Plate Margin Research Articles

Early Neoproterozoic (Tonian) subduction-related magmatism and tectonothermal activity in Shetland and northern mainland Scotland: implications for the tectonic evolution of NE Laurentia and Rodinia reconstructions

The tectonic setting of Tonian orogenic events recorded in the present-day circum-North Atlantic region is uncertain. U–Pb zircon geochronology shows that the Yell Sound and Westings groups (Shetland) and metasedimentary rocks of the Naver Nappe (northern mainland Scotland) were deposited between c. 1050 and 960 Ma and intruded by mafic, intermediate and felsic igneous rocks at c. 965–950 Ma. Chemical discrimination diagrams and Hf and Nd isotope data together suggest that the protoliths of the mafic meta-igneous rocks were emplaced as relatively juvenile crustal contributions in an active plate margin. Zircon growth at c. 920 Ma within the Yell Sound Group correlates with high-grade metamorphism documented previously in Shetland. Further zircon growth and Pb loss at c. 470–460 Ma indicates overprinting during the Ordovician Grampian orogenic event. Similar age successions of Ellesmere Island, Svalbard and East Greenland also contain evidence for Tonian magmatism (some calc-alkaline), deformation and metamorphism. The new data favour Rodinia reconstructions that incorporate subduction-related magmatism and associated tectonism along the margin of NE Laurentia during the Tonian. The Yell Sound Group and correlative peri-Laurentian successions were intruded by subduction-related magmas and deformed and metamorphosed during development of the Valhalla exterior accretionary orogen, part of a more extensive peri-Rodinian subduction system.

Exploring frictional properties of upper plate fault reactivation in subduction zones: The Atacama Fault System in northern Chile

The Maule 2010 and Tohoku-Oki 2011 earthquakes demonstrated how dormant upper plate faults can be reactivated as normal faults by plate margin relaxation following megathrust slip. However, the reactivation mechanisms of these types of faults are yet unexplored. To provide a better understanding of these mechanisms, we collected fault core samples from fault segments of the Atacama Fault System in northern Chile. The sampled fault segments have clear morphological evidence of Quaternary reactivation as normal fault. We performed laboratory experiments to measure the fault strength, stability and dynamic weakening. We consider low-velocity tests for exploring the frictional strength and velocity dependence of friction via a double-direct shear apparatus and ii) high-velocity tests for investigating the frictional properties at seismic velocities via a rotary shear apparatus. The experiments revealed that fault cores have low frictional strength, velocity-strengthening behaviour and strong dynamic weakening. Additionally, a novel experimental procedure that simulates stress relaxation by stepwise reducing of the normal stress on the sample assembly showed: 1. Accelerating creep towards dynamic weakening in chlorite-rich gouges and 2. oscillatory sliding in fault gouges enriched in illite. By extrapolating our experimental observations to natural conditions, we conclude that stable sliding is favoured during the interseismic phase of the subduction earthquake cycle, whereas unstable sliding is favoured during the coseismic and postseismic phases. The latter occurs via normal stress reduction during the shift from interseismic compression to co- and postseismic tension at the plate margin.

Deposition and diagenesis of the Triassic-Jurassic limestone successions of the northwestern passive margin of the Indian plate, Sulaiman Fold-Thrust Belt, Pakistan

Deposition and diagenesis of the Triassic-Jurassic limestone successions of the northwestern passive margin of the Indian plate, Sulaiman Fold-Thrust Belt, Pakistan

Rapid India–Asia Initial Collision Between 50 and 48 Ma Along the Western Margin of the Indian Plate: Detrital Zircon Provenance Evidence

