Subduction Direction Research Articles

Shear wave splitting and mantle dynamics of the southern Great Xing'an orogenic belt

We deployed a linear seismic array consisting of 50 broadband stations in the southern part of the Great Xing'an orogenic belt to investigate seismic anisotropy in the Songliao Basin, the orogenic belt and the Erlian Basin by making SKS wave splitting measurements. Different features of anisotropy are revealed on the two sides of the north-south gravity lineament. The anisotropy on the east side of the gravity lineament primarily results from mantle flows in the big mantle wedge above the flat Pacific slab in the mantle transition zone, and the SKS fast polarization direction is generally NW-SE, being consistent with the subduction direction of the western Pacific plate. On the west side of the gravity lineament, the SKS splitting is attributed to freezing anisotropy in the stable lithosphere under the Erlian Basin. The SKS splitting delay times show some changes with azimuths of the teleseismic events, but the fast polarization directions are generally the same, indicating that the anisotropies below these seismic stations have the same origin in the continental lithosphere and active flows in the big mantle wedge.

A new tectonic model for the Itmurundy Zone, central Kazakhstan: linking ocean plate stratigraphy, timing of accretion and subduction polarity

The Itmurundy Zone of Central Kazakhstan is a key structure in the core of the Kazakh Orocline representing a typical Pacific-type orogenic belt hosting accretionary complex, ophiolite massifs and serpentinite mélange. The main controversies in the existing tectonic models of the Itmurundy Zone are about the timing of subduction and accretion, the direction and kinematics of subduction and the number of oceanic plates. A new model for the early Paleozoic tectonic story of the Itmurundy Zone is postulated in this paper, based on new detailed geological and U–Pb detrital zircon age data, combined with previously documented geological, U–Pb age, microfossil, geochemical and isotope data from igneous rocks, deep-sea sediments and greywacke sandstones. The present study employs the Ocean Plate Stratigraphy (OPS) model to explain the tectonic processes involved in the evolution of the Itmurundy Zone and to present a holistic story of Ordovician oceanic plate(s), which accretion formed an accretionary complex. The detailed mapping allows distinguishing three types of OPS assemblages: (1) Chert-dominated, (2) OIB-hosting, and (3) MORB-hosting. The U–Pb ages of detrital zircons from sandstones of OIB and Chert types show unimodal distributions with similar main peaks of magmatism at 460–455 Ma in the provenance, and their maximum depositional ages (MDA) span 455–433 Ma. Two samples from OPS Type 3 show the peaks of magmatism both at ca. 460 Ma and the MDA of 452 Ma and 459 Ma, respectively. The MDA of sandstones and microfossils data from chert show the younging of strata to the south and SE in Types 1 and 2 and to NEE for Type 3 (in present coordinates) suggesting double-sided subduction to the NNW and SEE and, accordingly, the co-existence of pieces of two oceanic plates in Ordovician time. The U–Pb zircon data from both igneous and clastic rocks indicate a period of subduction erosion in early Ordovician time. As a whole, the accreted OPS units of the Itmurundy Zone record the timing of subduction and accretion from the early Ordovician to the early Silurian, i.e., 60 Ma at shortest.

Paleo-Pacific plate subduction direction change (122−118 Ma): Insight from late kinematic plutons in the Wulian metamorphic core complex, Jiaodong Peninsula, eastern China

The late Mesozoic subduction of the paleo-Pacific plate (Izanagi plate) beneath Eurasia involved complex changes in the direction and angle of subduction, which have been controversial in recent decades. This study investigated Early Cretaceous late kinematic plutons of the Wulian metamorphic core complex that formed in the southern Jiaodong Peninsula. Based on geochemical and Sr-Nd isotopic analyses, we conclude that the granites in the Heshan (122 Ma) and Kuaijishan (118 Ma) plutons are I-type granites that formed in a back-arc extensional environment, and tensile fractures played an important role in the process of magma emplacement. Gravity anomaly inversion revealed the significant role of tension fractures in the uplift of the late kinematic plutons. Analysis of magnetic fabrics showed that the dominant plunging direction of magnetic lineation in the late kinematic plutons is NW-SE (127°−307°), while the plunging direction of the mineral stretching lineation in the wall rock is nearly E-W (102°−282°). We posit that the extensional stress field in the Jiaodong Peninsula underwent a significant transformation during the Early Cretaceous period, specifically from 135 Ma to 122 Ma, when it was dominated by a regional E-W or WNW-ESE extensional stress field. Later, from 122 Ma to 118 Ma, the stress field was controlled by a NW-SE extensional stress field. This variation in the regional stress field in the Jiaodong Peninsula indirectly indicates the changing subduction and slab rollback direction of the paleo-Pacific plate. From 135 Ma to 122 Ma, the paleo-Pacific plate subducted beneath Eurasia along a WNW-ESE direction. However, back-arc extension and slab rollback led to extensive extension in the Jiaodong Peninsula. Subsequently, the subduction direction of the paleo-Pacific plate shifted to the NW, with rollback occurring eastward at a high angle under the Jiaodong Peninsula from 122 Ma to 118 Ma. This shift resulted in extensive magmatism and widespread gold mineralization. The findings of this study offer new insights into the complex geodynamic processes that occurred during the Early Cretaceous period in the region.

