Dipping Subduction Research Articles

Makran Subduction Zone: A Review and Synthesis

This review synthesizes existing research to elucidate the factors driving the distinct tectonic behaviors in the western and eastern Makran subduction zone, focusing on seismic activity, uplift rate, convergence rate, coupling, and subduction angle. The literature identifies the asymmetry in pressure and the variation in subduction angles between the western and eastern parts of the Makran as key factors in defining the region’s tectonic patterns. The western region has a steeper subduction angle, resulting in lower pressure, reduced coupling, and decreased seismic activity. This disparity arises from different interactions between the subducted and overriding plates. This article offers an overview of the Makran subduction zone, identifies some knowledge gaps, and suggests directions for future research to improve our understanding of this complex geological region. The review highlights the need for more comprehensive GPS stations and targeted studies on subduction dip angles to better understand the region’s tectonic dynamics.

Eocene high-K magmatic flare-up in a context of south dipping subduction and strike-slip tectonics: Insights from the Talysh Massif, NW Iran

The geodynamic significance of Cenozoic magmatism of the Central Tethyan orogenic belt linking the tectonic zones of Iran, South Armenian Block (Lesser Caucasus) and Turkey remains a subject of debate. This study is focused on the thick geological succession of high potassic magmatic rocks erupted in Talysh Massif, NW Iran. We report a comprehensive geochemical study that demonstrates that this succession is comprised of shoshonitic mostly basaltic and some rhyolitic pyroclastic rocks emplaced during an Eocene magmatic flare-up. Forsterite rich olivine (Fo 70–93), combined to high-Mg clinopyroxene (Mg# 0.6–0.8) and high-Ti calcic amphibole (magnesiohastingsite) are the dominant mafic minerals in these rocks. Mafic lavas are enriched in LILE and depleted in HFSE, with high Th/Yb, Ba/Th and Nb/Zr ratios, suggestive of an enriched mantle source following Tethys subduction history. Interlayered felsic lavas allowed to date the series by U-Pb on zircon at 43.17 ± 0.40 Ma, with one relict core displaying an inherited age of 580 Ma. eHfi CHUR of Eocene zircons range from 1.7 to −0.9 (with corresponding TDM ages around 1.0 Ga). The latter, along with the detection of a Neoproterozoic zircon xenocryst, indicates a crustal contamination by a Neoproterozoic (and older) lower crust. These data are in agreement with the partial melting of a subduction-modified subcontinental mantle. This magmatic flare-up could have been triggered by an Asthenosphere upwelling related to the onset of south-dipping subduction in the Transcaucasus region and transcrustal strike-slip faults with a more significant crustal contamination than in the Lesser Caucasus region.

Progressive weakening within the overriding plate during dual inward dipping subduction

The evolution of dual inward dipping subduction (DIDS) is crucial to understand multiple slab interaction. Yet, how DIDS influences the thermo-mechanical behaviour of the overriding plate remains unclear, as previous DIDS investigations all applied a compositional or Newtonian rheology that excludes temperature dependency. Here we apply a composite rheology, including temperature dependent creep deformation mechanisms, in 2-D thermo-mechanical numerical modelling to investigate how DIDS modifies the rheological structure of the overriding plate. Three variables on plate sizes are investigated to understand what may control the maximum degree of plate weakening. We find that reducing the initial length or initial thickness of the overriding plate and increasing the initial thickness of the subducting plate can enhance the viscosity reduction within the overriding plate. The progressive weakening can result in a variety of stretching states ranging from 1) little or no lithosphere thinning and extension (<5% accumulation of strain), to 2) limited thermal lithosphere thinning (<30% accumulation of strain), and 3) localised rifting followed by spreading extension. Compared with single sided subduction, DIDS further reduces the magnitude of viscosity in the overriding plate. It does this by creating a dynamic fixed boundary condition for the overriding plate and forming a stronger upwelling mantle flow, both of which promote the progressive weakening in the overriding plate. The result implies that these generic DIDS effects are important aspects to consider to understand extension developed in natural DIDS cases. We also demonstrate that both temperature dependent creep rheologies and yielding deformation mechanism contribute significantly to the continuous viscosity reduction. The finding may also have a broader implication for more general processes that involve plate scale weakening, strain localisation and the formation of new plate boundaries.

