Active Margin Research Articles

Seismic imaging of the Java subduction zone: New insight into arc volcanism and seismogenesis

The Java subduction zone is located in the southern part of Southeast Asia, where the strong plate convergence results in clustered arc volcanoes and intense seismicity. To better understand the arc magmatism and seismogenesis in this region, we determine a high-resolution 3-D P-wave velocity (Vp) model of the crust and upper mantle down to 600 km depth beneath Java. The geometry of the subducting Australian slab is taken into account in the starting model to obtain a better tomographic result by inverting a large number of travel-time data of local and teleseismic events. Our results show lower intra-slab Vp anomalies at depths of ∼100–200 km than other parts of the slab, which may reflect dehydration embrittlement and a slab hole. Feeble low-Vp anomalies appear below the subducting Australian slab, which is probably due to the slab hole where subslab hot materials flow into the mantle wedge. This feature may be related to the occurrence of potassium-rich back-arc volcanoes and the lack of giant megathrust earthquakes (Mw ≥ 8.5) in Java. Our tomographic results also support the hot finger model for explaining the arc magmatism in Java.

Rapid along-strike variations of shallow crustal structure in response to Indo-Burma subduction: Constraints from multi-type passive seismic data

Characteristics of the subducting crust and the attached sedimentary cover play a key role in shaping the structure and processes in the hanging wall of a convergent plate boundary. Such influence must be significant in the oblique Indo-Burma subduction zone. Yet, the poorly resolved shallow crustal velocity structure of the overriding Burma plate, especially its lateral variation along the subduction margin, hinders a comprehensive understanding of the Indo-Burma subduction dynamics. Here, we take advantage of the new and dense broadband seismic data in Myanmar to image the regional S-wave velocity (Vs) structure in the top 10 km of crust by joint inversion of Rayleigh-wave ellipticity, receiver function and P-wave polarization. The new velocity model not only matches surface geology and seismic reflection profiles, but also reveals new and rapidly varying near-surface features, including (1) the exhumed high-velocity metamorphic belt of the Indo-Burma accretionary prism, which is more prominent in the south with a shallower depth extent and greater Vs of ∼3.5 km/s; and (2) contrasting Vs structure beneath the two Quaternary volcanoes, that is, Vs of typical crystallized igneous rocks (∼3 km/s) beneath Monywa versus Vs slower than the adjacent basin sediments (<2.5 km/s) under Mt. Popa. Combining with existing knowledge, we propose that the observed along-strike changes in the shallow forearc structure could be attributed to the northward increase of the off-scraped/underplated Ganges-derived Bengal sediments in the inner accretionary prism and the uneven distribution of the sediment-derived fluids, the majority of which may be trapped within the prism in the north but be subducted to deeper sub-arc level in the south. These eventually result in slower exhumation of the metamorphic belt and less active shallow crustal magmatism in the north than the south. Further, we speculate that the Bengal sediment behavior and the upper plate crustal structure might be modulated by a transition of the incoming Indian plate from the continental lithosphere beneath Tibet and northern Myanmar to thin continental/oceanic nature in the south.

Causes of permanent vertical deformation at subduction margins: Evidence from late Pleistocene marine terraces of the southern Hikurangi margin, Aotearoa New Zealand

Theoretical studies of the seismic cycle at convergent plate boundaries anticipate that most coseismic deformation is recovered, yet significant permanent vertical displacement of the overriding plate is observed at many subduction margins. To understand the mechanisms driving permanent vertical displacement, we investigate tectonic uplift across the southern Hikurangi subduction margin, Aotearoa New Zealand, in the last ∼200 ka. Marine terraces preserved along the Wellington south coast have recently been dated as Marine Isotope Stage (MIS) 5a (∼82 ka), 5c (∼96 ka), 5e (∼123 ka) and 7a (∼196 ka) in age. We use these ages, together with new reconstructions of shoreline angle elevations, to calculate uplift rates across the margin and to examine the processes responsible for their elevation. The highest uplift rate—1.7 ± 0.1 mm/yr–and maximum tilting—2.9° to the west–are observed near Cape Palliser, the closest site to (∼50 km from) the Hikurangi Trough. Uplift rates decrease monotonically westward along the Palliser Bay coast, to 0.2 ± 0.1 mm/yr at Wharekauhau (∼70 km from the trough), defining a gently west-tilted subaerial forearc domain. Locally, active oblique-slip upper-plate faults cause obvious vertical offsets of the marine terraces in the axial ranges (&amp;gt;70 km from the trough). Uplift rates at Baring Head, on the upthrown side of the Wairarapa-Wharekauhau fault system, are ∼0.7–1.6 mm/yr. At Tongue Point, uplift on the upthrown side of the Ōhāriu Fault is 0.6 ± 0.1 mm/yr. Dislocation and flexural-isostatic modelling shows that slip on faults within the overriding plate—specifically the Palliser-Kaiwhata Fault and the Wairarapa-Wharekauhau fault system—may dominate uplift in their immediate hanging walls. Depending on their slip rate and geometry, slip on these two upper-plate fault systems could plausibly cause &amp;gt;80% of late Pleistocene uplift everywhere along the south coast of North Island. Our modelling suggests that subduction of the buoyant Hikurangi Plateau contributes uplift of 0.1–0.2 mm/yr and uplift due to sediment underplating at Tongue Point and Wharekauhau is likely ≤0.6 mm/yr but could be significantly lower. Earthquakes on the subduction interface probably contribute ≤0.4 mm/yr of late Pleistocene uplift, with ≤10% of uplift due to each earthquake being stored permanently, similar to other subduction zones. These results indicate a significant contribution of slip on upper-plate faults to permanent uplift and tilting across the subduction margin and suggest that in regions where upper-plate faults are prevalent, strong constraints on fault geometry and slip rate are necessary to disentangle contributions of deeper-seated processes to uplift.