Northward expansion of the Jiaolai Basin during the Early Cretaceous: Insights from source‐to‐sink reconstruction

AbstractThe Jiaolai Basin, situated in the northern Sulu orogenic belt along East Aisa continental margin, preserves evidence of the extensional events in East Asia and the post‐orogenic evolution of the Sulu orogenic belt during the Cretaceous period. In this study, multiple provenance analyses were employed to reconstruct the source‐to‐sink system of the Laiyang Group within the Jiaolai Basin. These studies reveal a history of northward expansion dictated by two significant rift events. During the early Early Cretaceous period (ca. 135–121 Ma), the Zhucheng and Gaomi sags in southern region developed initially. Subsequently, in the late Early Cretaceous period (ca. 120–113 Ma), the Laiyang sag in northern region emerged. Furthermore, these sags were fed by independent source‐to‐sink systems in their early stages but eventually shared a similar source‐to‐sink system towards the end of the Laiyang Group deposition (ca. 113 Ma). The provenance analysis results indicate that ca. 121 Ma, ultrahigh‐pressure rocks in the northern segment of the Sulu orogenic belt experienced rapid exhumation, while those in the southern segment might have remained concealed until ca. 113 Ma. The two rift events in East Asia, coupled with the alteration in the direction and magnitude of extension documented in the Jiaolai Basin, suggests that trench retreat and the change in subduction direction from E–W to NW–SE of the Izanagi plate played a principal role in driving the extensional events in East Asia during the Early Cretaceous. Our findings imply that the change in Izanagi subduction direction may have occurred ca. 121 Ma.

Late Cretaceous Sevier Versus Laramide Orogenies in Wyoming‐Utah‐Colorado, USA: New Insights From Basin Subsidence History

AbstractVariability in subsidence rates within Upper Cretaceous strata of the Western Interior Basin offers crucial insights into the response of surface sedimentation styles to Sevier‐to‐Laramide tectonics and related deep mantle processes. The formation mechanisms of the Late Cretaceous Western Interior Basin in North America have long been a subject of debate. A re‐evaluation of the basin's subsidence history reveals rapid subsidence pulses lasting ca. 2 Myr within longer‐term (average 5.7 Myr) progradational or aggradational clastic wedges. The timing of these wedges, especially the widespread marine flooding resulting from subsidence, is constrained through the calibration of ammonite zonation with absolute dates. Sevier wedges exhibit a different architecture compared to the Laramide wedges. The former recorded initial rapid and widespread marine transgressions followed by long‐term coastal progradation, whereas the latter developed by initial erosional and progradational growth followed by aggradation and long‐term coastal transgression. The Sevier clastic wedges, initially accumulated within a N‐S elongated, long‐wavelength tectonic subsidence zone close to the thrust belt, gradually migrated cratonward. Starting in the early Campanian (ca. 82 Ma), the Laramide Orogeny developed along a NW‐SE trend and then migrated northeastward, roughly consistent with coeval long‐wavelength frontal basin subsidence. The spatio‐temporal variations in long‐wavelength tectonic subsidence indicate a shift in the dynamic subsidence's migration direction from eastward to northeastward, driven by changes in Farallon subduction direction and mode. Our work shows how repeated subsidence behavior in the Sevier‐to‐Laramide transition records evolving architectural responses and the trajectory of coeval dynamic topography.