Early Cretaceous subduction of an oceanic plateau at the Northern Andes; geochemical, metamorphic, and cooling age constraints of the Raspas Metamorphic Complex

The Raspas Metamorphic Complex is a high-pressure unit that includes eclogites, blueschists, and metapelites, with minor garnet amphibolites and greenschists. Although its geochemistry and ages are relatively well known, metamorphic constraints and the origin of its protolith remain unclear. In this study, we present new geochemical, metamorphic, and cooling age data to understand the protolith, metamorphic conditions, and the tectonic implications of this feature for the Northern Andes.Geochemical analyses reveals rocks with N- to E-MORB signatures and trace element ratios typical of a heterogeneous mantle source, like oceanic plateau lavas. Blueschists originated from a deep enriched source, similar to primitive mantle, while eclogites are similar to MORB. Iterative thermodynamic modeling of an eclogite indicates a low T/P gradient of ∼406 °C/GPa during the formation of the assemblage garnet (core), quartz, katophorite, and omphacite cores at ∼598 °C and 1.43 GPa, followed by the formation of atoll garnets and zoisite at 661 °C and 1.63 GPa, around 133 Ma. Peak metamorphic conditions were attained after isobaric heating at 709 °C and 1.61 GPa, resulting in the assemblage of garnet (rim), zoisite (rim), omphacite (rim), and katophorite. 40Ar/39Ar ages around 129 Ma indicate fast exhumation rates of 1.25 to 0.5 cm/yr.We propose that the Raspas Metamorphic Complex represents the interaction of the South American plate with an oceanic plateau during the Early Cretaceous. During the early stages of subduction, it created a flat slab geometry around 130 Ma. As the plateau transformed to eclogite, the decrease in buoyancy caused an increase in subduction dip during a regional roll-back at 129–123 Ma, which triggered the exhumation along the high-pressure belt.

Dynamics of Oceanic Slab Tearing During Transform‐Fault Horizontally‐Oblique Subduction: Insights From 3D Numerical Modeling

AbstractOceanic‐plates vertical tearing is seismically identified in the present‐day Earth. This type of plate tearing is frequently reported in horizontally‐oblique subduction zones where transform‐faulted oceanic plates are subducting (or subducted). However, the mechanisms behind vertical slab tearing are still poorly understood, thus we utilize 3D time‐dependent Stokes' flow thermo‐mechanical numerical models to further study this problem. We find that (a) the age offset of transform fault and (b) the horizontal obliqueness of subduction fundamentally control the tearing behavior of two generic, materially homogeneous oceanic slabs separated by a low‐viscosity zone. The two slabs sequentially bend, which combined with the age‐thickness difference between slabs, causes the differential sinking of them. Based on the modeling results, well‐developed slabs vertical tearing would happen when the oblique angle of subduction is ≥30° or the age ratio of the secondly bent to firstly bent slab being ∼&lt;0.6. Quantifying the horizontal distance‐vector between sinking slabs, we find that subduction at medium‐low horizontal‐obliqueness angles (≤40°) of young lithosphere (slabs‐average ∼15 Myr) tends to produce fault‐perpendicular tearing. Contrastingly, old‐age slabs (average ≥ 30 Myr) with medium‐large obliqueness angles (∼&gt;20°) tend to produce fault‐parallel tearing, related to differential slab‐hinge retreat or rollback. Correlations between slabs' (a) computed tearing horizontal‐width and (b) scaling‐theory forms of their subduction‐velocity differences, are reasonable (0.76–0.97). Our numerically predicted scenarios are reasonably consistent with plate‐tear imaging results from at least four natural subduction zones. Our modeling also suggests that continual along‐trench variation in subduction dip angle may be related to a special case of oblique subduction.

Geodynamic controls on the Paleo-Asian oceanic plate evolution: Constraints from Paleozoic intrusive suites along the northern margin of the North China Block

Identifying the spatio-temporal variations of arc-relation magmatism is critical for determining the polarity, geometry, localization, and onset of subduction zones on Earth. When and how the Paleo-Asian oceanic plate subducted beneath the northern margin of the North China Block (NCB) have been widely debated. Here we conducted systematic petrological, geochronological, and geochemical investigations on the Paleozoic magmatic rocks in the northern NCB. The Early Paleozoic magmatic rocks (ca. 490–420 Ma) are only distributed in the Bainaimiao arc and show typical continental arc geochemical characteristics, indicating a south dipping subduction zone. The Devonian alkaline rocks (ca. 400–370 Ma) constitute a ∼ 900 km-long magmatic belt in the northern margin of the NCB. Combined with a striking ca. 370–355 Ma magmatic quiescence and the absence of high-pressure rocks during that period, a flat-slab subduction model is proposed to better account for the tectonic evolution of the northern NCB. Subsequently, the Carboniferous to Permian calc-alkaline arc magmatism (ca. 355–250 Ma) began to emerge and showed northward younging and migration, suggesting slab retreat with roll-back of the subducting Paleo-Asian slab and again opening of mantle wedge along the northern NCB. Our findings therefore suggest that the Paleozoic magmatism in the northern NCB records an entire spatio-temporal evolution of the Paleo-Asian oceanic plate from early subduction angle deepening to flat-slab subduction, then slab rollback and finally normal subduction. This is tentatively interpreted as a combined effect from the Paleo-Asian oceanic plate subduction angle change and a migrating keel in the mantle wedge region.

