Active Margin Research Articles

Did the Western and the Eastern Vardar ophiolites originate through a single intra-oceanic subduction? Insight from numerical modelling

The ophiolites of the Balkan Peninsula are distinguished into the Western and the Eastern Vardar ophiolites and it is generally accepted that both of them resulted from the latest Jurassic/earliest Cretaceous closure of the Vardar branch of the Tethys Ocean (Vardar Tethys). This study is aimed at testing the possibility that the origin and emplacement of these two ophiolite belts can be explained in terms of a single NE-dipping intra-oceanic subduction, which commences in mid Jurassic and lasts until the lowermost Cretaceous, and then transitions into collision between the African and the European plate. The subduction is accompanied by creation of a new oceanic lithosphere in the back-arc. In order to test this hypothesis, we use 2D numerical thermomechanical modeling for investigating the nucleation of the East Vardar Ocean in the back-arc and intra-arc region. Our numerical results suggest that a single intra-oceanic subduction can indeed be simultaneously responsible for both the westward obduction of the Western Vardar ophiolites and for complex active margin processes along the western rim of the European plate. The simulations show that the single intra-oceanic subduction zone is able to produce a pronounced extension and oceanic spreading along the European margin, which can be associated with the formation of the Eastern Vardar ophiolites in this subduction related setting (SSZ and BAB). The comparison of the numerical modeling results with available natural data suggests that our models are capable to approximate the regional geodynamic evolution during and immediately after the closure of Vardar Tethys.

The role of crustal-scale shear zones in SW Gondwana consolidation – transatlantic correlation

Abstract We present the first correlation between South American and African shear zones based on a reconstruction model of SW Gondwana continental crust, correlating 57 Brasiliano–Pan-African crustal-scale shear zones that sutured this palaeocontinent at c. 500 Ma. The final amalgamation and consolidation of the SW Gondwana continental crust were attained by an anticlockwise rotation of three cratons (Kalahari, Angola and São Francisco) in relation to the clockwise rotation of the Río de la Plata Craton in the Early Paleozoic. These relative movements were accommodated by transcurrent shear zones active from 585 to 500 Ma within the Pan-African–Brasiliano belts that surround these cratons. This kinematic interaction resulted in the initiation of a long-term active margin starting with the Cambrian Pampean orogeny and ending with the Permian–Triassic Gondwanide orogeny.

Evidence of crustal growth during the Svecofennian orogeny: New isotopic data from the central parts of the Paleoproterozoic Central Finland Granitoid Complex

The Central Finland Granitoid Complex (CFGC) is the Paleoproterozoic core of the Svecofennian orogeny composed mainly of plutonic rocks varying from diorites to granodiorites and granites. Common mixing and mingling textures, synplutonic dykes, and mafic magmatic enclaves highlight the simultaneous nature of mafic, mantle derived magmatism and more evolved felsic magmatism in the CFGC. New U-Pb geochronological data support crustal growth during 1.90–1.87 Ga for the CFGC. Both Sm-Nd (εNd -1.7–+2.5) from whole rocks and Lu-Hf (εHf -8–+9) from zircons point towards relatively juvenile sources for the plutonic rocks without Archean crustal component. On the whole, isotopic evidence collectively points to crustal growth with maturing arc characteristics at 1.90–1.87 Ga, and a possible amalgamation of island arcs during the Svecofennian orogeny. In a larger scale, the CFGC was 1.9 Ga ago located in the active margin of the Great Proterozoic Accretionary Orogen when supercontinent Columbia (Nuna) formed.