Differential Uplift Triggered Basin‐And‐Range System: Evidence From Low‐Temperature Thermochronology in Eastern NE Asia

AbstractSince the Mesozoic, eastern NE Asia has experienced multiple tectonic events, resulting in a complex structure and forming one of the world's largest Meso‐Cenozoic lacustrine basin systems. Presently, basin evolution models require further elucidation regarding the simultaneous generation of diverse rift basins and the potential impact stemming from the closure of the Mudanjiang Ocean, whose oceanic closure demarcated the boundary between the Songliao Basin and the eastern basins, raises questions about its influence on the development of the basin‐and‐range system. To address these questions, we augment new low‐temperature thermochronological data on basement highs separating the eastern NE Asia basins to investigate the shallow‐deep coupling process of tectonic evolution since the Mesozoic. The new cooling age pattern shows non‐overlapping and spatial differences among major basement highs. Inverse thermal modeling revealed five‐stage cooling episodes among the basement highs, but with different onset and cooling rates of each episode, indicating a significant differential uplift mode. A major reburial stage was identified throughout eastern NE Asia during the mid‐Cretaceous. Compiling cooling age patterns and inverse thermal modeling, we note the existence of a proto‐basin covering an area much larger than the previously contemplated “Pan‐Sanjiang” Basin. In general, our study indicates the final closure of the Mudanjiang Ocean occurred at ca. 150–140 Ma. Since the Early Cretaceous, with changes in the subduction direction, two‐stage flat slab subduction of the Paleo‐Pacific plate and the consequent subduction of the Pacific plate co‐dominated the basements' differential uplift and the formation of the eastern NE Asia basin‐and‐range framework.

Petrogenesis of the Morobe Granodiorite and their shoshonitic mafic microgranular enclaves in Maramuni arc, Papua New Guinea

Abstract The Miocene tectonics of Papua New Guinea, where subduction, arc-continent collision, and changes in subduction direction are considered to have occurred, is very complex and various tectonic models have been proposed. The Maramuni arc, active in the Miocene, is composed of a chain of granitoid bodies. As the chain-like distribution indicates the generation of igneous activities in a wide range of the same tectonic settings, the study of the Maramuni arc magmatism is important for elucidating the geologic events of the time. We provide data on the petrological and geochemical characteristics of the Morobe Granodiorite that form part of the Maramuni arc. The Morobe Granodiorite consists of metaluminous I-type granitoids, belonging to the medium-K to high-K series. The whole-rock major element variations in the granitoids can be explained by the fractionation of hornblende and plagioclase. They are generally within the composition range of experimental partial melts of amphibolites, and the whole-rock trace element compositions have characteristics of slab failure magma rather than arc. This suggests that the granitoids were generated by partial melting of the torn slab after slab failure. The mafic microgranular enclaves (MMEs) in the granitoids are classified as shoshonite, and their trace element compositions suggest that they were formed by partial melting of phlogopite-bearing mantle. The occurrences of native gold and barite within the MME show that MMEs transport Au from the mantle metasomatized by slab-derived sediment melt and/or fluid to the crustal magma chamber.

Opposite rotations in the Central Andes Bend: Tectonic scenario compared to other cases of opposite rotations and implications for long-term subduction at continental arcs

A review confirms opposite paleomagnetic rotations on the northwest and southeast limbs of the Central Andes Bend in Peru, Bolivia, northern Chile and Argentina. Counter-clockwise rotations occur NW of an ENE-oriented axis extending from Arica on the Chile coast to Santa Cruz, Bolivia, with clockwise rotations to the south and SE. Rotations have been measured in all the curvilinear provinces from the Western Cordillera via the Altiplano/Puna to the Eastern Cordillera, InterAndean and SubAndean belts. Although several processes have been postulated, the zone coincides with flat-slab subduction, possibly related to underthrusting of oceanic plateaux. The curvilinear subduction zone is convex in the direction of subduction. Shortening associated with compression and uplift migrated east-northeastwards towards the SubAndean belt from Mid Eocene to Recent and is greatest at the axis of the bend. The area may be segmented with rotations facilitated by strike-slip fault zones: CCW rotations relate to sinistral strike-slip whereas CW rotations are associated with dextral strike-slip. Additional compartmentalisation is here suggested based on volcano distribution, regularly spaced giant copper porphyry deposits, offsets of thrust anticlines, and isolated foreland basins. This tectonic scenario is contrasted with other identified cases of opposite microplate rotations which are usually linked to rollback of a subduction zone which is concave in the direction of subduction. In these cases, CW rotations also relate to dextral strike-slip, and CCW rotations to sinistral strike-slip, but their distributions change if the slab changes from convex to concave in the direction of subduction. These observations provide a model for long-lived subduction at continental arcs where flat-slab subduction switches to rollback over time, such as Pacific China/Russia from 200 to 70 Ma, SW Gondwana from 220 to 145 Ma, western North America from 90 to 0 Ma, or Mexico from 16 to 0 Ma.