Constraining the collision timing of India and Asia requires reliable information from the coeval geological record along the ~2400 km long collisional margin. This study provides insights into the India–Asia collision at the westernmost margin of the Indian Plate using combined U-Pb geochronological data and sandstone petrography. The study area is situated in the vicinity of Fort Munro, Pakistan, along the western margin of the Indian Plate, and consists of the Paleocene Dunghan Formation and Eocene Ghazij Formation. The U-Pb ages of detrital zircons from the Dunghan Formation are mainly clustered between ~453 and 1100 Ma with a second minor cluster between ~1600 and 2600 Ma. These ages suggest that the major source contributing to the Dunghan Formation was likely derived from basement rocks and the cover sequence exposed mainly in Tethyan Himalaya (TH), Lesser Himalaya (LH), and Higher Himalayan (HH). Petrographic results suggest that the quartz-rich samples from the Dunghan Formation are mineralogically mature and have likely experienced log-distance transportation, which is possible in the case of an already established and well-developed river system delivering the sediments from the Craton Interior provenance. Samples of the overlying Ghazij Formation show a major detrital zircon age clustered at ~272–600 Ma in the lower part of the formation, comparable to the TH. In the middle part, the major cluster is at ~400–1100 Ma, and a minor cluster at ~1600–2600 Ma similar to the age patterns of TH, LH, and HH. However, in the uppermost part of the Ghazij Formation, ages of <100 Ma are recorded along with 110–166 Ma, ~400–1100 Ma, and ~1600–2600 Ma clusters. The <100 Ma ages were mainly attributed to the northern source, which was the Kohistan-Ladakh arc (KLA). The ~110–166 Ma ages are possibly associated with the TH volcanic rocks, ophiolitic source, and Karakoram block (KB). The Paleozoic to Archean-aged zircons in the Ghazij Formation represent an Indian source. This contrasting provenance shift from India to Asia is also reflected in the sandstone petrography, where the sample KZ-09 is plotted in a dissected arc field. By combining the U-Pb ages of the detrital zircons with sandstone petrography, we attribute this provenance change to the Asia–India collision that caused the provenance shift from the southern (Indian Craton) provenance to the northern (KLA and KB) provenance. In view of the upper age limit of the Ghazij Formation, we suggest the onset of Asian–Indian collision along its western part occurred at ca. 50–48 Ma, which is younger than the collision ages reported from central and northwestern segments of the Indian plate margin with 70–59 Ma and 56 Ma, respectively.

Tectono-magmatic evolution of the Indian crust in western Himalayas during Paleoproterozoic: Insights from Nanga Parbat and Indus syntaxis in northern Pakistan

The Paleoproterozoic tectono-magmatic history of the Indian plate has been modelled mostly based on investigations of the Indian parts with limited studies considering the Pakistani domains of the mountain range. In this study we investigate basement rocks and younger intrusions of the Indian crust from within the core of the Indus Syntaxis and the Nanga Parbat Syntaxis in northern Pakistan, with the goal of establishing a chronology for the entire tectono-magmatic progression of northwestern Indian plate margin, including the source rocks that were ultimately involved in the formation of younger magmatic pulses. We present in-situ zircon U-Pb geochronology and O-isotopes complemented by whole rock geochemistry of granitoids and paragneisses that yielded evidence for two distinct, large-scale Paleoproterozoic magmatic events that took place in ∼ 1860 Ma and ∼ 2200 Ma in the Indian crust. Later, leucogranites (6.4 ± 0.1 Ma) intruded into the Nanga Parbat basement that were the result of melting of the basement induced by rapid uplift. The δ18OSMOW compositions of zircons from our basement samples range between 7.5 to 9.1 ‰, indicating the derivation of parental magma from the crustal source while the younger leucogranite is somewhat lighter with δ18O ranging between 7.06 to 8.23 ‰. Geochemical data show extensional tectonic settings for the basement rocks that have an A-type affinity, again pointing to a crustal precursor. We propose little to no δ18O exchange took place between the young anatectic melt and basement rocks during crustal evolution. Moreover, geochemical signatures record the crystallization of basement rocks dominated the northwestern margin of Indian plate in a post-orogenic setting in an overall extensional tectonic realm. Based on these observations, we infer that the northwestern Indian margin has experienced a large-scale magmatism during Paleoproterozoic associated with the amalgamation of the Columbia supercontinent.

Provenance and tectonic evolution of the forearc basin along the northwestern Central Qilian belt, northeastern Tibet