Shallow subduction of Indian slab and tectono-magmatic control on post-collisional porphyry mineralization in southeastern Tibet

The 1500-km long Gangdese belt in southern Tibet shows uneven distribution of giant to large porphyry deposits over 45 Mt Cu mainly in the middle-east segment between longitude 87–92°E. However, the western (80–86°E) and eastern end (92–96°E) is generally barren. The recent discovery of Demingding, Cuibaizi and Tangbula porphyry deposits in the east of 92°E has attracted great attention on their origin. Recent high-resolution seismic imaging shows the torn Indian lithosphere subducts with diverse angle. In contrast to the middle-east segment with moderate angle Indian plate subduction, the southeastern Tibet is characterized with a shallow dip of subduction (eastern of Comei rift), where Demingding, Cuibaizi and Tangbula deposits are located. To reveal the formation mechanism of these deposits as well as the Indian slab motion pattern beneath them, we compiled reported whole rock major and trace elements, Sr and Nd isotope signature, zircon trace element, U-Pb ages and Hf isotope data of Miocene ore-related adakitic rocks from Demingding and Tangbula to the north, and Oligocene ore-barren adakitic rocks from Linchi to the south. There is a clear negative correlation between zircon U-Pb ages of rocks and their distance to Indus-Yarlung Zangbo suture: Linchi is around 20 km away from the suture, with an average U-Pb age of 27.1 Ma, while Demingding and Tangbula rocks are 50 and 90 km away from the suture, with ages of 20.3 and 19.7 Ma respectively. Granitoids from Linchi have less compatible element Ni and Cr contents of 4.07 and 5.84 ppm on average respectively, while those of Demingding (20.1 and 43.4 ppm) and Tangbula (5.52 and 7.90 ppm) are much higher. Ni and Cr as compatible elements are concentrated in mafic minerals, and are thus proper indicators for mantle imprint indicators. In addition, as shown by Sr-Nd isotopes, Linchi granitoids have features of ancient crust [(87Sr/86Sr)i = 0.7059 to 0.7085, εNd(t) = − 6.20 to −2.90], while Demingding and Tangbula granitoids show less radiogenic features [(87Sr/86Sr)i = 0.7059 to 0.7068, εNd(t) = − 3.27 to −1.70], which are more similar to coexisting juvenile arc. Demingding and Tangbula granitoids also show signature of high fO2 and hydrous melt condition, as indicated by calculated fO2 of ΔFMQ + 2, and amphibole fractionation indicated by decreasing Dy/Yb with SiO2, since amphibole favors MREE (Dy) rather than HREE (Yb). Combining above, our work shows that the Indian slab constantly subducted shallowly beneath these deposits, given no mantle-like signature of near-suture Oligocene adakitic rocks. We suggest the cause to the formation of Demingding, Tangbula and Cuibaizi deposits is the reactivation of pre-enriched arc root materials, triggered by constant northward migration of Indian slab. This is supported by similar isotope signature between Miocene Demingding and Tangbula ore-related porphyries and coexisting arc, and absence of coeval pre-collisional porphyry deposits. The essential components for these porphyry deposits, high fO2 and water content, were also likely contributed from the pre-enriched juvenile arc root.

Crustal Lg Attenuation Beneath the Iranian Plateau: Implications for Cenozoic Magmatism Related to Slab Subduction, Slab Break‐Off, and Mantle Flow