Clockwise rotation of SW Japan and timing of Izanagi–Pacific ridge subduction revealed by arc migration

Igneous rocks associated with the Cretaceous to Paleogene volcanic arc in SW Japan show ages that young from west to east in a direction parallel to the Median Tectonic Line suggesting corresponding translation of a heat source traditionally interpreted in terms of oblique subduction of a spreading ridge. However, recent oceanic plate reconstructions suggest ridge subduction may be younger than the main arc activity. Age compilations of 1227 points of felsic to intermediate Cretaceous and Cenozoic igneous rocks from the Japan arc show arc magmatism that can be separated into an early active period 130–60 Ma (stage 1), a subsequent period of quiescence 60–46 Ma (stage 2), which is followed by a resumption of igneous activity from 46 Ma onward (stage 3). In southwest Japan, the orientations of the magmatic arcs of stages 1 and 3 show and angular discordance of about 20°. The lack of active arc magmatism and the occurrence patterns of adakitic and high-Mg andesitic magmas indicate that ridge subduction occurred during stage 2. The arc age distribution pattern of stage 1 is explained by the slab shallowing related to a younging of the subducting slab as the ridge approaches. Furthermore, the obliquity of the arcs formed at stages 1 and 3 is explained by a 20° clockwise rotation of the inner zone of southwest Japan during the ridge-subduction phase. Oceanic plate reconstructions show counterclockwise rotation in the subduction direction after the ridge subduction phase, and coupling of the subducting oceanic plate with the upper plate would support microplate rotation in the inner zone. The new proposed tectonic reconstructions provide a framework to related Paleogene subduction of an active spreading ridge along the east Asia margin not only to the distribution of granitic bodies but also to rift-related basin formation on the eastern margin of the Eurasian continent and to rotation of crustal blocks indicated by paleomagnetic data of Cretaceous terranes.

Gravity inversion constrained by OBS receiver function reveals crustal structure in Ryukyu Trench

The Ryukyu Trench is located in the northern Philippine Sea. The subduction of the Philippine Sea Plate to the Eurasia Plate along the Ryukyu Trench in the NNW direction leads to extremely complex tectonic processes such as subduction, collision, accretion, and back-arc expansion. It is of great significance to obtain the crustal structure of the Ryukyu Trench for understanding the crustal deformation characteristics, subduction direction and the scale of the Philippine Sea Plate. The ocean bottom seismometer (OBS) could provide important information on the deep structure of the oceanic area, and the wide coverage of gravity anomaly can provide more constraints for the regional crustal structure. In this study, we put the time-delay term of the sedimentary layer into the H-κ stacking formula, which eliminates the influence of the sedimentary layer in traditional H-κ stacking. Based on the accurate crustal thickness at OBS locations, a gravity inversion method is proposed to solve the problem of non-uniqueness in gravity data and limited distribution of OBSs. By using this method, we obtain a crustal structure with higher resolution than Crust1.0 model in Ryukyu Trench. The results show that the crustal thickness in the Ryukyu Trench region thickens along the NNW subduction direction, revealing that the Philippine Plate’s subduction direction towards the Eurasian Plate is in the NNW direction.

Cretaceous source to sink system reconstruction of northeastern Asian continental margin: Insight from integrated detrital geochronology in NE China