Authigenic Carbonate Precipitation at Yam Seep Controlled by Continuous Fracturing and Uplifting of Four‐Way Closure Ridge Offshore SW Taiwan

AbstractHydrocarbon seeps are common manifestations of gas leakage from the seafloor. However, the fate of methane seepage within the gas hydrate stability zone at active margins is poorly constrained. This study presents a 40‐thousand‐year record of hydrocarbon seepage archived by a ∼5‐m long core composed of authigenic carbonate from the Yam Seep area, Four‐Way Closure Ridge off SW Taiwan. Different carbonate microfacies could be distinguished: Consolidated microcrystalline aragonite representing lithified host sediments intercalated by pure aragonite present in 10–50 cm thick intervals in the core. These aragonite intervals are interpreted as having precipitated within former fractures in the host rock. High resolution U‐Th dating of these aragonites is interpreted to record the minimum age of the opening of these fractures. The chronology of aragonite precipitation throughout the core suggests a record of continuous seepage from ∼41 to 2 ka that fluctuated in intensity over this time period. The chronology of putative fracturing events and observed carbonate precipitation suggest (a) an active period of fracturing and seepage from ∼37 to 27 ka, (b) a more quiescent period from ∼27 to 16 ka, followed by (c) another active period from ∼16 to 12 ka. A schematic model illustrates the evolution of carbonate formation within the core influenced by faulting, fracturing, erosion, gas hydrate accumulation, and aragonite precipitation and provides a unique 40,000‐year‐old record of methane seepage and crucial insights into the dynamics of long‐term seepage systems at active margins.

The Late Mesozoic Intracontinental contraction–Extension Transition in the Beishan Fold‐Thrust Belt, Central Asia: Constraints From Structural Analysis and Apatite (U–Th)/He Thermochronology

AbstractThe late Mesozoic Beishan fold‐thrust belt occupies a key position in central Asia, figuring importantly in our understanding of intracontinental deformation in response to far‐field plate‐boundary forces. To determine when and how the Beishan fold‐thrust belt was developed, we conducted detailed field mapping, structural analysis, and apatite (U–Th)/He (AHe) thermochronology in the Hongliudaquan area, southeastern Beishan Range. The study area exposes the Hongliudaquan fold and thrust system, consisting of the southern Hongliudaquan thrust nappe and autochthonous fold sets. The nappe experienced >16 km northward thrust‐induced translation. The autochthonous fold sets strongly deformed Lower–Middle Jurassic strata showing diverse trends of kilometer‐scale basins and ranges. We interpret that this fold interference pattern was produced in a constrictional deformation field, which underwent synchronous shortening in two orthogonal directions. This stain history was related to synchronous imbricate thrusting, duplex formation, and strike‐slip faulting at different structural levels. Restoration of the Hongliudaquan fold and thrust system yields a minimum of ∼27% (∼10.1 km) north–south shortening strain. Thrusting and nappe emplacement within the Beishan fold‐thrust belt may have started in the late Middle Jurassic and persisted until the Early Cretaceous (∼140–133 Ma). Our AHe data also record ∼124–115 Ma cooling related to footwall exhumation during subsequent normal faulting, indicative of the late Mesozoic intracontinental contraction–extension transition. The Beishan fold‐thrust belt initiated in central Asia via the late Mesozoic reactivation of the Paleozoic–early Mesozoic Beishan orogen as a result of the far‐field responses to multi‐directional plate convergence around Asia.

Происхождение и структурная позиция Камчатского срединного массива по данным глубинных геолого‑геофизических исследований