AbstractCenozoic magmatism is present across the Iranian Plateau but, despite playing an important role in the geodynamic evolution of the young plateau, remains poorly understood. In this study, the crustal thermal structure beneath the Iranian Plateau is investigated by high‐resolution Lg‐wave attenuation tomography using a newly compiled data set, which consists of records from three recent temporary seismic arrays (for a total of 197 stations) and previous regional networks. Strong Lg attenuation is mainly observed beneath the Zagros orogenic belt, Urumieh‐Dokhtar magmatic arc (UDMA), Makran magmatic arc, Lut magmatic zone, and Alborz reararc magmatic belt, which covers not only tectonic boundary belts but also the active internal zones of the Iranian Plateau. Combining deep seismological observations and geochemical data, the strong crustal attenuation characteristics beneath the northwestern and southeastern UDMA suggest that Miocene–Quaternary magmatism was activated by slab detachment‐induced upwelling after the cessation of oceanic subduction. Western Makran is characterized by strong Lg attenuation with a NE trend. The age of the Paleogene–Quaternary Makran volcanism decreases toward the NE and is likely related to the change in subduction dip. Diffuse magmatism and strong crustal attenuation are present in the northern Lut Block rather than the entire Lut–Afghan collision zone, which is likely associated with the accumulation of mantle magmas driven by the potential northward asthenospheric flow. In northern Iran, a strong attenuation anomaly spans the entire E‒W Alborz magma belt, suggesting that partial crustal melting and mantle thermal sources may exist in this region.

The Geodynamic Implications of Passive Margin Subduction in Northwest Turkey

AbstractThe number of subduction zones that facilitated the northward translation of the Anatolide‐Tauride continental terrane derived from Gondwana to the southern margin of Eurasia at the longitude of western Turkey is debated. We hypothesized that if two north dipping subduction zones facilitated incipient collision in western Turkey, a late Cretaceous arc would have formed within the Neotethys and along the southern margin of Eurasia. To determine if an island arc formed within the Neotethys we investigated the sedimentary record of the Central Sakarya basin, which was deposited along the southern margin of Eurasia from 85 to 45 million years ago. Detrital zircon deposited within the lower levels of the Central Sakarya basin (the Değirmenözü Formation) are associated with south to north‐directed paleocurrents and exhibit a unimodal late Cretaceous age peak sourced from isotopically juvenile mantle melts. Zircon maximum depositional ages from the Değirmenözü Formation cluster between 95 and 90 Ma and are 5–10 Myr older than biostratigraphic depositional ages. Between 95 and 80 Ma, a 12‐unit shift from mantle to crustal derived εHf values occurs in the overlying Yenipazar Formation. We explain the absence of Paleozoic, Eurasian‐sourced detrital zircon, the rapid shift from mantle to crustal derived εHf values, and lag time in terms of passive margin subduction within an isolated intra‐oceanic subduction zone, whose island arc was reworked from south to north into the Central Sakarya basin during incipient collision. Thus, widely outcropping late Cretaceous plutonic rocks within Eurasia must have belonged to an additional convergent margin.

The Carboniferous shoshonitic (s.l.) gabbro–monzonitic stocks of Veiros and Vale de Maceira, Ossa-Morena Zone (SW Iberian Massif): Evidence for diverse subduction-related lithospheric metasomatism

Post-collisional potassium-rich rocks are a critical petrogenetic and geodynamic marker across the European Variscan Belt. We present a detailed characterization of the only potassium-rich mafic intrusions identified so far in the Ossa-Morena Zone (Iberian Massif). Using new major, trace and SmNd isotopic whole-rock analyses, as well as mineral chemistry analyses, we discussed the petrogenesis of the early Carboniferous potassium-rich Veiros and Vale de Maceira (Portugal) stocks in the geotectonic framework of the southern Iberia, marked by the collision of the Ossa-Morena Zone, a Gondwanan terrane, with the South Portuguese Zone, a Laurussian terrane. The Veiros and Vale de Maceira shoshonites (s.l.) range from gabbros to syenites, with a predominance of monzonites and monzogabbros, characterized by different proportions of olivine, clinopyroxene, orthopyroxene, plagioclase, alkali feldspar, phlogopite and brown hornblende. The Veiros stock is mainly composed of fine-grained ultrapotassic rocks, and the Vale de Maceira stock is composed chiefly of medium-grained shoshonite rocks. A small group of hornblende-bearing medium-grained calk-alkaline rocks was also identified. The stocks have high contents of K2O, MgO, total alkalis, Th and other LILE, high ratios of LREE/HREE and LREE /HFSE, and show a Nb-Ta-Ti negative anomaly, typical of orogenic settings. Sr and Nd isotopes yielded moderate radiogenic Sr and unradiogenic Nd, that might suggest the involvement of a crustal component. However, the combined use of isotopic and elemental compositions rules out significant contamination by crustal materials upon ascent and emplacement. Instead, a source metasomatised by subduction-related fluids and hydrous melts from the slab is proposed. This diversely metasomatised source was a phlogopite-amphibole-bearing and (phlogopite-free) amphibole-bearing lherzolite at the upper part of the garnet stability field in the sub-continental lithospheric mantle. The Devonian mafic volcanic rocks point to a northward dipping (present coordinates) subduction of the Rheic oceanic plate underneath the Ossa-Morena Zone during the Variscan Orogeny. However, the Veiros and Vale de Maceira magmas originated during the post-collisional stage from the partial melting of the supra-subduction metasomatized lithosphere. The melting occurred during a period of enhanced heat due to asthenospheric upwelling related to the slab break-off of the Rheic's slab. The stocks are concomitant with the profusion of granitoid intrusions, which is compatible with a post-collisional tectonic setting. They are also coeval with the opening of the Beja-Acebuches back-arc, likely related to slab roll-back of a segment of the Rheic oceanic crust, implying polyphasic subduction and a complex geodynamic setting for the late stages of oceanic closure in southern Iberia.