The Cretaceous sedimentary successions in NE China are largely incomplete as the basins are inverted and younger strata are eroded. Except for the Songliao Basin, whose depositional record has remained relatively intact, the burial record to the east was interrupted in the late Early or Late Cretaceous. There is still controversy on the possible connection between the Songliao Basin and the eastern basin group, as well as the extent of the previously suggested “pan-Sanjiang” proto-basin that covered most current eastern satellite basins. To address these questions, we studied modern river catchments of variable sizes from sediment-supplying basement highs that cover most of eastern NE China to track the intense denudation events and provenance changes through the Cretaceous. We found a great inconsistency between detrital age patterns of the modern river sands and the spatial proportions of the source units. The detrital age distributions allow to re-evaluate the basement units that are mostly composed of the Triassic–Jurassic igneous suite and some metasedimentary units with Paleozoic ages, but the contribution from Proterozoic formations is negligible. Combining the newly dated modern catchment detrital U-Pb ages, the region-wide compilation of the basement emplacement ages, and the U-Pb ages in the Cretaceous formations in NE China, we systematically refine the Cretaceous provenance history. In the Early Cretaceous, the eastern satellite basins accumulated sediments from the Lesser Xing’an range (LXR) in the west, Zhangguangcai range (ZGCR) in the south, and the Nadanhada terrane (NT) in the east. The sediment of the southern Songliao Basin derived from the North China Craton (NCC) and from ZGCR but barely from the Great Xing’an Range (GXR). The Jiamusi uplift (JU) did not provide sediments to the eastern satellite basins and the Songliao Basin.In the Late Cretaceous, the eastern satellite basins received sediments from the exhumed JU but barely from ZGCR and LXR. The Songliao Basin still accumulated the sediments from the NCC, some south part of ZGCR, and GXR but minor from the central and northern ZGCR and LXR. Between the latest Early Cretaceous and early Late Cretaceous, a much wider proto-basin was suggested in NE China than previously assumed. It covered during its maximum extension period the current Songliao Basin, the eastern satellite basins, the LXR, ZGCR, JU, and partial GXR basement areas. The provenance change is induced by the Paleo-Pacific plate motion change, i.e., with the subduction direction change, the roll-back, and two-stage limited flat slab subduction trigged the formation of the basin-and-range system in NE China since the Early Cretaceous. Moreover, we suggest conducting modern sand provenance analysis to detect, verify or re-classify the ages of the zircon-bearing units, especially at a reconnaissance prospecting on areas covered by imprecise large-scale geological maps only to reveal the catchment better.

The Wakayama earthquake swarm in Japan

An earthquake swarm in the Wakayama prefecture, Japan, is known as the most active and persistent swarm, with > 95,000 earthquakes (M ≥ –1.3) occurring during the 2003–2020 period. However, no systematic studies have highlighted the source of this intriguing non-volcanic earthquake swarm to date. This study systematically investigates the temporal and spatial evolution of the Wakayama earthquake swarm and estimates the seismic velocity structure around the Kii peninsula, where we observe series of anomalous geophysical and geochemical signatures, such as high 3He/4He ratios, deep low-frequency earthquakes, and hot springs with high salinity and solute concentrations. We reveal that seismicity associated with the Wakayama earthquake swarm occurs almost evenly in both time and space, and that the majority of the earthquakes in the northern part of the swarm activity occur along well-defined planes that dip to the west at 30–45°. The seismic tomography results reveal that a northwestward-dipping low-velocity zone exists beneath the Wakayama swarm and the low-velocity zone is sandwiched by high-velocity anomalies in the continental crust interpreted as impermeable and rigid materials on both sides in the subduction direction. This unique tectonic setting controls a pathway of the upward migration of slab-derived fluids to the surface, with the high fluid concentration in the dipping low-velocity zone. Therefore, we infer that the location of the Wakayama swarm is controlled by deep crustal heterogeneities rather than by the major structures of geological accretionary complexes. This study suggests that the anomalous geophysical and geochemical signatures observed across the Kii peninsula are different manifestations of the frictional and hydrological processes during the upward migration of the slab-derived fluids. We further propose that the valley-shaped geometry of the Philippine Sea slab beneath the Kii peninsula is caused by the rigid materials in the continental crust.Graphical

Unilateral Magma Emplacement of the Telimbela Batholith in the Central Ecuadorian Arc: Implications for Kinematics of Oblique Subduction of the Farallon‐Nazca Plate

AbstractThe N‐S elongated granodioritic Telimbela batholith (TB, ∼220 km2) was emplaced at the restraining bend of the dextral strike‐slip Calacalí‐Pujilí‐Pallatanga Fault (CPPF) developed at the ca. 120 km wide Tertiary Ecuadorian arc. Our and previous zircon U‐Pb ages indicate that the batholith has a westward younging trend, with the oldest and youngest ages of 25.5 ± 0.5 and 17.5 ± 0.3 Ma, respectively, and a minimum duration of 8 ± 0.6 Ma was acquired. The Al‐in‐hornblende geobarometer results show that the TB was emplaced in the upper crust (∼4.7–5.3 km). The anisotropy of magnetic susceptibility (AMS) results of TB show three distinct groups of magnetic foliations, with a mean orientation of (64°/SE ∠ 39°) (0°/E ∠ 83°), and (16°/NW ∠ 83°) in the eastern, central and western parts of TB, respectively. As the TB was syntectonically emplaced with the evolution of CPPF, the vorticity numbers (Wn) of eastern, central, and western parts are determined by the angle between CPPF and mean orientation of AMS with Wn values of 0.79, 0, and 0.53, respectively. According to previous geodynamic studies on Northern Andes, the early and late kinematic transitions recorded in the TB may correspond to the change of plate subduction direction and the decrease in subduction angle, respectively. Comparing the estimated growth rate of TB to the tempo of Tertiary Ecuadorian arc magmatism revealed by detrital zircon ages and previous plate convergence velocity models, the good consistency among them will help to restore a detailed kinematic evolution of the Ecuadorian arc.