The article shows the results of deep research along the profile of the settlement of Nizhnyaya Oblukovina – the city of Andrianovka, crossing the northern part of the Kamchatka median massif. A geological and geophysical model of the structure of the earth’s crust and upper mantle has been constructed, where the structural position of the object under study is presented and an assumption is made about its origin. The model highlights a fragment of paleosubduction (slab), which was part of the most ancient convergent boundary in western Kamchatka. The final stage of subduction blocking and its displacement to the east at a distance of ~60 km in the Early Eocene is associated with the entry into the accretionary complex of a terrane in the form of an island-arc plate 6–9 km thick. At the site of the maximum inflection of the subsequent slab, an extension zone was formed – a rift zone, along which the rise of mantle material and high-temperature fluid occurred. Approximately 52 million years ago, the processes of metamorphism, focal melting and intrusion of granites into the upper layers of the crust took place. As a result, in the eastern part of the plate and its flanks, a granitoid massif was formed with a rock density of 2.58 g/cm3, which is significantly lower than the environmental density. Density deficiency led to a violation of isostatic equilibrium and, as a result, to a rise in this part of the structure. The most intense uplift occurred at the end of the Oligocene, as a result of which a ledge was formed, which the authors recommend giving the name: “Middle Kamchatka ledge” instead of the rooted “Kamchatsky median massif”. The genetic relationship of the Shanuch ore region with the features of the deep structure of the lithosphere has been revealed. The results of the research indicate a hidden (buried) distribution of the island-arc plate beyond the boundaries of the mapped outcrops of metamorphids. Intrusions of the main composition, promising for the opening of sulfide copper-nickel ores, are located in the marginal parts of the ledge.

Discovery of Permian–Triassic eclogite in northern Tibet establishes coeval subduction erosion along an ~3000-km-long arc

Abstract Eclogite bodies exposed across Tibet record a history of subduction-collision events that preceded growth of the Tibetan Plateau. Deciphering the time-space patterns of eclogite generation improves our knowledge of the preconditions for Cenozoic orogeny in Tibet and broader eclogite formation and/or exhumation processes. Here we report the discovery of Permo-Triassic eclogite in northern Tibet. U-Pb zircon dating and thermobarometry suggest eclogite-facies metamorphism at ca. 262–240 Ma at peak pressures of ~2.5 GPa. Inherited zircons and geochemistry show the eclogite was derived from an upper-plate continental protolith, which must have experienced subduction erosion to transport the protolith mafic bodies to eclogite-forming conditions. The Dabie eclogites to the east experienced a similar history, and we interpret that these two coeval eclogite exposures formed by subduction erosion of the upper plate and deep trench burial along the same ~3000-km-long north-dipping Permo-Triassic subduction complex. We interpret the synchroneity of eclogitization along the strike length of the subduction zone to have been driven by accelerated plate convergence due to ca. 260 Ma Emeishan plume impingement.

Tracing a Neoproterozoic subduction to collision magmatism in the eastern Pernambuco–Alagoas domain, northeastern Brazil

The Pernambuco–Alagoas (PEAL) Domain, southern Borborema Province, northeastern Brazil, bears the most voluminous Cryogenian–Ediacaran granitic rocks in this province, which was formed during the convergence of the São Francisco Craton and the Borborem Province, during the western Gondwana amalgamation. New geochemical and isotopic data along those from the literature, for one of the largest intrusions, the Correntes composite batholith, at the southern part of this domain, indicate that the granitic rocks are intermediate to acid, oxidized, metaluminous to peraluminous, magnesian, and are high-K calc alkalic to shoshonitic with I-type signature. They are enriched in large-ion lithophile elements, exhibit lightly negative Eu in rare-earth element patterns and negative Ta–Nb, P, and Ti anomalies in spidergrams. Average initial 87Sr/86Sr is 0.7055 and rocks present values of whole-rock εNd (0.6 Ga) that fall into two groups. The pre-to syn-collisional, 635–628 Ma plutons present εNd (0.6 Ga) values from +1.3 to −2.5, with Nd model ages varying from 1.29 to 0.92 Ga, while the late-to post-collisional, 602–580 Ma, plutons show more negative values (−8.2 to −9.5), and older Nd model ages from 1.95 to 1.81 Ga. This Nd-isotope pattern is also observed in other plutons of the southern PEAL Domain and indicates a change in the lower crust source rock with time, with larger contributions of crustal components in the late-to post-collisional rocks. The chemical and isotopic characteristics of the studied granitic rocks and other granitic batholiths in the southern PEAL Domain are compatible with modern magmatic arcs of continental active margins. We propose that in earlier stages of convergence, melting has further affected the younger (Stenian –Tonian), likely less refractory rocks associated with abundance of volatiles, whose segregation would have differentiated to form the oldest plutons. Slow cooling allowed the accumulation of heat from mantle-derived magma in the pre-collisional period, which, together with radiogenic heat from thickened crust, led to partial melting of older crustal rocks, and the formation of magmas whose segregation and differentiation of the melt would originate the younger plutons in this part of the PEAL Domain. Older plutons formed during a Cryogenian-Ediacaran flare-up event that occurred in the southern PEAL Domain.