Pervasive carbonation of peridotite to listvenite (Semail Ophiolite, Sultanate of Oman): clues from iron partitioning and chemical zoning

Abstract. Earth's long-term cycling of carbon is regulated from mid-ocean ridges to convergent plate boundaries by mass transfers involving mantle rocks. Here we examine the conversion of peridotite to listvenite (magnesite + quartz rock) during CO2 metasomatism along the basal thrust of the Semail Ophiolite (Fanja, Sultanate of Oman). At the outcrop scale, this transformation defines reaction zones, from serpentinized peridotites to carbonated serpentinites and listvenites. Based on a detailed petrological and chemical study, we show that carbonation progressed through three main stages involving the development of replacive textures ascribed to early stages, whilst carbonate (± quartz) veining becomes predominant in the last stage. The pervasive replacement of serpentine by magnesite is characterized by the formation of spheroids, among which two types are identified based on the composition of their core regions: Fe-core and Mg-core spheroids. Fe zoning is a type feature of matrix and vein magnesite formed during the onset carbonation (Stage 1). While Fe-rich magnesite is predicted to form at low fluid XCO2 from a poorly to moderately oxidized protolith, our study evidences that the local non-redox destabilization of Fe oxides into Fe-rich magnesite is essential to the development of Fe-core spheroids. The formation of Fe-core spheroids is followed by the pervasive (over-)growth of Mg-rich spheroids and aggregates (Stage 2) at near-equilibrium conditions in response to increasing fluid XCO2. Furthermore, the compositions of carbonates indicate that most siderophile transition elements released by the dissolution of primary minerals are locally trapped in carbonate and oxides during matrix carbonation, while elements with a chalcophile affinity are the most likely to be leached out of reaction zones.

Slab tearing in non-collisional settings: Insights from thermo-mechanical modelling of oblique subduction

The propagation of slab break-off (slab tearing) is usually attributed to laterally variable plate convergence systems with a spatial transition between simultaneous oceanic subduction and continental collision. To study the process of slab tearing in a non-collisional geodynamic context, here we use a 3D thermo-mechanical numerical approach to model the oblique subduction of a homogeneous oceanic plate. We investigate the effects of the following parameters: (1) subduction obliquity angle, (2) age of oceanic slab, and (3) partitioning of boundary velocities (i.e., the ratio between the subduction component and the advance of the overriding plate in the total convergence). In our simulations, the retreat of the subduction zone leads to a thinning of the fore-arc and back-arc lithosphere, which are decoupled from the subducting slab by the rise of the hot asthenosphere from the underlying mantle wedge. As a consequence of the initial obliquity of the active plate margin, slab roll-back velocities are subject to progressive along-trench variations. Consistent with the gradual rotation of the trench, the front of the decoupling between the overriding and downgoing plates (together with predicted magmatic activity and topographic uplift) migrates in a horizontal direction. In the experiments with low angles of subduction obliquity (< 15°), relatively old subducting plates (> 50 Ma), and in the absence of the subduction component in the overall shortening, slab detachment either develops simultaneously along the entire length of the subduction zone or does not occur at all. In contrast, with higher subduction obliquity (≥ 15°), younger slabs (≤ 50 Ma) and in the presence of a boundary push on the oceanic side, the initial slab break-off is followed by the gradual growth of the “tear” window in the direction opposite to the migration path of the previously established plates decoupling. The sharp contrast in trench retreat rates between subduction zone segments affected and unaffected by slab detachment results in the arcuate shape of the trench. Furthermore, the direction of slab tearing may change from horizontal to vertical, eventually leading to the formation of a transform fault on the subducting plate. Our results show striking similarities with several features – such as trench curvature, subduction zone segmentation, magmatic production, lithospheric stress/deformation fields, and associated topographic changes – observed in many subduction zones (e.g., Marianas, New Hebrides, Mexico, Calabrian).