The age and properties of detrital zircon grains in a forearc basin provide crucial information on the history of convergent plate margins. These data can be used to constrain provenance dispersal patterns, establish source-to-sink relationships, and reveal the tectonic framework and subduction history. The Qilian orogen in the northeastern margin of the Tibetan Plateau records the tectonic history of the Proto-Tethys Ocean from its initial spreading and subduction to final closure. However, the timing of subduction initiation and the subduction polarity of the Proto-Tethys Ocean remain controversial. The Qilian orogen includes the North Qilian, Central Qilian, and South Qilian belts. A succession of sedimentary rocks along the northwestern margin of the Central Qilian belt, previously considered to be Precambrian in age, has been redefined based on detailed field investigation and geochronology. U-Pb analysis of detrital zircon grains from metasedimentary rocks of the Gongcha Group of the northwestern Central Qilian belt yielded Paleozoic ages between 530 and 470 Ma, with prominent Mesoproterozoic and Paleoproterozoic ages between 1800 and 1000 Ma. In contrast, the overlying Duoruonuoer Group yielded predominantly early Paleozoic ages between 465 and 434 Ma, with a few older age populations. This change in source is reflected in a substantial increase in volcanic detritus within the Duoruonuoer Group as compared to the Gongcha Group. The detrital zircon age spectra and lithologic characteristics indicate the Gongcha and Duoruonuoer Groups constitute the lower and upper volcanic-sedimentary strata in the forearc basin. Based on resemblances of statistical ages and εHf(t) values of zircon, we interpret that the early Paleozoic intrusive and volcanic units and Precambrian basement rocks of the Central Qilian belt were the main source of the Gongcha and Duoruonuoer Groups, verifying the southward subduction of the North Qilian Ocean, a portion of the larger Proto-Tethys Ocean, beneath the Central Qilian belt. The detrital zircon ages reveal that the initial subduction of the Proto-Tethys Ocean in the Qilian orogen occurred ca. 530 Ma and closed ca. 435 Ma. These forearc sedimentary units constitute the tectonic boundaries between the Central Qilian belt and North Qilian belt.

Surgical management of costal margin rupture associated with intercostal hernia: Evolution of techniques

INTRODUCTION Costal margin rupture (CMR) injuries in association with intercostal hernia (IH) are rare and symptomatic and provide a significant surgical challenge. Surgical failure rates up to 60% are reported, and optimal techniques are unclear. We have characterized these injuries and describe the evolution of our surgical management techniques. METHODS Patients characterized by the Sheffield Classification where CMR and IH were both present, either CMR-IH or transdiaphragmatic intercostal hernia (TDIH), were identified prospectively, and injury characteristics, patient management, and follow-up data were recorded. Surgical techniques evolved according to patient outcomes from suture repair without and then with extrathoracic mesh, to three iterations of double-layer mesh repair (DLMR). The third iteration involved DLMR with biologic mesh, titanium buttress plates applied to the ribs adjacent to the IH with intercostal nerve-sparing suture placement. Associated surgical stabilization of rib fractures, or surgical stabilization of nonunited rib fractures, was performed when required, with costal margin plate fixation where possible. RESULTS Of 25 patients with CMR-IH and 11 with TDIH, 25 patients underwent surgery, with 6 reoperations in 5 patients. There were 8 suture repairs and 3 extrathoracic mesh repairs: DLMR was performed in 14 patients (3 Mark [Mk] 1, 5 Mk 2, and 6 Mk 3) with 2, 1, and 0 reoperations, respectively. Costal margin stabilization with titanium plates was successful twice at the level of the seventh but failed twice out of three times at the ninth costal cartilage. Reoperation after a failed mesh repair is particularly challenging and may require the placement of titanium buttress plates, surgical stabilization of rib fractures, and the use of stainless steel wire sutures. CONCLUSION Repair of CMR-IH/TDIH is challenging, but experience-based evolution of techniques has led to a durable and reproducible Mk 3 repair. LEVEL OF EVIDENCE Observational; Level III.