Tectonic Evolution of the JLJB, North China Craton, Revisited: Constraints from Metamorphism, Geochemistry and Geochronology of the Ji’an Group and Related Granites

The Jiao-Liao-Ji Belt (JLJB) is the most representative Paleoproterozoic orogenic belt in the North China Craton (NCC). The sedimentation, metamorphism and magmatism of the Ji’an Group and associated granites provide significant insights into the tectonic evolution of the JLJB. In this study, we have synthesized published geochemistry and geochronology data on metasedimentary, metavolcanic and igneous rocks. According to the available data, the protoliths of the metasedimentary rocks are sets of shale, wacke, arkose, quartz sandstone and carbonate, while the protoliths of the metavolcanic rocks are calc-alkaline basalt, basaltic andesite, andesite, dacite and rhyolite. The rock assemblages indicate a transformation of the tectonic environment from a passive margin to an active continental margin following the onset of plate convergence and subduction. The A2-type gneissic granite (Qianzhuogou pluton) is formed in a subsequent back-arc basin extension setting at 2.20–2.14 Ga. The Ji’an Group was finally deposited in an active continental margin during the closure of a back-arc basin at 2.14–2.0 Ga. Then, the sediments were involved in a continent–arc–continent collision between the Longgang and Nangrim blocks at ~1.95 Ga. This process was accompanied by HP granulite-facies metamorphism at ~1.90 Ga. The subsequent exhumation and regional extension resulted in decompression melting during 1.90–1.86 Ga, producing metamorphism with an isothermal decompression clockwise P–T path. The resulting metapelites are characterized by perthite + sillimanite, and mafic granulites are characterized by orthopyroxene + clinopyroxene. The S-type porphyritic granite (Shuangcha pluton) is formed during the crustal anatexis. Meanwhile, extensive anatexis produced significant heating and triggered prograde to peak metamorphism with an anticlockwise P–T path. Cordierite-bearing symplectites around the garnet in the metapelites indicate a superposed isobaric cooling metamorphism. The ages of monazites and anatectic zircons suggest that the post-exhumation cooling occurred at 1.86–1.80 Ga. The Paleoproterozoic magmatism, sedimentation and metamorphism suggest a process of subduction back-arc basin extension and closure, collision and exhumation for the tectonic evolution of the JLJB.

Meso-Neoproterozoic basic-intermediate mafic granulites (metabasites) from the ∼1 Ga granulitic southwestern Oaxacan complex, Mexico, crustal evolution and phase equilibria modeling

The basement of eastern Mexico is made up of Grenvillian granulite-facies metamorphic rocks, which constitute an inferred microcontinent named Oaxaquia. Proto-Oaxaquia oceanic arc reconstructions are related to the transition from Columbia to Rodinia supercontinents (∼1.5 Ga). This study focuses on the petrography, geochemistry, and phase equilibria modeling of metabasites from the southwestern Oaxacan Complex, the largest Oaxaquia outcrop. Southwestern Oaxacan Complex protoliths mainly correspond to gabbro/basalt. They constitute an assemblage apart from the rest of the Oaxacan Complex metabasites due to their Ti enrichment and SiO2 depletion related to post-magmatic/metamorphic processes. Southern study site metabasites correspond to the primitive root of the arc, mostly related to a divergent geotectonic setting. The other metabasites are related to the magmatic back/forearc active margin between proto-Oaxaquia and Amazonia craton. Phase equilibria modeling point to Zapotecan (last Grenvillian cycle) metamorphic peak conditions of 850 ± 25 °C and 1.0 ± 0.15 GPa from one garnet-amphibole bearing metabasite, and a later hydration episode (post-Grenvillian) of 760 ± 10 °C and 0.8 ± 0.20 GPa derived from one amphibole bearing metabasite. The time of incipient partial melting of the southwestern Oaxacan Complex is a pre-Zapotecan event (∼1 Ga). Mineral textures do not display significant processes related to the post-Zapotecan metamorphic retrogression, and only a few whole-rock light rare element patterns display strong enrichments.