Seismicity and Stress State in the Ryukyu Islands Subduction Zone

Based on the newly compiled and mostly complete unified earthquake catalogue for China’s seas and adjacent areas, further information was obtained about the structural shape and dip angle of the Benioff zone in the Ryukyu Islands subduction zone during the different subduction stages. In addition, using the damped regional stress tensor inversion method, we were able to investigate the complex stress field characteristics and the dynamic significance of the shallow and intermediate earthquakes in the Ryukyu Islands subduction zone. The results show that the tectonic stress field of the Ryukyu Islands subduction zone was extensional along the subduction direction in the northern area of the Tokara Strait and was compressional along the subduction direction in the southern area of the Tokara Strait. The R value of the shallow stress field of the Okinawa Trough was low, and the σ3 was stable in the NNW direction with a small dip angle (>30°). The type of stress field in the shallow part of the Okinawa Trough transitioned from strike-slip type to normal fault type from north to south, reflecting the difference in the degree of development of the trough, and the southern segment of the trough began to transform into the expansion stage. The northeastern portion of the study area and southeast Taiwan constituted the high R value (0.68–0.87) region where the σ2 had tensile components. The stress state was biaxial tension–uniaxial compression, and the principal compressive stress was determined to be in the SEE direction with a large dip angle (>30°). The σ1 in northeast Taiwan exhibited a nearly vertical (>60°) plunge, while the σ2 and σ3 were nearly horizontal. The σ2 was thrust in the ENE–WSW direction, and the σ3 was extended in the NNW direction. Through this research, a greater understanding has been gained of the seismicity characteristics and shape of the Ryukyu Islands subduction zone. Supplementary research has also been completed on the focal mechanism solution and stress field of the Ryukyu Islands subduction zone. Finally, this research is important for earthquake hazard analysis and earthquake engineering safety evaluation in this area.

Discussion on Mesozoic Geochemical Characteristics and Genesis of Nadanhada Terrane

Through field geological survey and drilling exploration in Sanjiang Basin, combined with geochemical analysis, the tectonic thermal evolution history of intrusive rocks is studied. The results show that the Early Mesozoic siliceous rocks in Qianjin Depression of Sanjiang Basin are widely distributed. The siliceous rocks are of post diagenetic hydrothermal metasomatic origin, and the hydrocarbon source rock indicators are good, which is expected to be a new formation for oil and gas exploration in Sanjiang area. The time temperature thermal history simulation of fission track shows that there have been two rapid cooling events (late Early Cretaceous Late Cretaceous and Paleocene Eocene) in the Nadanhadaling area since the Mesozoic era, with cooling rates of 3.42~4.81 ℃/Ma and 1.43~1.83 ℃/Ma, respectively. The two rapid cooling processes were caused by the subduction of the Pacific plate. The rapid cooling during the Early Cretaceous to the Late Cretaceous was caused by the change of the subduction direction of the Pacific plate and the collision and superposition of the Okot Motsk block and the East Asian continental margin; The rapid cooling from Paleocene to Eocene was mainly caused by the subduction and retreat of the Pacific plate, which made the East Asian continental margin in an extensional environment and caused denudation and cooling in most parts of Northeast China.