Cold-subduction biogeodynamics boosts deep energy delivery to the forearc

Metamorphic fluids in subduction zones carry C–H–N–O–P–S species, which are crucial for sustaining subsurface microbial life at shallower crustal depths in the forearc region. Upwards migration of deeply released fluids to shallower levels, where temperatures permit the persistence of microbial life, is recorded by metasomatic rocks formed along the plate interface. Variations in the redox state and component speciation of metamorphic fluids – from local to secular, and highly dependent on thermal gradients and redox state of subduction inputs – may strongly control microbial pathways or even the possibility for metamorphic fluids to sustain microbial communities in the subsurface biosphere at convergent plate margins. We show that metamorphic fluids containing reduced energy sources for microbial life – e.g., CH4, H2 – are common in Phanerozoic, high-pressure/low-temperature plate-interface metasomatic rocks such as jadeitites and albitites worldwide. Based on the stability fields of minerals hosting CH4, H2 and graphite inclusions, we pinpoint the protracted, probably episodic migration of energy sources in the mantle wedge via fluid circulation being mediated by jadeitites from > ca. 35 km depth, and by their retrogressed counterparts forming from between 35–15 km depth. These fluids can cross the so-called biotic fringe – whose limit is the depth corresponding to ca. 122–135 °C (as deep as ca. 13 km depth depending on geothermal gradients) – as suggested by previous documentation of slab-derived fluids reaching subsurface microbial communities. Thermodynamic modeling indicates that cool thermal gradients, possibly combined with increased inputs of organic matter-rich sediments into subduction, favor the abundance of reduced energy sources relative to more oxidized species (e.g., CO2), thus promoting the proliferation of subsurface microbial life at convergent margins.

A potential deep geothermal reservoir in eastern Ladakh as inferred from the upper crustal geoelectric structure of the region

The Ladakh Himalaya is a repository of the evolutionary history of the India-Eurasia convergent plate margin. We present the results of a magnetotelluric (MT) study carried out in Eastern Ladakh along a 40-km-long profile traversing across various tectonic domains of this plate margin. We modeled the MT dataset by two-dimensional (2-D) and three-dimensional (3-D) inversion algorithms. The upper crustal geoelectric model obtained by 2-D inversion of distortion corrected and decomposed data of 18 sites shows an excellent match with the surface geology. In this model, the Indus Suture Zone (ISZ) appears as a steeply dipping contact and the Ladakh batholith is about 10 to 12 km thick. The model yields a prominent electrical conductor of ∼6 km diameter at 4 km depth beneath the Tso Morari Crystallines with an offshoot of a small conductor that rises upward along the ISZ. The geoelectrical model obtained by 3-D inversion is broadly consistent with the 2-D model but also reveals off-profile features and a connectivity of the shallow conductors with a widespread conductive zone at deeper level of the upper crust. In view of the presence of Puga and Chumathang hot springs towards west and Demchok hot springs towards east of our study region, we infer these conductors to be representing a potential major geothermal reservoir system connected to the widespread deeper conductive zone of partial melts, which extends from the southern Tibet to the eastern Ladakh region.

Upper mantle structure beneath the Mongolian region from multimode surface waves: Implications for the western margin of Amurian plate

Multimode phase speeds of surface waves are used to build a new radially anisotropic S wave model in the eastern Eurasian and Mongolian regions. Our dataset includes seismic waveforms of over 1655 teleseismic events (Mw≥5.8) from 2009 to 2021, recorded at permanent and temporary stations in and around Mongolia. The multimode dispersion curves of Love and Rayleigh waves were extracted using the nonlinear waveform fitting method for individual seismograms. Then, we retrieved phase speed maps for each mode and frequency, incorporating finite-frequency effects. Finally, localized multimode dispersion curves extracted from the phase speed maps were inverted for local 1-D SV and SH wave profiles, which are combined into a radially anisotropic 3-D shear wave model. Our new model exhibits significant lateral variations of S wave speeds at 70–100 km depth beneath Mongolia, i.e., slow anomalies in the tectonically active western Mongolia in contrast to fast anomalies in stable eastern Mongolia. In the radial anisotropy model, SH waves are faster than SV waves in most areas of the Mongolian lithosphere above 100 km depth, except for the northeast of the Altay Mountains. The Hangay Dome region is characterized by significantly slower velocities that may relate to its uplifting. A large-scale low velocity beneath the northeast of the Hangay Dome with a slower SV wave speed than SH may indicate the existence of partially molten layers. This study also reveals distinct lateral variations of S wave speeds across the boundary between the Amurian and Eurasian plates, characterized by the fast anomaly in eastern Mongolia, corresponding to the lithosphere in the western Amurian plate.

He-Ar isotopic signatures of the Mesozoic granitoids in South Korea: implications for genesis of the granitic magma and crustal evolution in NE continental margin of the Eurasian plate

He-Ar isotopic signatures of the Mesozoic granitoids in South Korea: implications for genesis of the granitic magma and crustal evolution in NE continental margin of the Eurasian plate

Reducing the Wallacean and the Haeckelian Deficits in Apobaetis Day, 1955 (Ephemeroptera: Baetidae).