Paleogene marine bivalves of the deep-water Keasey Formation in Oregon, Part II: The pteriomorphs

Systematic documentation of pteriomorph bivalves of the late Eocene–early Oligocene Keasey Formation in western Oregon follows previous monographic treatments of the anomalodesmatans and heteroconchs. It includes material from coeval Cascadia Margin strata in southwestern Washington in the context of major molluscan faunal turnover during the dramatic doubthouse climate transition from an ice-free tropical marine environment to the establishment of permanent polar ice caps and a cold temperate marine climate. The families represented are Crenellidae, Parallelodontidae, Glycymerididae, Limopsidae, Isognomonidae, Pectinidae, Propeamussiidae, and Limidae. New taxa include the genus Bathyisognomon and five new species: Limopsis squiresi, Bathyisognomon smithwickensis, Delectopecten kieli, Delectopecten keaseyorum, and Propeamussium (Parvamussium) mistensis. Small to minute mud pectens and glass scallops are among the most abundant and frequent taxa in the Keasey fauna, although they are poorly preserved and have been overlooked heretofore. Pteriomorphs are prominent elements of recurring associations of species in mudstone and fine-grained siltstone facies adjacent to three Keasey cold methane seeps. Previously described Keasey anomalodesmatans and heteroconchs in these seep-adjacent strata have living relatives adapted to dysoxia and toxic geochemistry. Because the unique macrofaunal associations in these settings are neither seep fauna nor part of the background fauna, this paper introduces and defines the concept of a transitional peri-seep biotope. Relationships, monophyly, systematic resolution within major pteriomorph groups, and global biogeographic distributions are reviewed in each systematic treatment. Recurring patterns include occurrences in active margin settings, inferred oscillatory deepening of the calcium carbonate compensation depth, late Eocene origin of two-layered oceans following establishment of the Antarctic Circumpolar Current, and biogeographic gateway changes that disrupted earlier Paleogene larval dispersal patterns.

Cross‐Orogen Granite Migration as an Indicator of Slab Rollback Along Eastern Gondwana

AbstractSlab rollback during subduction plays a key role in controlling continental growth at convergent plate boundaries. The dynamics of currently subducting slabs can be precisely constrained using geophysical techniques. In contrast, ancient episodes of slab rollback can be difficult to constrain, yet are critical to unlocking the tectonic evolution of long‐lived orogens such as the Phanerozoic Australian Tasmanides of eastern Gondwana. Recognition of ancient slab rollback relies on the identification of the progressive migration of magmatic arcs. Here, we investigate the timing and isotopic variation of ∼90 km of the trans‐orogen migration of the Carboniferous Bathurst Batholith as a potential indicator of slab rollback. U–Pb–Hf isotopes, combined with a regional zircon Hf isotope data set, suggest that the eastward migration of the batholith over ∼18 Myr, from 340.1 to 322.4 Ma, records a maximum slab steepening rate of ∼0.6–1.2° Myr−1 in a relatively stable trench setting. These results provide a magmatic record of Carboniferous slab rollback and establish a missing link between the long‐lived Lachlan and New England orogens of the eastern Gondwanan Tasmanides.