A comprehensive re‐look into Jurassic‐Cretaceous Neotethyan active margin in Iran: Evidence of deep mantle flow into subduction magma factory and formation of transitional basalts

Subduction zones display the most diverse magma compositions from various calc‐alkaline differentiates to alkaline mafic associations, but mechanisms generating Nb‐Enriched Basalts (NEB) and High‐Nb Basalts (HNB) in such settings have received less attention from the researchers. Occurrences of NEB and HNB magmas in the Mesozoic Neotethyan active margin in Iran, known as the Sanandaj‐Sirjan Zone, provide an excellent natural lab for examining this interesting petrologic puzzle. The Middle‐Jurassic typical calc‐alkaline arc plutons exhibit a NW‐SE subduction parallel trend. In the northwestern sector of the zone, the Late Jurassic–Early Cretaceous magmatic rocks (JCMR) define a chain that transects the Middle‐Jurassic arc trend. The two cross‐cutting magmatic chains are chemically and genetically different. The JCMR include three distinct compositional groups: (i) tholeiites that are depleted in high‐field‐strength elements (HFSE) and show subduction‐related geochemical fingerprints, (ii) NEB with higher abundances of Nb and other HFSE, and (iii) HNB with the highest abundances of Nb and other HFSE. The latter two groups do not display typical arc signatures and are transitional between arc and plume‐type magmas. These three groups of rocks show linear major and trace element, and Sr–Nd isotopic variation trends extending from Mid‐Ocean Ridge Basalts‐like tholeiites to Oceanic Island Basalts‐like HNB. Hence, a two‐component mixing process is suggested for explaining the compositional variation of the JCMR. We propose that a vertical window resulted from slab tear created the geodynamic environment for juxtaposition of magmas originated at different mantle depths and formation of the transitional basalts by magma mixing.

Clastic carbonates in the South Sistan Basin (SE Iran): implications for subduction vergence and timing of collision

ABSTRACT The N–S-trending Sistan Suture Zone in East Iran results from the collision of the Lut and Afghan continental blocks. The southern Sistan Basin hosts Eocene terrigenous deep-marine turbiditic sequences with carbonate olistoliths, debris-flow sheets and calcareous turbidites. These carbonates represent one or several early to middle Eocene carbonate platforms fragmented and redeposited in a deep-marine environment. Palaeocurrent measurements on the terrigenous and calcareous turbidites show that the source carbonate platform was located on the Afghan active margin. Volcanic debris in the limestone and surrounding turbidites corroborate an active magmatic margin on the Afghan side. The formation of limestone debris-flow sheets is attributed to increased erosion and retreat of the carbonate platform, potentially triggered by the closure of the Sistan Basin and continental collisions in the late middle Eocene, after subduction of the Sistan Ocean beneath the Afghan Block.

Silurian inverted Barrovian-type metamorphism in the Western Sierras Pampeanas (Argentina): a case of top to bottom heating?

AbstractThis paper focuses on one orogenic belt that formed during the Rinconada phase on the final stage of the Famatinian orogeny, between 445 and 410 Ma, which is well exposed at Sierra de Ramaditas and neighbouring ranges in western Argentina. The Ramaditas Complex is formed by metasedimentary and meta-ultrabasic rocks and amphibolites. This complex forms the upper nappe of a thrust stack resulting from westward thrusting. Deformation consists of an early high-temperature S1 foliation (stromatic migmatites), coeval with thrusting and metamorphism. Metamorphism attained peak P–T conditions of 6.0–6.9 kbar and 795–810 °C, at c. 440 Ma, i.e. coincident with the Rinconada orogenic phase. The lower unit and intermediate nappes crop out in the nearby sierras of Maz and Espinal and underwent low- to medium-grade Silurian metamorphism, respectively, together with the upper nappe, defining an inverted Barrovian-type metamorphism with T decreasing and P increasing downwards across the thrust stack (i.e. westward). We argue that the Rinconada orogenic phase developed near the continental margin of SW Gondwana, during a magmatic lull following accretion of the Precordillera terrane to the continental margin at c. 470 Ma. The active margin jumped to the west after accretion, and flat-slab subduction resumed in the early Silurian, provoking thrusting and imbrication of nappe stack under the still hot root (800–900 °C) of the older Famatinian magmatic arc. This ‘hot-iron’ process explains both the inverted Barrovian-type metamorphism and the missing overburden of 21 to 24 km implied by the P estimate.