During an ongoing project aiming to understand the dynamics of Ephemeroptera biodiversity in the Doce River Basin after the disaster, an unusual large number of Apobaetis Day, 1955 were analyzed. The occurrence of Apobaetis is quite relevant, since the combination of factors such as its small body size and its habitat preferences makes their collection difficult and its presence often neglected. For the first time in science, adults of Apobaetis irai De Lima, Massariol, Cruz & Hamada, 2022 and Apobaetis fiuzai Salles & Lugo-Ortiz, 2002 are described and diagnosed. Apobaetis irai can be easily distinguished, among other characteristics by turbinate portion of compound eyes touching each other along midline; abdominal terga II-III and V-VII with well-marked lines on lateral and posterior margins; posterior margin of subgenital plate with projection broadly rounded to slightly truncate. Apobaetis fiuzai are diagnosed by turbinate portion of compound eyes touching each other posteriorly; mesothorax brown, except for broad cream marks on anterior margin of PSp and SLS, SSLi tinged with white, posterior corner of SMS with a red spot; abdominal terga II-VI translucent orangish-brown, terga VII-X brown, terga II-III and VI with conspicuous red medial mark; posterior margin of subgenital plate with subtriangular projection, apex narrowly truncate; forceps base wider at base, inner irregular edges. The new records of A. irai and A. fiuzai extend the distribution of Apobaetis within Minas Gerais and Espírito Santo States, Brazil. Furthermore, it is reported for the first time cases of gynandromorphs for Apobaetis.

Abrupt structural deformation changes from the boundary to the interior of a craton basin: Implications for the long-term stability of cratonic blocks

Stable intraplate cratonic blocks usually have less structural deformation and fewer earthquakes than other locations on Earth, but with strong compressional deformation around their periphery. Investigating how and why this different deformation occurred is beneficial for understanding why the cratonic block is so stable and how the intraplate in-plane stress is transmitted. In this work, we first investigated the structural deformation changes from the margin to the interior of the western Ordos block (one of the most tectonically stable areas in China) via seismic data. The results show abrupt structural deformation changes from the margin to the interior of the Ordos block in terms of the deformation strength (from strong to weak), structural orientation (high-angle oblique relationships), and kinematics (from compression to wrenching). Our investigation also shows that such phenomena are widespread in cratonic blocks worldwide. The abrupt changes are probably induced by special in-plane stress transfer inside the cratonic block: When far-field stress is transmitted into continental interiors from active plate margins, the weak belt around the cratonic block filters and accommodates the in-plane stress. Consequently, this decreases the stress, changes the stress direction, and transmits the in-plane stress along a shallower layer (probably less than 1500 m). Furthermore, the compressional stress from the plate margin is converted into shear stress within the cratonic block. This stress transmission manner makes reactivation of deep preexisting faults difficult under far-field horizontal plate-boundary stresses in the cratonic block without vertical forces from the mantle, guaranteeing long-term stability and low seismicity. This understanding can provide a new perspective for the interpretation of earthquakes in stable continental regions. It can also be applied to appraise the long-term stability of sites for the storage of CO2.

The Paleoproterozoic Trans-Australian Orogen: Its magmatic and tectonothermal record, links to northern Laurentia, and implications for supercontinent assembly

The 2.1−1.79 Ga Trans-Australian and Canadian Trans-Hudson orogens preserve a common record of Himalayan-scale orogenesis and voluminous Cordilleran-style magmatism behind which turbidite-dominated sedimentary sequences evolved in a backarc or retroarc foreland setting. Successive cycles of subduction retreat and advance drove the orogenic process, culminating in continent-continent collision and closure of a shared and formerly contiguous ocean basin—the Paleoproterozoic Diamantina and Manikewan oceans. Cordilleran-style arc magmatism in proto-Australia commenced along the southern reaches of the Diamantina Ocean with emplacement of the 2005−1975 Ma Dalgaringa Batholith along the leading edge of the Pilbara Craton (Gascoyne Province) before both it and its host craton docked against the Yilgarn Craton, resulting in the Glenburgh Orogeny. After a brief episode of post-kinematic granitic magmatism from 1965 Ma to 1945 Ma, tectonic activity switched to the opposing margin of the Diamantina Ocean in what is now northern Australia, where three more cycles of upper plate orogenesis and Cordilleran-style magmatism occurred from 1890 Ma to 1850 Ma, 1840 Ma to 1810 Ma, and 1810 Ma to 1760 Ma along a convergent continental margin extending from the Kimberley and Pine Creek regions southward through the Mount Isa domain into the eastern Gawler Craton. Batholiths developed along this margin include granites of both low- and high-Sr/Y composition, with the more adakitic varieties interpreted to have been intruded during periods of enhanced asthenospheric upwelling accompanying the opening of one or more slab windows following slab breakoff, tearing, and/or subduction of an actively spreading oceanic ridge. Terminal collision between the North and South Australian (Mawson) cratons at ca. 1790 Ma brought this succession of subduction-related events to a close, although neither this event nor the corresponding Trans-Hudson Orogen need equate to final assembly of the Nuna supercontinent. Instead, the 1870 Ma peak in global compilations of magmatic and detrital zircon ages may be interpreted more simply as the result of elevated tectonism and magmatism along a Paleoproterozoic Cordilleran-style continental plate margin that was transcontinental in scale and continued uninterrupted from proto-Australia into northern Canada and beyond.