40Ar–39Ar geochronology and palaeostress analysis using lamprophyre dikes and quartz veins in the Sizhuang gold deposit: new implications for Early Cretaceous stress regime in the Jiaodong Peninsula, North China Craton

AbstractLamprophyre dikes and quartz veins in the Sizhuang gold deposit are used to date and unravel the Early Cretaceous stress regime in the Jiaodong Peninsula, North China Craton. The lamprophyre dikes are grouped into two major sets, trending NNW–SSE and NNE–SSW, respectively, and a subsidiary one, trending WNW–ESE, whereas the quartz veins trend mainly NNE–SSW. The age of lamprophyre dikes’ intrusion was robustly calculated at c. 119 Ma by phlogopite 40Ar–39Ar dating. The fuzzy clustering technique defined four stress states, which were grouped into three stress regimes in Sizhuang. Furthermore, these stress regimes were interrelated with two regional far-field stress regimes, ST1 and ST2, defined by the available published fault-slip data near Sizhuang by applying the separation and stress inversion TR method (TRM). The palaeostress reconstruction indicates that extension and strike-slip tectonics (i.e. transtension) affect the emplacement of the lamprophyre dikes and quartz veins. More precisely, the dike emplacement occurred under unstable stress conditions related to the shift from E–W pure extension–transtension to WNW–ESE transtension, whereas the quartz veins formed under relatively stable WNW–ESE transtension with the variant and increasing fluid pressure ( ${P_{\rm{f}}})$ giving rise to the strike-slip against dip-slip kinematics along the faults. The change from the ST1 to ST2 stress regimes reflects the significant clockwise rotation in the palaeo-Pacific plate subduction direction and defines the switching time at c. 119 Ma as precisely obtained by the lamprophyre dating.

Cenozoic tectonic evolution of the Bohai Bay Basin: Constraints from strike-slip activities of the Wangjiagang fault zone, NE China

The Wangjiagang (WJG) fault zone is regarded as the northward propagation of the Tan-Lu fault zone (TLFZ) in the Bohai Bay Basin (BBB). We used three-dimensional seismic and borehole data, as well as structure and isopach maps, to describe the structural properties and tectonic development of the WJG fault zone. The properties and thickness of the mechanical layer are the principal factors of differences in the deformation of the strike-slip fault zone. Extensional tectonics and strike-slip faulting occurred in WJG. Not only does strike-slip faulting regulate sand body distribution, but it also plays a critical role in oil migration. WJG provides a record of the tectonic evolution of the BBB and how it responded to plate interactions around the Pacific-Eurasian-Indian plates and/or the strike-slip faulting of the TLFZ. The BBB extended in the early Paleocene as a result of relative relaxation between the Pacific and Eurasian plates and the upwelling of the asthenosphere mantle. Before and after the late Eocene (ca. 42 Ma), the tectonic stress regimes at a nearly N-S-oriented extensional to dextral strike-slip faulting along TLFZ coincided with the strike-slip faulting of the pre-existing fault in WJG. The Pacific Plate's change in direction and rate of subduction, as well as the far-field effect of a hard collision between the Indian and Eurasian plates, caused the TLFZ's two walls to move in opposite directions. The resulting force couple caused a strong simple shear in the TLFZ and adjacent areas, including the WJG fault zone.

Petrogenesis and Tectonic Setting of the Gold-Bearing Magmatic Rocks of the North-Western Melfi Massif, Central Chad

This paper presents petrographic and geochemical data on magmatic rocks of the north-western Melfi massif (part of the Gera Massif at the center of Chad) in the Adamawa-Yadé domain. The study area consists of syenite and granite, all crosscut by minor dykes and quartz veins. Syenite and granite show porphyritic texture and are dominated by K-feldspar and plagioclase phenocrysts, biotite, a variable proportion of quartz, and often few amphiboles. These rocks are characterized by low Mg#, Ni and Co, high K2O, Na2O, LREE/HREE ratio, I-type geochemistry as well as positive Eu anomaly, suggesting an origin from partial melting of crustal rocks (here garnet-amphibolite) and metasomatized mantle-derived component with the input of sediments. The compositional evolution from syenites to granite raises the fractional crystallization process in their petrogenesis and involves the fractionation of amphibole, biotite, plagioclase, apatite, magnetite, and ilmenite. Studied samples are affected by the post-magmatic hydrothermal alteration with an assemblage made of epidote, biotite, sericite, and opaques minerals. In addition, they are characterized by considerable gold concentration (Au = 0.6-3.8 ppm) suggesting that the north-western part of the Melfi massif in the center of Chad is a preferential site of gold mineralization. The K-rich calc-alkaline to shoshonitic geochemistry and the pronounced Nb, Ta, and Ti negative anomalies of these rocks, combined with their high Nb/Ta (7.55-32.10), La/Nb (1.80-11.73), and La/Ta (21.53-376.60) ratios but low Rb (57.60-138.30 ppm) indicate that they are subduction-related or arc-related magmas belonging to the Great Central Gondwana Arc (GCGA) formed further to east-directed subduction direction.