The Atlas of Morocco: A Plume‐Assisted Orogeny

AbstractWe explore the connections between crustal shortening, volcanism, and mantle dynamics in the Atlas of Morocco. In response to compressional forces and strain localization, this intraplate orogen has evolved far from convergent plate margins. Convective effects, such as lithospheric weakening and plume‐related volcanism, contributed in important ways to the building of high topography. We seek to better understand how crustal and mantle processes interacted during the Atlas' orogeny by combining multiple strands of observations, including new and published data. Constraints on crustal and thermal evolution are combined with new analyses of topographic evolution, petrological, and geochemical data from the Anti‐Atlas volcanic fields, and a simple numerical model of the interactions among crustal deformation, a mantle plume, and volcanism. Our findings substantiate that: (a) crustal deformation and exhumation accelerated during the middle/late Miocene, contemporaneous with the onset of volcanism; (b) volcanism has an anorogenic signature with a deep source; (c) a dynamic mantle upwelling supports the high topography. We propose that a mantle plume and the related volcanism weakened the lithosphere beneath the Atlas and that this favored the localization of crustal shortening along pre‐existing structures during plate convergence. This convective‐tectonic sequence may represent a general mechanism for the modification of continental plates throughout the thermo‐chemical evolution of the supercontinental cycle.

Terrazas marinas levantadas por deformación tectónica costera activa a lo largo del este de Argel: implicaciones para la convergencia de las placas africana y europea y las curvas del nivel del mar

Marine terraces are geological features composed of elevated, flat surfaces and steep slopes. The evolution of these terraces is largely influenced by active coastal tectonic deformation. This study investigates the uplift of marine terraces along the east coast of Algiers and its implications for the African and European plate convergence and sea-level curves. The objective of studying marine terraces in the Zemmouri area is to gain a deeper understanding of the evolution of the Earth's coastlines and how they have been shaped over time by natural processes such as sea level changes, tectonic activity, geomorphic parameters, and erosion. Marine terraces provide valuable information about the history of sea level changes and the rate of tectonic uplift or subsidence of the coastal areas. Secondary data sources, including qualitative information and high-resolution satellite imagery (SRTM 30 m and ALOS 12.5 m), were used to analyze the impact of sea level on marine terraces. Fieldwork using GPS and altimeter measurements at the site of the Mw=6.8 Zemmouri earthquake that occurred on May 21, 2003, revealed the presence of two well-preserved marine terraces, which serve as important markers for measuring the long-term fault slip rate. The analysis highlights the significance of active coastal tectonic deformation in shaping the coastlines. This research offers new insights into the ongoing processes of plate convergence and sea level change along the east coast of Algiers, contributing to our overall comprehension of coastal evolution and the potential for seismic hazards in the region.

Long‐Term Offshore Borehole Fluid‐Pressure Monitoring at the Northern Cascadia Subduction Zone and Inferences Regarding the State of Megathrust Locking

AbstractThe Cascadia subduction megathrust off the Pacific Northwest follows an “end member” seismogenic behavior, producing large (up to moment magnitude 9) but infrequent (every several hundred years) earthquakes and tsunamis. Crustal deformation associated with the ongoing plate convergence has been characterized by land‐based geodetic observations, but the state of locking across the full breadth of the seismogenic fault is poorly constrained. We report results of offshore monitoring of borehole fluid pressure, as a proxy for formation volumetric strain, at a site ∼20 km landward of the Cascadia subduction deformation front since 2010. The multi‐depth pressure records were plagued by hydrologic noise, but noise at the deepest monitoring level (303 m sub‐seafloor) abated in 2015. Subsequently, including at the times of regional large earthquakes that caused significant dynamic stressing, no persistent pressure transients are present above a threshold of 0.08 kPa imposed by unremovable oceanographic signals, corresponding to a strain detection limit of ∼16 nanostrain. Simple dislocation models using local megathrust geometry suggest a resolvable slip of <1 cm along a trench‐normal corridor beneath the borehole for a range of slip‐patch dimensions. A large slip patch can be well resolved even at considerable along‐strike distances from the borehole; for instance, ∼10 cm slip is detectable over a 200‐km strike range for a slip‐patch radius of ∼50 km. This high sensitivity for detecting slip, along with the lack of observed events, stands in stark contrast to observations at other subduction zones, and suggests that the Northern Cascadia megathrust is most likely fully locked.