Neoproterozoic tectonic shift along the western margin of the Yangtze block, South China craton: Implications for its paleogeographic position during the Rodinia−Gondwana transition

Neoproterozoic igneous rocks in the Panxi region from the western margin of the Yangtze block, South China craton, record major changes in tectonic evolution and crustal thickness, and provide important constraints on the paleogeographic position and climate changes of the block in Rodinia and Gondwana reconstructions. Our new dating results show that the Pengguan complex from the Panxi region comprises ca. 743 Ma adakitic granodiorites and ca. 741 Ma A-type granites. The former is plagioclase-rich and has relatively low SiO2 content (65.22−67.36 wt%), high Mg# values (44.40−52.33), Sr/Y (23.22−61.94) and (La/Yb)N ratios (normalized to chondritic values; 13.41−24.00), with positive whole-rock εNd(t) values (+1.17 to +5.15), and high zircon εHf(t) (+5.53 to +8.83) and low zircon δ18O (4.21‰ to 5.51‰) values, indicating derivation from the partial melting of thickened juvenile mafic lower crust. By contrast, the granites are rich in K-feldspar and have high SiO2 contents (78.71−84.62 wt%), 10,000 × Ga/Al ratios (>2.6), and Fe/Mg ratios, with negative εNd(t) values (−1.47 to −0.02), high zircon εHf(t) (+2.94 to +9.77), and high saturation temperatures (828−920 °C). These features indicate fractional crystallization and partial melting of continental crust in a low-pressure, high-temperature, extensional environment. Integrated with the zircon Eu anomalies (Eu/Eu*) data and Hf-O isotopic data on pre-740 Ma magmatism in the Panxi region, we find that a rapid increase of crustal thickening (820−740 Ma) was synchronous with a gradual decrease of zircon δ18O and an increase of zircon εHf(t) values. These variations indicate an increase in the degree of hybridization between mantle sources and subducting slab-derived materials, thus generating a great number of low-δ18O melts, and continuous crustal thickening through the underplating of juvenile magmas. In addition, thinning of the crust since ca. 740 Ma, along with the gradual increase in zircon δ18O and εHf(t), suggests a shift from regional compression to extension. This documented shift, together with regional data from along the western margin of the Yangtze block, indicates a change from the well-developed and long-established Panxi convergent plate margin arc system (at least ca. 870−750 Ma) to passive-margin development and Cryogenian (720−635 Ma) glaciation. The long-lived subduction zone requires a peripheral location of South China that was linked to northern India during the Rodinia breakup. The subsequent and contemporaneous passive-margin development of the Yangtze block and India, along with the similar tectonic affinities of the Cathaysia block with India, indicate that South China lay outboard of the northern Gondwana margin during the assembly of this supercontinent.

