaceous could explain these debated geological features. The evidence includes petrological and geochemical variations in magmatism, the presence of ophiolite that indicates tectonic sutures between the Cordillera and Craton, and seismic tomography images showing high-velocity bodies within the underlying convecting mantle that are interpreted as slab remnants from the westward subduction. Here we use 2-D upper mantle-scale numerical models to investigate the dynamics associated with westward subduction and Cordillera-Craton collision. The models demonstrate the controls on slab breakoff (remnant) following collision including: (1) oceanic and continental mantle lithosphere strength, (2) variations in density (eclogitization of continental lower crust and cratonic mantle lithosphere density), and (3) convergence rate. Our preferred model has a relatively weak mantle lithosphere, eclogitization of the lower continental crust, cratonic mantle lithosphere density of 3250 kg/m3, and a convergence rate of 5 cm/yr. It shows that collision and slab breakoff result in an ∼2 km increase in surface elevation of the Cordilleran region west of the suture as the dense oceanic plate detaches. The surface also shows a foreland geometry that extends >1000 km east of the suture with ∼4 km of subsidence relative to the adjacent Cordillera.

The Early Cretaceous tectonic evolution of the southern Great Xing'an Range, NE China: New constraints from A2-type granite and monzodiorite

The widespread Early Cretaceous plutons intruding along the southern Great Xing’an Range (SGXR) provide evidence for tectonic evolution of the region. Petrological, geochemical, zircon U–Pb geochronology and zircon Hf isotopic studies are conducted on intrusions from Bianjiadayuan and Hongling areas. These suites classify as A2-type granites and monzodiorites, respectively. The 138–133 Ma A2-type granites originated from partial melting of continental crustal materials at high temperatures and shallow depths with significant addition of juvenile mafic lower crust sourced from a metasomatized mantle. The 136–134 Ma monzodiorites originated from the partial melting of an enriched mantle that was modified by melts of a previously subducted slab coupled with crustal contamination. The Early Cretaceous magmatism in the SGXR occurred in two periods: ∼145–136 Ma (peak at ∼139 Ma; εHf (t) = 5 to 10) and ∼136–130 Ma (peak at ∼131 Ma; εHf (t) = −10 to 15). The Early Cretaceous granite–monzodiorite suite in the SGXR suggests a bimodal magmatism in an extensional setting. The ∼145–130 Ma magmatism may have been triggered by asthenospheric upwelling induced by the Mongol–Okhotsk oceanic slab breakoff and large-scale lithospheric delamination resulting from post-orogenic extension. The variation of subduction direction of the Paleo-Pacific Ocean likely triggered a change in stress regime at ca. 136 Ma and likely promoted the lithospheric delamination beneath the SGXR resulting in intense magmatism originating from various sources. As such, the Paleo-Pacific Oceanic subduction likely played an important role in the Early Cretaceous magmatism in the SGXR.

Eocene thickening without extra heat in a collisional orogenic belt: A record from Eocene metamorphism in mafic dike swarms within the Tethyan Himalaya, southern Tibet

Knowledge of the nature of the earliest metamorphism experienced by collisional orogenic belts is essential for reconstruction of tectonic processes that build high mountain chains and their environmental consequences. Understanding the metamorphic nature of Eohimalayan-phase orogeny of the Himalayan orogen, one of the typical examples of orogenic belts worldwide, could provide some important constraints to test different tectonic models (shallow continental subduction vs. slab breakoff) for the early phases of the development of large-scale orogenic belts. As exhumed middle- to lower-crustal rocks in the Kangmar gneiss dome, the garnet amphibolites with a protolith age of 176.4 ± 3.6 Ma experienced a phase of metamorphism at 47.2 ± 1.8 Ma with an increase in pressure as well as temperature from 3−5 kbar and 550−600 °C to over ∼11 kbar and 650 °C. This suggests that the middle- to lower-crustal rocks experienced heating at least by ∼50 °C while they underwent compression and thickening. Heat-flow estimation further demonstrates that the self-produced heat was high enough to achieve the observed pressure-temperature conditions recorded by the garnet amphibolite. Therefore, an additional heat supply is not required during early Eocene metamorphism. A breakoff of the leading part of the subducting Indian continental slab, if it occurred, should be younger than ca. 47 Ma.

Protracted intra- and inter-pluton magmatism during the Acadian orogeny: evidence from new LA-ICP-MS U-Pb zircon ages from northwestern Maine, USA