Upper plate exhumation during continental collision: An example from the paranapanema paleocontinental active margin, SE Brazil

The Southern Brasília Orogen is the result of Ediacaran continental collision between the active margin of the Paranapanema paleocontinent and the passive margin of the São Francisco paleocontinent. The active margin was deformed into the Socorro-Guaxupé Nappe and the associated Embu Domain. This paper presents new U–Pb zircon data and geothermobarometric estimates that allow to reconstruct the metamorphic evolution in these upper plate units. A pre-collisional metamorphism M1 with an age peak in metamorphic rims of zircon grains in the range 680–630 Ma is related to the development of the Socorro-Guaxupé magmatic arc with batholiths dated in a similar time span, 690 - 630 Ma. This metamorphism reached medium pressure granulite facies, now exposed by the tilted uplift and erosion due to the collision. Another peak is detected in metamorphic rims of zircon grains, in the range 630–590 Ma, labeled M2, related to the collision and corresponds to the main metamorphism in the lower plate and accretionary wedge-like nappe stack, underlying the Socorro-Guaxupé Nappe. This nappe stack was affected by greenschist facies metamorphism in the autochthonous underlying craton, grading to high pressure granulite facies in the upper nappe, with local occurrences of retroeclogite lenses. In the Socorro-Guaxupé Nappe and Embu Domain this main metamorphism (M2) is characterized by three metamorphic zones decreasing in grade with distance from the suture: a granulite facies orthopyroxene bearing zone; a high temperature amphibolite facies zone and a lower amphibolite facies zone. Geothermobarometric results in mineral associations related to M2 show temperatures decreasing from about 770 °C to ∼620 °C and pressures decreasing from about 9 kbar close to the suture to about 5 kbar in the southern and western parts of the studied region. On the upper Paranapanema plate, relative to the Socorro-Guaxupé Nappe and Embu Domain units, at the base of the plate, an older metamorphism (M1) is verified with temperatures between 960 and 890 °C and pressures between 11 and 9 kbar, reequilibrated at approximately 700 °C with a pressure of about 9 kbar (M2, continental collision). For this reason, it can be assumed that the rim of the Paranapanema plate was exhumed during the continental collision, since the temperature and pressure of the metamorphic peak determined during the continental collision are lower than those determined for the pre-collisional magmatic arc metamorphism.

Mid-Pleistocene drainage rearrangement of the Dadu River in response to plate convergence in southeastern Tibet

AbstractThe rearrangement of drainage basins provides critical insight into crustal deformation and geodynamic mechanisms. Near the southeastern boundary of the Tibetan Plateau, the Dadu River abruptly shifts from south- to east-flowing, providing important implications for regional tectonogeomorphic development since the mid-Pleistocene. South of the bend, the headwaters of the Anning River occupy an unusually wide valley. Field investigations show that large quantities of fluvial/lacustrine sediments are widespread along the Dadu and Anning rivers and are exposed at their drainage divide. Detrital zircon U-Pb age patterns confirm that these fluvial/lacustrine sediments are the remnants of the paleo-Dadu River, which strongly suggests that the paleo-Dadu River originally flowed southward into the Anning River. The cosmogenic nuclide burial ages of the lacustrine sediments along the Dadu and Anning rivers suggest deposition of these sediments from separate dammed lakes ca. 1.2 Ma ago, ca. 0.6 Ma ago, and ca. 0.9 Ma ago from north to south, respectively. Provenance and burial-age studies indicate that reorganization of the Dadu drainage occurred within the last 0.6 Ma. We propose that this drainage reorganization in southeastern Tibet resulted from progressive convergence between the India and Eurasian plates during the Pleistocene.

Frontal wedge variations and controls of submarine landslides in the Negros–Sulu Trench System, Philippines