Tectonic Evolution of the Southern Dabie Orogenic Belt, China: Insights from Peak P–T Conditions and U–Pb Zircon Dating of the Susong Metamorphic Complex

The Susong metamorphic complex (SSC) in the southern margin of the Dabie orogenic belt (DOB) in central-eastern China is a key metamorphic unit for understanding subduction and exhumation processes in the DOB. However, the formation age and metamorphic grade of the SSC remain uncertain, hampering our understanding of the mechanism of the formation of the DOB. An integrated study of field survey, regional metamorphic petrology, geothermobarometry, and U–Pb dating of zircon was carried out in this study. Our results reveal that the SSC was metamorphosed under epidote amphibolite- to amphibolite-facies conditions with average metamorphic P–T values of 0.98 ± 0.07 GPa and 531 ± 35 °C. The smooth spatial variation in peak P–T conditions and an apparent geothermal gradient of ~17 °C/km indicate that the SSC as a whole fall into Barrovian-type metamorphic environments. Zircon U–Pb dating for garnet–mica schists of sample ZT003, ZT005 and ZT006 yield five (Groups I to V), six (Groups I to VI) and five (Groups I to V) age groups, respectively, concentrating on the Meso-Neoarchean, early-middle Paleoproterozoic, middle Mesoproterozoic, early Neoproterozoic, Palaeozoic and Triassic-lower Jurassic. Therein, a 259–190 Ma (Group V) from zircons with Th/U ratios of &lt;0.1 in sample ZT006 record the timing of both peak and retrograde metamorphism for the SSC. All other ages are detrital zircon ages, and from age provenances in the DOB or the Yangtze Block (YZB), indicating the YZB affinity of the SSC. The two youngest age populations of 427–415 Ma (Group VI) and 475–418 Ma (Group V) from samples ZT005 and ZT006, respectively, suggest that the formation age of the SSC could be Middle Devonian. The similarity of formation age and peak P-T conditions of the SSC to Foziling Group, located in the northernmost DOB, implies that both units formed the sedimentary cover on the passive continental margin of the YZB during the late Palaeozoic, and subducted into the middle-lower crust of 20–40 km depth as a whole, corresponding to the shallow subduction. Compared to the deep subduction defined by high-pressure (HP) and ultrahigh-pressure (UHP) units, larger differences in peak P–T conditions, age and geothermal gradient between two different tectonic environments happen. Accordingly, it is speculated that a transitional subduction from shallow to deep levels occurred at Moho depths during the Early Triassic, and is due to a change in subduction dip angle.

Along-Strike Variation in Accretion, Non-accretion, and Subduction Erosion Recorded in Rocks of the Jurassic-Neogene Convergent Plate Margin of California

Post-subduction dextral faulting was restored to evaluate the spatial distribution of units of the Franciscan subduction complex of California that formed as a result of subduction accretion. The Franciscan and related rocks of western California exhibit significant along-strike variation in its recording of subduction-accretion processes. Most notably, two segments 830 km apart record subduction erosion associated with low-angle subduction events that took place at ca. 120 Ma and ca. 80–70 Ma in the north and south, respectively. This spatial relationship is not affected by restoration of post-subduction dextral slip because none of the slip passes between the two inboard tectonic windows. Between these segments the subduction complex records net accretion from ca.175 Ma to 12 Ma, but includes horizons recording non-accretion. None of the accreted units of the subduction complex are preserved over the entire length of the subduction complex. One unit, however, correlated on the basis of its structural level in the subduction complex and distinctive detrital zircon age spectra, accreted at about 80–83 Ma extends a strike length of 580 km, an amount increased to 850 km with restoration of post-subduction dextral faulting. The long-strike length of accretion of this unit demonstrates that detrital zircon age populations of subduction complex clastic rocks are poor indicators of strike-slip displacement. Some reaches of the subduction complex include schistose blueschist facies rocks (most Franciscan blueschist facies rocks are not schistose), whereas others do not, and some reaches lack blueschist facies rocks altogether. The significant along-strike variation in the Franciscan and related rock units reflects temporal and spatial differences in history of accretion, non-accretion, subduction erosion, and probably subduction dip. Similar time and space variation in processes and resultant geologic record should be expected for other subduction complexes of the world.