A synthesis of geochemistry of Mesozoic igneous rocks in NE China and tectonic superposition and transformation of the easternmost Central Asian Orogenic Belt

As the product of the amalgamation of multiple microcontinental massifs, Northeast China is located in the easternmost segment of the Central Asian Orogenic Belt (CAOB), which underwent multi-stage tectonic superposition in Mesozoic. Extensive and intensive igneous activities occurred throughout northeast China and the related igneous rocks can be considered as an effective probe for constraining tectonic evolution. In this study, numerous available ages and geochemical data are collected from published literature. The spatial-temporal distribution, rock assemblage, and geochemical characteristics of Mesozoic igneous rocks are systematically reassessed and summarized. A conclusion is drawn that NE China experienced the superposition and transformation of the Paleo-Asian Ocean (PAO) domain, the Mongol-Okhotsk Ocean (MOO) domain, and the Paleo-Pacific Ocean (PPO) domain in Mesozoic and detailed tectonic evolution is presented as follows. Firstly, NE China was under the joint effect of the MOO and the PAO during Early-Middle Triassic (ca. 252–237 Ma). Arc-type igneous rocks in the Erguna-Xing’an Block were induced by the southward subduction of the MOO Plate and adakites with high Sr/Y and (La/Yb)N ratios in the south segment of NE China were emplaced in the post-collision orogenic environment related to the closure of the PAO. Secondly, during Late Triassic, the influence of the PAO domain was gradually weakened, and igneous activities developed sporadically along the Solonker-Xar Moron-Changchun-Yanji suture zone. The MOO continued to subduct south until the Early Jurassic (ca. 220–190 Ma), and continental igneous rocks with arc-like features were widely distributed in the Erguna-Xing’an Block. During ca. 220–200 Ma, the Jiamusi-Khanka Massif was slipped away from the Songnen-Zhangguangcai Range Massif and this progress may be related to the PPO domain. Thirdly, in the Early Jurassic-Early Cretaceous (ca. 200–120 Ma), NE China came under the joint influence of the MOO and PPO tectonic domains. The subduction and closure of the MOO Plate and corresponding thickening of the lower crust resulted in widespread and intensive igneous activities in the Erguna and the Great Xing’an Range (GXAR). Two-stage subduction of the PPO and between which a period of change of subduction direction, induced the two-stage arc igneous activities of ca. 185–164 Ma and ca. 135–120 Ma, and the magmatic hiatus between ca. 163.5–135 Ma in the Zhangguangcai Range (ZGCR) and its east. Fourthly, the influence of the MOO domain was completed at ca. 120 Ma and numerous volcanic rocks corresponding chemically to A-type granite began to erupt violently, due to the rollback of the Paleo-Pacific oceanic slab. Since then, NE China has been completely under the control of the PPO domain.

Deep mass redistribution prior to the 2010 Mw 8.8 Maule (Chile) Earthquake revealed by GRACE satellite gravity

Subduction zones megathrust faults constitute a considerable hazard as they produce most of the world's largest earthquakes. However, the role in megathrust earthquake generation exerted by deeper subduction processes remains poorly understood. Here, we analyze the 2003 – 2014 space-time variations of the Earth's gravity gradients derived from three datasets of GRACE geoid models over a large region surrounding the rupture zone of the Mw 8.8 Maule earthquake. In all these datasets, our analysis reveals a large-amplitude gravity gradient signal, progressively increasing in the three months before the earthquake, North of the epicentral area. We show that such signals are equivalent to a 60km3 water storage decrease over 2 months and cannot be explained by hydrological sources nor artefacts, but rather find origin from mass redistributions within the solid Earth on the continental side of the subduction zone. These gravity gradient variations could be explained by an extensional deformation of the slab around 150-km depth along the Nazca Plate subduction direction, associated with large-scale fluid release. Furthermore, the lateral migration of the gravity signal towards the surface from a low coupling segment around −32.5∘ North to the high coupling one in the South suggests that the Mw 8.8 earthquake may have originated from the propagation up to the trench of this deeper slab deformation. Our results highlight the importance of observations of the Earth's time-varying gravity field from satellites in order to probe slow mass redistributions in-depth major plate boundaries and provide new information on dynamic processes in the subduction system, essential to better understand the seismic cycle as a whole.