Frontal wedge characteristics provide clues to the efficiency of the overriding slab for large displacement during megathrust and upper-plate earthquakes, whereas submarine landslides along active margins may trigger or amplify tsunamis. The lack of clear precursors of submarine failures poses difficulty in monitoring and providing real-time alert warning systems. With that, delineating submarine features along active margins, their spatial distribution, and controls provide valuable information in identifying regions susceptible to large submarine landslides and tsunami hazard assessments. In this study, we performed terrain and morphometric analyses on 20 m resolution bathymetry data to map submarine landslides, submarine canyons, and lineaments in the forearc margin of the Negros–Sulu Trench System in the Philippines. Lineaments are distributed mainly along the frontal wedge, where previous seismic surveys revealed that the mapped ridges are morphotectonic expressions of thrusted sediments. The morphological variations of the four frontal wedge segments were attributed to heterogeneous sediment influx, convergence rates, and subduction processes. More than 1,200 submarine landslides and their morphometric parameters were delineated, and exploratory spatial analyses indicate clustering and underlying controls. The tendencies of prolate submarine landslides (high L/W) to significantly cluster along submarine canyons while oblate morphologies (low L/W) along the frontal wedge reflect the different environments and geomorphological conditions to form these contrasting shapes. Ubiquitous small submarine landslides are mainly controlled by submarine canyon systems at relatively shallow depths of &amp;lt;2 km, where high sediment influx from inland sources preconditions instability. Large submarine landslides (&amp;gt;0.5 km3), on the other hand, are significantly most clustered where the Cagayan Ridge seamount collides and subsequently subducts beneath the northernmost frontal wedge. This suggests the dominant role of seamount subduction and related tectonic processes causing slope steepening to mainly induce large submarine landslides. This study unveiled how submarine landslides vary morphologically depending on their spatial, geomorphological, and tectonic controls in the active margin. This new information provides clues in identifying offshore areas susceptible to large submarine landslides that may induce damaging tsunamis in the Negros–Sulu Trench System as well as in other active margins of similar underlying controls.

Petrography and geochemistry of successions from northwest Bolivia

A complete section of Paleozoic sedimentary rocks has been sampled in northwest Bolivia to investigate compositional changes based on petrographic but mainly geochemical data. Studies of Paleozoic succession in southern Peru, northern Chile and northwest Argentina allowed to decipher the paleotectonic history of the region for the Paleozoic (e.g. compilations in Bahlburg et al., 2009). However, a few is only known about the area geographically in between. We focus here on an area in the vicinity of the capital La Paz, where the Paleozoic belongs paleogeographically to the so-called ‘altiplano’ and the ‘Cordillera Oriental del Norte’ and tectonically to the ‘Faja de Huarina del Norte’. The objective of the study is, hence, to establish a first complete stratigraphic insight of the Paleozoic to enable further studies on key formations or horizons. While the abovementioned areas to the north and south do reflect in the sedimentary record the emergence of a continental arc during the Ordovician, followed by a significant tectonic quiescence during the late Silurian and Devonian and succeeded by the evolution of a second active margin with a continental arc during the late Paleozoic, these events are not visible in the Bolivian samples which is sandwiched between the westerly Arequipa Massif and the eastern Sunsas belt, both composed of mainly metamorphic rocks.The Paleozoic section in northwest Bolivia shows highly recycled quartz-arenites of Ordovician age followed by Silurian and Devonian successions mainly composed of subangular to surrounded detritus, moderately sorted, devoid of significant typical metamorphic detritus. During the late Paleozoic, the rocks tend to a larger variability, partly even poorly sorted, higher amount of angular grains and the occurrence of volcaniclastic debris. Geochemically the same trend can be observed, with strongly recycled successions during the Ordovician (Zr/Sc 20–200) followed by moderately recycled rocks (mainly Zr/Sc 10–20100) with typical trace element composition for unrecycled upper continental crust (UCC). During the Permian, sediments which are less weathered and slightly less fractionated in their overall geochemical composition, have been deposited. The geochemistry may point to the evolution of late Paleozoic volcanism, recorded in overlying, but however, Triassic rocks. Significant increase or even typical UCC values of compatible elements or ratios are absent besides Cr and Ni increase in Ordovician rocks. The absence of any significant input of arc related detritus can be explained by not effective sediment dispersal system to transport arc related detritus into the depositional area. The low compositional and low to moderate textural maturity throughout the entire post-Ordovician stratigraphy implies relatively proximal sources and insignificant intra-basinal recycling. This together with the absence of metamorphic detritus, allows in proposing thick homogeneous sedimentary successions with low or no metamorphic overprint as major sources. Those covered possibly the today exposed metamorphic successions of the aforementioned basin surroundings. Sedimentary or any other lithologies older than the Ordovician and younger than Mesoproterozoic are nearly unknown in the region, besides one exception (Chilla beds; Bahlburg et al., 2020). Those hypothetical deposits would be at this stage of knowledge the best explanation.

Early stages of trench-slope basin development: Insights from mass-transport deposits and their interactions with turbidite systems (southern Hikurangi margin, New Zealand)