Mapping paleoelevations along active continental margins with igneous geochemistry: A case study from South America

Mountains and mountain ranges, often situated at convergent plate margins, play a pivotal role in many fields of the Earth, climate, and biological sciences. Reconstructing past episodes of mountain building from the geological rock record is one of the main challenges for unravelling the ancient physical geography of Earth’s surface. Established methods for quantifying past elevations traditionally relied on sedimentary rocks, but in recent years, alternative approaches have emerged on the basis that geochemical signatures of magmatic rocks formed in convergent settings correlate with crustal thickness or elevation. Consequently, methods based on igneous samples have the potential to allow quantitative mapping of past topographic change for large spans of space and time where existing maps are largely based on a qualitative approach. Here, we investigate the application of paleoelevation estimates derived from geochemistry using the western margin of South America as a case study. We investigate their consistency with independent indicators of past elevations such as stratigraphy, stable isotopes, fossils etc. for Cenozoic samples along the Andean margin. For older times, we compare the estimated paleoelevations with more general aspects of the geological record, as well as paleoelevations from global paleogeography models widely used in climate modelling studies, to evaluate the extent to which these models are consistent with the igneous geochemical proxies. We find that estimates based on multiple geochemical proxies, and which account for variations in elevation as a function of MgO, are preferable to those based on individual proxies. The multi-mohometer estimates yield estimates of elevation that better match both present-day topography and Cenozoic paleoelevations, and retain more samples during the data filtering stage. In deeper time, we show that igneous geochemistry quantifies changes in elevation related to documented phases of crustal thickening and thinning, and is thus likely to allow improvements to existing maps of paleotopography.

Controls on the genesis of a giant sand injection complex; insights into the Paleogene evolution of the stress of northern and central California

Giant sand injection complexes and localized swarms of sandstone intrusions are common in Upper Cretaceous to Miocene sedimentary successions of central and northern California within a distance of less than 100 km from the Pacific margin of the North America plate. One of the best preserved and extensively exposed injection complexes is the late Eocene Tumey Giant Injection Complex. The emplacement of sand injectites was driven by overpressure generated by thermal diagenesis of biosiliceous and smectite-rich mudstone host-rocks. The orientation and size distribution of sandstone intrusions was controlled by stress in which σ 1 and σ 3 were horizontal and, respectively, parallel and perpendicular to the present trace of the San Andreas Fault, and σ 2 was vertical. A strike-slip tectonic regime is inferred. Our analysis documents margin-orthogonal extension and provides support for a late Eocene phase of increase of strain, and possibly active slip, along a syn-subduction strike-slip fault zone. Comparison with other injection complexes in the region indicates that the near-field maximum principal stress rotated through time, from normal to parallel with respect to the plate margin, probably in relation to variations of the relative motion vector of the converging plates.

The role of continental heterogeneity on the evolution of continental plate margin topography at subduction zones

The nature of the overriding plate plays a major role for subduction zone processes. In particular, the highly heterogeneous continental lithosphere modulates intra-plate tectonics and the surface evolution of our planet. However, the role of continental heterogeneity is relatively under-explored for the dynamics of subduction models. We investigate the influence of rheological and density variations across the overriding plate on the evolution of continental lithosphere and slab dynamics in the upper mantle. We focus on the effects of variations in continental plate margin and keel properties on deformation, topographic signals, and basin formation. Our results show that the thickness, extent, and strength of the continental plate margin and subcontinental keel play a crucial role for the morphology and topography of the overriding plate, as well as the retreat of the subducting slab. We show that this lateral heterogeneity can directly influence the coupling between the subducting and overriding plate and determine the partitioning of plate velocities across the overriding plate. These findings suggest that back-arc extension and subsidence are not solely controlled by slab dynamics but are also influenced by continental plate margin and keel properties. Large extended back-arc regions, such as the Pannonian and Aegean basins, may result from fast slab rollback combined with a weak continental plate margin and a strong and extended continental keel. Narrow margins, like the Okinawa Trough in NE Japan, may indicate a comparatively stronger continental plate margin and weaker or smaller continental keel. Additionally, continental keel properties may affect the overall topography of the continental lithosphere, leading to uplift of the deformation front and the formation of intermontane basins.