Nickel Isotope Fractionation During Magmatic Differentiation

AbstractThe behavior of nickel (Ni) isotopes during magmatic differentiation is not adequately explored. Here, we find that tholeiitic rocks in the Kīlauea Iki (KI) lava lake, Hawai'i, show progressively lighter Ni isotopic compositions with increasing magmatic differentiation, whereas calc‐alkaline rocks from the thick Kamchatka arc (30–45 km), located at the convergent boundary of the Eurasian and Pacific plates show increasing Ni isotope values as MgO and Ni decrease. Forty‐three global intermediate‐felsic continental igneous rocks analyzed in this study display large Ni isotopic variations, with the Eoarchean samples having light Ni isotopic compositions that fall in the trend defined by the KI lavas, and the post‐Eoarchean samples showing systematically heavier Ni isotopic compositions overlapping those of Kamchatka arc rocks. The isotopic dichotomy results from the crystallization of isotopically heavy magnetite during low‐pressure differentiation of KI lavas, whereas the participation of sulfide separation that removes isotopically light Ni during high‐pressure differentiation of magmas traversing thick continental crust. Combined with Rhyolite‐MELTS and sulfur concentration at sulfide saturation simulations, we demonstrate that the Ni isotope fractionation during magmatic differentiation is mainly controlled by the tempo of magnetite crystallization and sulfide formation, which is a function of pressure, oxygen fugacity, and water activity. High‐pressure calc‐alkaline differentiation usually suppresses magnetite crystallization while stabilizing sulfide, leading to heavy Ni isotopic compositions for the evolved magmas, significantly deviating from the low‐pressure fractionation trend seen in the KI lavas. Ni isotopes can be used in the future as a tracer of magmatic differentiation and processes of continent formation and differentiation.

Developing geo-sequential reasoning about tectonic processes using computational simulations

ABSTRACT Explaining phenomena associated with a system involves describing a system’s structure and articulating the process through which the system’s structure changes over time. This paper defines geo-sequential reasoning in the context of plate tectonics and uses it to analyse how students explain the geological processes that occur along convergent boundaries as part of the plate tectonics system. This study was part of design-based research on an online Plate tectonics module that included simulation-based modelling developed for secondary school students. We analysed students’ explanations (n = 950) about phenomena found along a convergent boundary (1) as an oceanic plate and a continental plate move towards each other and (2) between two oceanic plates located on the opposite side of a tectonic plate from a divergent boundary. We also analysed images created by students of the simulation as evidence to support their explanations. We found that a majority of students used simulation-based evidence when describing the sequence of events along the convergent boundary and that the synced planet surface and cross-section views in the simulation supported students’ inclusion of processes responsible for the events. These findings have implications for how teaching and research with dynamic simulations can support reasoning built with temporal evidence.

Multichannel seismic evidence for tectonic migration in the Mariana subduction system

AbstractThe Mariana subduction system is an ideal place for ocean–ocean convergence boundary research. As a typical example of an extensional subduction model, the subduction process, subduction mechanism and subduction effect of this system have long been popular research topics. The seismic reflection information of oceanic sedimentary layers shown on multichannel seismic profiles is an intuitive record of small‐ to medium‐scale structural activity and sedimentary sequence evolution and is an important source for studying plate‐coupling processes and tectonic dynamic action. This paper is based on the raw multichannel seismic data of MGL1204 in the Mariana arc forearc region; these data were obtained by using linear interference filter technique noise attenuation technology and multiple velocity iterations to accurately image the oceanic sedimentary layer, and the imagery is used to study the Cenozoic fracture activity and residual strata thickness distribution characteristics of the middle part of the Mariana forearc region and to explore the tectonic migration characteristics of the Cenozoic sedimentary basins. We believe that due to the influence of regional stress field changes caused by the movement directions of the Pacific Plate and Philippine Sea Plate towards the Eurasian Plate, the development of secondary structures in the western Pacific subduction zone shows a characteristic west‐to‐east migration, and the structural characteristics of the Mariana forearc Cenozoic sedimentary basin are consistent with the regional tectonic features. The faults, stratigraphy and depocentres of the oceanic sedimentary layer show a migration pattern from west to east and from north to south overall; 25 and 13 Ma are identified as two key tectonic migration periods, and the tectonic migration characteristics and time records of the basin also provide direct evidence for the plate reconstruction model of trench retreat and Philippine Sea plate rotation.