Devonian granitoid plutons comprise a major part of the bedrock of northwestern Maine representing the magmatic expression of the Acadian orogeny in this part of the northern Appalachian orogen. They are petrographically diverse with minerals characteristic of both I- and S-type granites, in some cases within the same intrusion, and some are compositionally zoned. New LA-ICP-MS ages presented here elucidate the timing and duration of this magmatism. The earliest phase of granitoid magmatism began around 410–405 Ma with the emplacement of the Flagstaff Lake Igneous Complex, and the presence of contemporaneous mafic rocks suggests that mantle-derived magmas were also produced at this time. Late Devonian ages, ca. 365 Ma, for many intrusions, such as the Chain of Ponds and Songo plutons, reveal that magmatism continued for 45 million years during which compositionally diverse I- and S-type magmas were produced. In addition, there is evidence that intrusive activity was prolonged within some plutons, for example the Rome-Norridgewock pluton and the Mooselookmeguntic Igneous Complex, with 10–15 myr between intrusive units. The new ages suggest a break in magmatism between 400 Ma and 390 Ma apparently separating Acadian magmatism into early and late pulses. The production of lower crustal I-type magmas appears to have been concentrated later, ca. 380–365 Ma, although several S-type granitoids were also emplaced during this period. These Late Devonian plutons display abundant zircon inheritance with ages around 385 Ma, which suggests that the crust was experiencing enhanced thermal perturbations during this extended timeframe. The new data for granitoid plutons in northwestern Maine are consistent with tectonic models for other parts of Ganderia which propose initial flat slab subduction followed by slab breakoff and delamination.

Magmatic evidence for Late Carboniferous-Early Permian slab breakoff and extension of the southern Mongolia collage system in Central Asia

<p>Oceanic subduction and its last underthrusted part can both triggers arc-like magmatism. As the existence of multi-subduction zones in the Central Asian Orogenic Belt, controversy still surrounds on when and especially how the subduction of the (Paleo-Asian Ocean) PAO terminated. We present geochronological, geochemical, and Lu-Hf isotopic data for a suite of basalt-andesites, dacite-rhyolites and later trachyandesite-mugearitic dykes from the Khan-Bogd area in the Gobi Tianshan Zone (GTZ) of the southern Mongolia. U-Pb dating of zircons indicate the basalt-andesites and dacite-rhyolites were formed at ~334-338 Ma, and the dykes at ~300 Ma. These Early Carboniferous volcanic rocks display high U/Th, Ba/Th, low La/Sm and variable Zr/Nb ratios, implying the involvement of subduction fluids or sediment melt. They display arc geochemical features such as calc-alkaline and metaluminous nature and positive Ba and U and negative Nb, Ta and Ti anomalies. Moreover, their continental geochemical signals (e.g. positive Pb, K anomalies) and some old captured zircons implying a continental arc setting. Comparatively, the ~300 Ma dykes are characterized by high alkaline contents, which are common for coeval (~320-290 Ma) and widespread post-subductional granites there. Given a mainly crust-derived magma source for those granites, these dykes likely reflect a mantle disturbance due to: (1) their relative low SiO<sub>2 </sub>(51.71-55.85 wt. %) and high Mg# (40.3-67.3) values, and (2) positive zircon Ɛ<sub>Hf</sub>(t) (most > 12). Considering a slab rollback model during the Carboniferous and Triassic, the mantle disturbance was possibly induced by the oceanic slab breakoff. Combined with previous work, this ~320-290 Ma slab breakoff-induced extension marks the closure of a wide secondary ocean (North Tianshan-Hegenshan ocean) north of the main ocean basin of the PAO. This research was financially supported by NSFC Projects (41730213, 42072264, 41902229, 41972237) and Hong Kong RGC GRF (17307918).</p>

Closing the Neotethys Ocean in western Anatolia: Insights from forearc and foreland sedimentary basin records

<p>Across the Tethyan realm, subduction zones are characterized by phases of forearc and backarc extension, and subsequent collisions are protracted and polygenetic, often resulting in significant discrepancies among proxies of collision age. The closure of the northern branch of the Neotethys Ocean along the İzmir-Ankara-Erzincan suture in Anatolia has been variously estimated from the Late Cretaceous to Eocene. It remains unclear whether this age range results from a protracted, multi-phase collision or disparities between proxies and geographic location. Near-continuous Jurassic through Eocene deposition in the forearc-to-foreland Central Sakarya Basin system in western Anatolia makes it an ideal location to integrate pre-collisional extension and multi-stage collision into a holistic reconstruction of subduction through collision. The Central Sakarya Basin system is located north of the Izmir-Ankara-Erzincan suture, where the Gondwanan-derived Anatolide and Tauride terranes to the south collided with the Laurasian-derived Pontide terrane in the north. By integrating new sandstone petrography and detrital zircon U-Pb and Hf isotopes with other geologic proxies, we demonstrate four phases of evolution of subduction and collision. (1) Magmatism began on the Pontides at 110 Ma, potentially the signal of subduction (re-)initiation, and is coincident with extension in the forearc. (2) Forearc obduction began around 94 Ma with initial subduction of lower plate continental lithosphere. Extension migrated to the backarc and opened the Black Sea. (3) The onset of intercontinental collision at 76 Ma is marked by gradual arc shutdown, basement exhumation, and uplift of the suture zone. (4) This first contractional phase is followed by thick-skinned deformation and basin partitioning starting around 54 Ma, coeval with regional syn-collisional magmatism. The 20-Myr protracted collision in western Anatolia could be explained by three non-exclusive mechanisms that produced a change in plate coupling: relict basin closure, progressive underthrusting of thicker lithosphere, and slab breakoff.</p>