Extreme geochemical fractionation during mantle melting: Insights from Hf-Nd isotopically ultra-depleted eclogite

Extreme fractionation of elements and isotopes in mafic igneous rocks is abnormal in deciphering the source nature and melting conditions of mafic magmatism. Especially, identification of geochemically ultra-depleted mafic melts and their mantle sources has great bearing on the property of crust-mantle differentiation at plate margins. This is illustrated by extreme Hf-Nd isotope fractionation in ultrahigh-pressure eclogites from the Sulu orogen in east-central China. In addition to the previous finding of ultrahigh εNd(0) values, we report here new data of whole-rock trace elements and Lu-Hf isotopes in eclogites and related rocks from the same region. The present results show extremely high Lu/Hf ratios and abnormally high εHf(0) values of up to 576 for the eclogites, significantly different from the garnet amphibolites and other eclogite-facies metamorphic rocks in the same orogen. This feature is coupled with the ultrahigh εNd(0) values as well as the severe depletion of light rare earth elements (LREE) and high field strength elements (HFSE). Because HFSE and LREE are immobile in aqueous solutions and the effect of melt extraction is insignificant during the continental deep subduction, the extreme fractionation of Lu/Hf and Sm/Nd indicate their origination from a geochemically ultra-depleted mantle source. These eclogites have the depleted mantle Hf model ages of 1.27 Ga to 1.61 Ga, similar to the depleted mantle Nd model ages of 1.39 Ga to 1.67 Ga as previously reported. This suggests that the protolith of the extremely high εHf-εNd eclogites was a kind of mafic igneous rocks derived from fractional crystallization of geochemically ultra-depleted mafic melts, which were produced by partial melting of the highly refractory lithospheric mantle during a series of seafloor spreading initiation-failure cycles at a divergent plate margin after the breakup of supercontinent Columbia in the Early Mesoproterozoic. The mafic igneous rocks were located in a passive continental margin in the Late Paleozoic and experienced deep subduction and exhumation in the Triassic, giving rise to the presently studied eclogites. The ancient geochemical signatures were retained without considerable influence by mantle convection, providing insights into the nature of crust-mantle differentiation during the tectonic transition from supercontinental breakup to seafloor spreading beneath the sub-ridge lithospheric mantle.

Magmatic degassing and fluid metasomatism promote compositional variation from I-type to peralkaline A-type granite in the late Cretaceous Fuzhou felsic complex, SE China

Abstract A-type granites generally have much lower water, higher temperature, and incompatible element concentrations than I-type granitoids. Yet it remains unclear why I-A-type granitic complexes occur in convergent plate margins. Here we conduct geochemical analyses on apatite and mafic minerals from the late Cretaceous I-A-type granitic complex in Fuzhou area, SE China, aiming to decipher differentiation, fluid metasomatism, and degassing that primarily control the compositional diversity of felsic magmas. Apatites in both rock types are F-rich and show large H2O and δD variations, i.e., 341–3892 ppm H2O and –325 to +336‰ δD in I-type granitoids; 67–1366 ppm H2O and –251 to +1439‰ δD in A-type granites. H2O in apatite is negatively correlated with La/Sm and Sr/Y in the I-type granitoids, whereas it is positively correlated with Ce and total rare earth element (REE) concentrations in the A-type granites. Once H2O increases up to hundreds of ppm, both rock types show a rapid decrease of H2O/Ce, an increase of F/Cl, and extensive H isotope fractionation. Arfvedsonite occurs as a late crystallizing mineral in the A-type granite and has much higher contents of Na2O, K2O, F, and high field strength elements (HFSE) than hornblende in the I-type granitoids, indicating the addition of F-HFSE-rich alkaline fluids during its magmatic evolution. The consumption of arfvedsonite and formation of aegirine further indicate the role of fluid metasomatism and H2 degassing via a reaction of 3Na3Fe5Si8O22(OH)2 + 2H2O = 9NaFeSi2O6 + 2Fe3O4 + 6SiO2+5H2. The combined geochemical data demonstrate that the systematic differences in mineral assemblage, whole-rock composition, magma temperature, H2O content, and δD of apatite between the I- and A-type granites are likely attributed to varying degrees of differentiation, fluid metasomatism and magmatic degassing. The I-type granitoids experienced hornblende, biotite, plagioclase, K-feldspar, and apatite fractionation and close-system degassing. The A-type granite was likely formed from the I-type monzogranitic magma that was metasomatized by the mantle-derived F-HFSE-rich alkaline fluids to produce the peralkaline magma, which further experienced K-feldspar + plagioclase + biotite + apatite fractionation and open-system degassing. Further numerical estimation indicates that the primary magma of Fuzhou granitic complex contained ~3.0 wt% H2O, and the lower water content of A-type granite was likely attributed to strong degassing during its emplacement. Our results indicate that some peralkaline A-type granites can be generated from relatively water-poor I-type granitic magmas by fluid metasomatism and degassing.