Geological complexity, progressive tectonic overprint and prevailing submarine location of subduction margins generally hinder investigating the early stages of development of their oldest trench-slope basins, witnesses of subduction initiation and early history. Along the eastern North Island of New Zealand, the inner portion of the Hikurangi subduction wedge is emerged, thereby offering a unique opportunity to examine, at outcrop-scale, the tectonostratigraphic evolution of a trench-slope basin (Castlepoint trench-slope basin) that was linked to the onset of subduction. We present new occurrences of lowermost Miocene gravity-driven systems, suggesting a more intricate depositional framework to the ones previously inferred during this key period. Results show that the early stages of development of a trench-slope basin coeval with the birth of a subduction margin may deviate from traditional models either comprising (1) sustainable sediment sources connected to the basin very early in the history of the margin and (2) a sedimentation-deformation feedback mechanism promoting the long-term development of an aggradational turbidite system downslope. Analysis of the mass-transport deposits (MTDs) also revealed that the outboard migration of deformation was discontinuous and uneven along the margin during the earliest Miocene. Two major tectonic events, separated by a period of reduced tectonic activity, were recorded at the inboard border of the Castlepoint trench-slope basin, each resulting in seaward motion, oversteepening and frontal denudation of a thrust sheet. Each thrust sheet provided contrasting failed material and morphometric characteristics to the associated deposits, thereby allowing us to discriminate the nature of the nappe and related controls responsible for shedding each MTD as well as refine the timing of nappe emplacement along this part of the margin. Overall, this study draws new insights on the early structural evolution and stratigraphic infill during the birth of subduction zones, insights which may, in turn, help improve understandings of active margin settings.

The Late Devonian contraction along the Precaspian Basin north margin: Transpressional range building in the intracratonic domain

The evolution of intraplate sedimentary basins located in the vicinity of an active convergent plate boundary is often controlled by the collisional dynamics of the adjacent orogen. The transfer of compression from the orogen to the platform's interior results in the formation of complex structural geometry and kinematics that often reactivate older crustal faults, focus far-field stresses and control the evolution of associated sedimentary basins. One place where this localisation can be optimally understood is the Precaspian Basin, situated at the SE periphery of the East European Craton and bordered to the east by the Uralian orogen. The Precaspian Basin and its northern margin experienced long-term extension and subsidence interrupted by several short-lived shortening episodes. To understand the impact and role of pre-existing basement structures on the geometry and kinematics of the subsequent deformation, we analysed subsurface data from the northern margin of the Precaspian Basin by the means of 3D seismic interpretation correlated with wells and structural modelling. The analysis results provide new insights into the kinematic effects of the Late Devonian contraction event. The superposition of stratigraphic units above and below the angular unconformity suggests an intra-Famennian age of the deformation event. Steeply dipping bi-verging reverse fault zones associated with variable amounts of sinistral strike-slip movement component display an arcuate geometry, trending from WSW-ENE to NW-SE. The distribution of deformation indicates that this complex kinematic pattern was driven by a NE-SW oriented contraction and transpression, where faults show the characteristics of a restraining bend area. This area is interpreted as part of a regional transcurrent fault system developed on the northern periphery of the Precaspian Basin. Furthermore, the study results suggest that intralithospheric stress localisation transmitted by the Paleouralian subduction zone resulted in the reactivation of pre-existing basement structures, propagation of faults, and localized short-term exhumation of the Precaspian Basin north margin. Generated by the collision of Magnitogorsk volcanic arc with East European Craton, far-field stress transfer produced a zone of oblique deformation. These data and interpretation demonstrate that the northern margin of the Precaspian Basin is an excellent natural setting to investigate and better understand mechanisms of far-field strain localisation and reactivation of deformational structures in stable platform areas resulting in the intracratonic mountain building process.

High magnesian schist, granitic gneiss, amphibolite and monzogneiss in the eastern Ama Drime Massif in South Tibet (China): A rifted Paleoproterozoic arc fringed the western Columbia supercontinent?

Understanding the Paleoproterozoic tectonic setting of the north Indian Craton—as either continental rifting or continent arc—is vital for locating the Indian Craton in the Columbia supercontinent. Herein, we present new petrological, geochemical, Nd–Hf isotopic, and geochronological data of high magnesian schist, granitic gneiss, amphibolite and biotite monzogneiss in the south-eastern margin of the Ama Drime Massif of South Tibet (China). The high magnesian schist contains 20.4 %–31.3 % phlogopite, 12.2 %–17.8 % clinochlorite, 10.5 %–13.4 % cordierite and 5.5 %–5.6 % loss on ignition, implying its protolith of Mg-rich clays and later alteration in the fault zone. Its medium-silicic composition and concentrative magmatic zircon age of ca. 1859 Ma indicates the origin of the clays from locally deposited mafic rocks. Moreover, its negative values of ƐNd(t) (−3.0 to −2.6) and zircon ƐHf(t) (−8.6 to +2.4) correspond to the metavolcanic rock in the same outcrop as well. Granitic gneiss, amphibolite and biotite monzogneiss exhibit their protolith of crystal ages between 1870 and 1849 Ma with characteristics of negative anomalies of Nb, Ta, and Ti in subduction zone setting. They have similar negative values of ƐNd(t) (−3.0 to −1.3) and zircon ƐHf(t) (−8.6 to +4.0), indicating various degrees of contamination in the continental crust. The geochemical characteristics of the granitic gneiss and amphibolite signify the generation of their protolith via various degrees (1.5–2.5 %) of partial melting of spinel lherzolite inside the shallow mantle. As the high alkalinity of biotite monzogneiss implies a greater contribution of water from the subduction plate, and the trace elements of zircons manifest their affinity of continent arc toward island arc, we interpret the origin of the protolith of these rocks in a setting of continental arc-related rifting at the north of the Indian Craton. The finding of this arc reinforces that the reconstruction of the western Columbia supercontinent was fringed with an active margin.