Geodetic constraints of the 2015Mw6.5 Alor, East Indonesia earthquake: a strike-slip faulting in the convergent boundary

SUMMARYOn 2015 November 4, an Mw 6.5 earthquake struck the east region of Alor Island, eastern Indonesia. Here, we use Sentinel-1 and ALOS-2 data to explore the coseismic surface displacement of this earthquake. Based on the ALOS-2 coseismic interferograms, the first fault model and coseismic slip distribution of the 2015 Alor earthquake are presented in this study. The preferred slip model links to a blind south-southeast striking, south-southwest dipping strike-slip fault with amount of normal slip component and a peak slip of 2.09 m at 2.34 km of depth. Considering the fact that the ascending and descending InSAR prediction of the distributed slip model does not fit the InSAR observations at the northwest and southeast tips with the simple planar fault model. We tried to construct a strike-variable fault model to further improve data fit. The results of the calculated Coulomb stress change imply that the regions with positive CFS changes mainly located at the northwest and southeast extremities of the rupture of the Alor earthquake, and in the lobes north and south of the rupture. The most striking discovery from the InSAR observations of the Alor earthquake is that most of the displacements occurred on a fault whose existence was unknown before the earthquake.

Transcrustal and source processes affecting the chemical characteristics of magmas in a hyperactive volcanic zone

Arc magma generation and evolution are affected by transcrustal processes, such as assimilation and fractional crystallisation, and source processes like slab influx into the mantle wedge. However, the relative contribution of these mechanisms remains contentious. The Pleistocene to Holocene Taupo Volcanic Zone (TVZ) is unusually dominated by felsic volcanism with more than 95% of the total eruptive volume corresponding to rhyolitic magmas. Despite the relatively thin crustal basement (∼16–30 km), combined wall rock assimilation and fractional crystallization (AFC) of primary basalt has previously been presented as a necessary means to generate the chemical characteristics of these magmas. This petrogenetic model is here assessed using a new, internally consistent set of major and trace element concentrations, and Sr-Pb isotope data from a suite of basaltic to rhyolitic samples that are representative of the entire TVZ with respect to geographical distribution and age. The dataset was modelled for AFC using the energy- and mass-constrained Magma Chamber Simulator (MCS) and the local Permian to Early Jurassic Torlesse composite terrane basement used as the principal assimilant. Regardless of the intensive parameters employed, the energy- and mass-constrained models presented here fail to realistically reproduce the combined major oxide, trace element, and isotope systematics of the sample set. The Pb isotope ratios of TVZ basalts to rhyolites reveal a broadly linear trend ranging from an unradiogenic (206Pb/204Pb: 18.77) to a more radiogenic crustal endmember (206Pb/204Pb: 18.84). This trend can be reproduced by a composite mélange dominated by the Torlesse terrane (representative of tectonically eroded forearc crust), with some contribution of subducting sediments (GLOSS-II), interacting with the sub-arc mantle to generate the source of the magmas observed in the TVZ. This model is also consistent with Sr-Pb isotope systematics. At New Zealand’s active margin, tectonic erosion has been reported based on geophysical data. Therefore, it is proposed that slab input can have a stronger relevance in the magma generation of the TVZ than previously thought. The broad compositional variations in the TVZ may result from source contamination by a subduction mélange, with differentiation processes within the overriding crust being subordinate.