Change in Subduction Dip Angle of the Indian Continental Lithosphere Inferred From the Western Himalayan Eclogites

The occurrence of ultrahigh-pressure (UHP) and high-pressure (HP) rocks in the Himalayan orogen has been conventionally attributed to the different subduction dip angles along the strike. The western Himalayan UHP eclogites point to a steep continental subduction in the Eocene. The present-day geophysical data show low subduction dip angles of the Indian lithosphere beneath southern Tibet and Karakoram, implying that a shift from steep to low-angle subduction probably happened in the western Himalaya. However, the timing and mechanism of such a subduction-angle change are still unknown. Here we present a combined analysis of zircon geochronology and geochemistry of eclogites and gneiss in the Stak massif, western Himalaya. Metamorphic zircons equilibrated with garnet and omphacite show flat heavy rare earth element patterns without Eu anomalies and, thus, yield similar eclogite-facie ages of ca. 31 Ma. The Stak HP eclogite-facie metamorphism is at least 15 Ma younger than those measured in the western Himalayan UHP eclogites, but broadly contemporaneous with other Himalayan HP rocks. Therefore, all the Himalayan HP rocks record higher peak geothermal gradients and younger ages than those of the UHP rocks. Our new data, combined with the magmatic lull observed in the Kohistan–Ladakh–Gangdese arc and with the convergent rate of the Indian plate, suggest a change in subduction dip angle over time. Consequently, we suggest that the entire Indian continental lithosphere experienced an approximately coherent shift from steep to low-angle subduction after the breakoff of the Neo-Tethyan slab since the middle Eocene. This critical change in subduction geometry is interpreted to be responsible for the transition from continental subduction to collision dynamics.

Early Results of P Wave Regional Tomography Study at Sunda-Banda Arc using BMKG Seismic Network

The plate movement, geological structure, magmatism, and seismic activity in the area of Bali to East Nusa Tenggara are mainly related with the subducting of Indo-Australian Plate underneath the Eurasian plate. The complexity is added with the recent collision of Australian continent lithosphere with the western Banda arc, along the islands of Flores, Sumba and Timor island. Our study area is known as the Sunda-Banda arc transition. With the aim of imaging subsurface structure, we perform seismic tomography inversion using regional events. We collected 5 years of earthquake data (January 2015 – December 2019) from the Indonesian Agency of Meteorology, Climatology, and Geophysics (BMKG). The output of our data processing is not limited to only P wave velocity model, but also relocated seismicity pattern in the region. In general, seismicity pattern shows dominant shallow events in the south that progressively shift into deeper events in the north down to a few 500 km, marking a dipping subduction zone in this region. A group of shallow events down to a depth of 50 km is also seen at the norther region that may relate to back-arc thrust activity. P wave tomogram model show a lower velocity perturbation at a depth of 30 km that could be associated with magmatic activity along the volcanic front line. Higher P wave perturbation model are spotted at two different zones, the first one is marking a dipping Indo-Australian plate down to depth of 400 km. We noticed that the angle of dipping is steeper in the Eastern part compared to the Western part. The second a relatively flat at shallow depth at the northern region from the island of Lombok to Nusa Tenggara Timur that may mark the back-arc thrust region

Opposite Symmetry in the Lithospheric Structure of the Alboran and Algerian Basins and Their Margins (Western Mediterranean): Geodynamic Implications