Geochemical features and seismic imaging of the tectonic zone between the Tibetan Plateau and Ordos Block, central northern China

The Tibetan Plateau is growing by both vertical uplift and horizontal extension. It is a continuing debate how the Tibetan Plateau interacts with its surrounding plates and blocks. Due to intense tectonic activity, which produced catastrophic earthquakes, the tectonic zone between the northeast margin of the horizontal extending Tibetan Plateau and the stable Ordos Block has garnered considerable interest. This study investigated the spatial distribution of gas geochemical anomalies (e.g., high flux of CO2 in correspondence of the main faults) at regional scale together with the seismic tomography in correspondence of this tectonic zone with the aim to figure out the domain of convergent boundary between the Ordos block and Tibetan plateau, and trace the tectonic discontinuities which are able to transfer fluids through the crustal layers between the two main geological units. From northwest to southeast, obvious difference of spatial distributions of geochemical and geophysical features in the tectonic zone between the northeast margin of the Tibetan Plateau and the Ordos Block is inferred. The northeast area (Zone A) is dominated by thrust and strike-slip faults with clear velocity boundary underneath, where low crack density (ε), saturation rate (ξ) and Poisson’ ratio (σ) in the middle-lower crust coincided with the low values of heat flow and CO2 emissions, tectonic compression and regional locked-fault can be inducements. The southeast area (Zone C) is dominated by extensional tectonics with roughly E-W fast-velocity direction (FVD) of P-wave azimuthal anisotropy, where high permeability and porosity can be deduced from crustal high ε, ξ and relatively high σ anomalies, resulting in high heat flow, CO2 concentrations and fluxes at the surface, and predominantly crustal-derived gases. The intermediate area (Zone B) also dominated by thrust and strike-slip faults is an extraordinary zone, where intensely locked-fault were clearly revealed, while the predominant anisotropic FVDs in the middle crust changed obviously, more contribution of shallow gas component was detected, and CO2 flux, heat flow, and regional ε, ξ, and σ in the upper crust were higher, compared with those in Zone A, which indicated the regional crushing fragmentation underneath Zone B. The adopted multidisciplinary approach demonstrated that Zone B is the convergent boundary between the Tibetan Plateau and the Ordos Block.

Peak Cenozoic warmth enabled deep-sea sand deposition

The early Eocene (~ 56–48 million years ago) was marked by peak Cenozoic warmth and sea levels, high CO2, and largely ice-free conditions. This time has been described as a period of increased continental erosion and silicate weathering. However, these conclusions are based largely on geochemical investigation of marine mudstones and carbonates or study of intermontane Laramide basin settings. Here, we evaluate the marine coarse siliciclastic response to early Paleogene hothouse climatic and oceanographic conditions. We compile an inventory of documented sand-rich (turbidite) deep-marine depositional systems, recording 59 instances of early Eocene turbidite systems along nearly all continental margins despite globally-elevated sea levels. Sand-rich systems were widespread on active margins (42 instances), but also on passive margins (17 instances). Along passive margins, 13 of 17 early Eocene systems are associated with known Eocene-age fluvial systems, consistent with a fluvial clastic response to Paleogene warming. We suggest that deep-marine sedimentary basins preserve clastic records of early Eocene climatic extremes. We also suggest that in addition to control by eustasy and tectonism, climate-driven increases in sediment supply (e.g., drainage integration, global rainfall, denudation) may significantly contribute to the global distribution and volume of coarse-grained deep-marine deposition despite high sea level.