AbstractThe geodynamic evolution of the Western Mediterranean for the past 35 My is a matter of debate. Present‐day structure and composition of the lithosphere and sublithospheric mantle may help in constraining the geodynamic evolution of the region. We use an integrated geophysical‐petrological modeling to derive and compare the present‐day thermal, density and compositional structure of the lithosphere and sublithospheric mantle along two NNW‐SSE oriented transects crossing the back‐arc Alboran and Algerian basins, from onshore Iberia to the northern Africa margin. The crust is constrained using available seismic data and geological cross‐sections, whereas seismic tomography and mantle xenoliths constrain the upper mantle structure and composition. Results show a thick crust (37 and 30 km) and a relative deep LAB (130 and 150 km) underneath the HP/LT metamorphic units of the Internal Betics and Greater Kabylies, respectively, which contrast with the 16 km thick magmatic crust of the Alboran Basin and the 10 km thick oceanic crust of the Algerian Basin. The sharp change in lithosphere thickness, from the orogenic wedge to the back‐arc basins, contrasts with the gentler lithosphere thickening toward the respective opposed margins. Our results confirm the presence of detached slabs ∼400 °C colder than upper mantle and a fertile composition than the continental lithospheric mantle beneath the External Betics and Saharan Atlas. Presence of detached quasi‐vertical sublithospheric slabs dipping toward the SSE in the Betics and toward the NNW in the Kabylies and the opposed symmetric lithospheric structure support an opposite dipping subduction and retreat of two adjacent segments of the Jurassic Ligurian‐Tethys realm.

Generation of syn-collisional S-type granites in collision zones: An example from the Late Triassic Tanggula Batholith in northern Tibet

Mechanisms of syn-collision magma generation are unclear due to the uncertainty in defining the term collision. We address this issue by presenting detailed petrographic, chronological, and geochemical data for the Tanggula Batholith located immediately to the north of the Longmu Tso–Shuanghu suture zone (LSSZ) in northern Tibet. Zircon LA–ICPMS U–Pb dating suggests that the Tanggula Batholith was emplaced at 235–226 Ma. Samples from the batholith show variable SiO2 of 66.6–80.6 wt% and are characterized by high normative corundum (1.09–4.35 wt%) and high aluminum saturation index (A/CNK = 1.05–1.40). These samples exhibit high initial 87Sr/86Sr (0.7165–0.7214), negative whole-rock εNd(t) (−10.1 to −8.5), εHf(t) (−8.7 to −6.3), enriched Pb isotope compositions, and variable zircon εHf(t) values (−29.9 to +5.6), as well as variable zircon δ18O value (6.90–9.22‰). These signatures are characteristic of S-type granites, and were derived from anataxis of metasedimentary rocks from the Qiangtang Terrane with input from mantle-derived components. We propose that the Tanggula Batholith was emplaced in a syn-collisional setting following closure of the Longmu Tso–Shuanghu Ocean along a north dipping subduction zone based on: the timing of high-pressure, low-temperature metamorphism at 244–221 Ma marking collision between Southern and Northern Qiangtang; the timing of exhumation of metamorphic rocks at ~223–204 Ma; and, bimodal volcanism (225–202 Ma) associated with slab breakoff at 220–215 Ma.

Lower Permian basaltic agglomerate from the Tsengel River valley, Mongolian Altai

A new occurrence of Permian volcanic and volcaniclastic rocks in the Mongolian Altai south of the Main Mongolian Lineament was described between soums of Tugrug and Tseel in Gobi-Altai aimag. Studied vitrophyric pyroxene basalt lies in a layer of agglomerate and amygdaloidal lavas, which is a part of NE–SW trending subvertical sequence of varicolored siltstones and volcaniclastic rocks in the Tsengel River valley. This high-Mg basalt is enriched in large ion lithophile elements, Pb and Sr and depleted in Nb and Ta. LA-ICP-MS dating on 44 spots reveals several concordia clusters. The whole rock geochemistry of sample fits volcanic arc characteristic in the geotectonic discrimination diagrams. Dominant zircon data yield Upper Carboniferous and Permian magmatic ages 304.4 ± 2.3 and 288.6 ± 1.9 Ma. Two smaller clusters of Upper Devonian (376 ± 4.7 Ma) to Lower Carboniferous ages (351.9 ± 3.5 Ma) indicate probably contamination of ascending magmatic material. Youngest Triassic age found in three morphologically differing grains reflects probably lead loss. Described high-Mg basalt lava represents sub-aerial volcanism in volcanic arc environment developed over the N dipping subduction zone in the southwestern Mongolia in the time span from Uppermost Carboniferous to Permian during terminal stage of its activity.