Widespread Cadomian–Pan-African Ediacaran magmatism across the Moroccan Meseta: Implication for the geodynamic evolution of the NW Gondwana margin

The Moroccan Meseta constitutes a key peri-Gondwanan and Variscan terranes linking the branches between the SW European Cadomian and the north West African Craton (WAC), anti-Atlas metacratonic Pan-African terranes. While the Paleozoic sedimentary and tectono-magmatic record of the different blocks of the Moroccan Meseta is well documented, there is still a lack of systematic absolute ages and geochemical isotopic data of putative Precambrian basement rocks from different Meseta blocks. Here we present new U–Pb zircon ages, petrological, and whole-rock major, trace, and Sr–Nd–Pb isotope geochemical data of (meta)igneous rocks from the Bou-Acila, Jbel-Hadid, Goaïda, Midelt and El Jadida, constituting the basement of two main Moroccan Meseta blocks. Basaltic rocks, unconformably overlain by Cambrian platform sediments, in the Western Meseta Bou-Acila basement yield U–Pb zircon age of c. 581 ± 9 Ma that unveils the existence of Ediacaran magmatism in the Moroccan basement of this area. Alleged early Paleozoic mafic sills in the metasedimentary series of the Midelt Sidi-Saïd yield U–Pb zircon ages of c. 554 ± 12 Ma reveals, for the first time, the presence of Ediacaran magmatism in the basement of the Moroccan Eastern Meseta. U–Pb zircon ages in the Jbel-Hadid and El Jadida rhyolitic basement rocks (c. 604–543 Ma and c. 575–559 Ma, respectively), as well as granitic pebbles (c. 573 Ma) in the Goaïda Cambrian limestone series, further attest for coeval Ediacaran magmatism in the basement of Western Meseta. The geochemical affinity of the investigated Moroccan Meseta Ediacaran magmatic rocks is mainly metaluminous, low-K tholeiites for mafic rocks, and peraluminous, high-K calc-alkaline for the felsic rocks, consistent with a back-arc tectonic setting. Geochemical and Sr–Nd–Pb isotopic data indicate that the source of the Moroccan Meseta Ediacaran magmatism involved sources from the depleted mantle and old WAC crustal basement. Altogether, these data show widespread Ediacaran magmatism in the Moroccan Meseta mostly likely formed in a back-arc south of the Cadomian active margin along the NW Gondwana during the waning stages of the Cadomian–Pan-African orogeny, recording a transitional stage between the Ediacaran subduction and early Paleozoic rifting.

Large-scale Permo-Triassic back-arc extensions of the Mongol- Okhotsk Ocean

The Late Paleozoic−Mesozoic Mongol-Okhotsk orogenic belt marks the final aggregation of East Asia. The geodynamics of the Mongol-Okhotsk oceanic plate subduction are still poorly understood due to its curved orogenic architecture, complex kinematics, and the protracted active continent margin that developed during oceanic subduction. Here, we report the discovery of an ophiolite within the southern paleo-active margin of the Mongol-Okhotsk orogenic belt. The ophiolitic slices are composed of serpentinites, metagabbros, and metabasalts, and interleaved with deformed volcaniclastic rocks. Using zircon U-Pb dating, we determined an age of 253 ± 2 Ma for the metagabbro, which suggests that the ophiolite formed during the Late Permian. Geochemical data and geologic relationships indicate that this ophiolite formed in a back-arc setting. The contemporaneous granitic dikes (ca. 255 Ma) intruding the basalts of the ophiolite were likely formed by crustal anatexis during the back-arc extension. Detrital zircon in sandstone associated with the ophiolite shows a prominent peak age of ca. 273 Ma that probably represents the sediments of the back-arc basin. Combining our discovery with the available data from the literature, we suggest that a &amp;gt;5000-km-long back-arc extension belt existed in the southwestern segment of the Mongol-Okhotsk belt, which indicates a probable Western Pacific-type active margin rather than the previously proposed Andean-type margin that formed during the Permo-Triassic period.

Impacts of Tectonic Subsidence on Basin Depth and Delta Lobe Building

AbstractChannel avulsions on river deltas are the primary means to distribute sediment and build land at the coastline. Many studies have detailed how avulsions generate delta lobes, whereby multiple lobes amalgamate to form a fan‐shaped deposit. These studies often assume a steady subsidence and uniform basin depth. In nature, however, lobe building is disrupted by variable subsidence, and progradation of lobes into basins with variable depth: conditions that are prevalent for tectonically active areas. Herein, we explore sediment dispersal and deposition patterns across scales using measurements of delta and basin morphology compiled from field surveys and remote sensing, collected over 150 years, from the Selenga Delta (Baikal Rift Zone), Russia. Tectonic subsidence events, associated with earthquakes on normal faults crossing the delta, displace portions of the topset several meters below mean lake level. This allogenic process increases regional river gradient and triggers lobe‐switching avulsions. The timescale for these episodes is shorter than the predicted autogenic lobe avulsion timescale. During quiescent periods between subsidence events, channel‐scale avulsions occur relatively frequently because of in‐channel sediment aggradation, dispersing sediment to regional lows of the delta. Avulsion settings for the Selenga Delta preserve discrete stratal packages that could contain predominately deep channels. Exploring the interplay between tectonic subsidence and sediment accumulation patterns will improve interpretations of stratigraphy from active margins and basin models.