Jurassic to Cenozoic Magmatic and Geodynamic Evolution of the Eastern Pontides and Caucasus Belts, and Their Relationship With the Eastern Black Sea Basin Opening

AbstractThe magmatic arcs of the Eastern Pontides and Lesser Caucasus lie in continuation from one another. A comparison of the subduction‐related magmatic rocks outcropping throughout this segment of the Northern Tethyan belt exhibits chronological disparities, questioning the common subduction history of the Eastern Pontides and the Lesser Caucasus regions. New data and observations including geochronological and geochemical data, relative to subduction‐ to collision‐related magmatic rocks, argue a novel paleogeographic reconstruction illustrating Mesozoic and Cenozoic evolution of this region. Jurassic to Early Cretaceous arc magmatism runs mainly from the Sochi‐Ritsa/Bechasyn regions (Greater Caucasus) toward the southeast to the Alaverdi region and further into the Lesser Caucasus. Late Cretaceous and Cenozoic arc magmatism is evidenced throughout the Eastern Pontides extending through the Bolnisi region to the Lesser Caucasus arc. East to west, Jurassic to Early Cretaceous and Late Cretaceous to Cenozoic portions of arc split to the north and south of the Eastern Black Sea, respectively. Throughout Cretaceous subduction, this segment of the magmatic arc of the Southern Eurasian margin was torn in two due to the oblique opening of the Eastern Black Sea as a back‐arc to intra‐arc basin, from west to east. This reconstitution implies that the Jurassic‐Early Cretaceous subduction‐related magmatic rocks of the Greater Caucasus are remnant portions of the Eastern Pontides and Lesser Caucasus arcs. This infers the emplacement of subduction‐ to collision‐related magmatic rocks throughout the Mesozoic and Cenozoic along the entire Southern Eurasian margin is solely due to a single long‐lasting north dipping subduction.

Geochemical and Stratigraphic Analysis of the Chisana Formation, Wrangellia Terrane, Eastern Alaska: Insights Into Early Cretaceous Magmatism and Tectonics Along the Northern Cordilleran Margin

AbstractThe Chisana Formation consists of Lower Cretaceous volcanic rocks that occur in the Nutzotin Mountains of eastern Alaska. New stratigraphic analysis indicates that the volcanic succession is &gt;2 km thick at the Bonanza Creek type section. We present stratigraphic, geochemical, Sr‐Nd‐Pb isotope, and U‐Pb age data from samples collected from various stratigraphic levels of the Chisana Formation. We demonstrate that the Chisana Formation can be divided into a lower subaqueous unit, a middle transitional unit, and an upper subaerial unit. Chisana Formation lavas range from transitional to subalkaline basalts through andesites. Trace element geochemistry shows high field strength element depletions relative to large ion lithophile elements and hydrous mineral assemblages with calc‐alkaline to tholeiitic chemistries, all consistent with a magmatic arc origin. Chisana lavas yield geochemical compositions and isotope characteristics that overlap with magmas from volcanic suites formed within juvenile continental crust and immature island arcs. Volcanism occurred between ~131 and 117 Ma judging from previously reported lava ages and new U‐Pb ages of detrital zircons recovered from sandstones that conformably underlie the lowermost Chisana Formation lavas. Our results support existing tectonic models in which an east dipping subduction zone existed beneath Wrangellia during Early Cretaceous time. The upsection shift from marine to terrestrial depositional conditions in the Chisana Formation and the overlying ~117–93 Ma Beaver Lake Formation was coincident with regional shortening. Together, the geologic evidence for shortening and terrestrial deposition are interpreted to reflect accretion/suturing of Wrangellia against inboard terranes.

Controls on Continental Strain Partitioning Above an Oblique Subduction Zone, Northern Andes

AbstractStrain partitioning onto margin‐parallel thrust and strike‐slip faults is a common process at obliquely convergent plate margins, leading to the formation and migration of crustal slivers. The degree of strain partitioning and rate of sliver migration can be linked to several factors including the angle of convergence obliquity, the dip angle of subduction, frictional coupling between the plates and the strength of the upper plate, among others. Although these factors are known to be important, their relative influence on strain partitioning is unclear, particularly at natural margins where the factors often vary along strike. Here we use a 3‐D mechanical finite‐element model to investigate the relationship between continental crustal strength, the convergence obliquity angle, the subduction angle, and strain partitioning in the Northern Volcanic Zone (NVZ) of the Andes (5°N–3°S). In the NVZ the subduction dip and obliquity angles both vary along strike, weaknesses in the continental crust may be present in suture zones or regions of arc volcanism, and strain partitioning is only observed in some regions. Thus, it is an ideal location to gain insight in which of the factors have the largest influence on deformation and sliver formation in the upper plate. Our numerical experiments confirm that a moderately high obliquity angle is needed for partitioning and that a continental crustal weakness is also required for movement of a coherent continental sliver at rates similar to geodetic observations from the NVZ. In contrast, the subduction dip angle is only of secondary importance in controlling strain partitioning behavior.