Zones Of Lithospheric Weakness Research Articles

Influence of Farallon Slab Loading on Intraplate Stress and Seismicity in Eastern North America in the Presence of Pre‐Existing Weak Zones

AbstractDespite the stability of the continental interior, eastern North America has hosted many significant historical earthquakes. Seismicity concentrates within tectonically inherited structures, which can act as weak zones where stress accumulates. Within these zones, systematic stress rotations may be explained by long‐wavelength sources. We test the hypothesis that mantle‐flow driven by the Farallon slab contributes to intraplate seismicity via the reactivation of pre‐existing faults. We model the stress field using seismically constrained global high‐resolution finite‐element flow models with CitcomS. To isolate the slab's effect, we vary its buoyancy between a case of neutrality and a case with full negative thermal buoyancy derived from tomography. Low‐viscosity lithospheric weak zones located at failed rifts, loaded by a mass anomaly at depth, transmit elevated stresses to the overlying crust. The sinking of the Farallon slab drives localized mantle flow beneath the central‐eastern US, generating a large stress amplification of 100–150 MPa peaking over the New Madrid Seismic Zone (NMSZ). This stress amplification exerts a continent‐wide clockwise rotation on the stress field, which in the presence of weak zones reproduces some observed deviations of the seismically inferred SHmax from the regional borehole SHmax, bringing optimally oriented faults, closer to failure, some of which are associated with major historical earthquakes, including the Reelfoot Fault in the NMSZ and the Timiskaming Fault in Western Quebec. However, stronger lithospheric viscosity gradients, shallower weak zones, or weaker faults are still needed to fully reproduce the observed stress field in some areas.

Heng-o Corona, Venus: Dyke swarms record evolution of its underlying mantle plume

Coronae are enigmatic, quasi-circular/elliptical tectono-magmatic features, common across the surface of Venus ranging in size from 60 to 2600 km This study undertook a detailed mapping (1:500,000 scale) of Heng-o Corona, the second largest on Venus. 24,600 graben-fissure lineaments were mapped, interpreted to be underlain by mafic dykes, and grouped into radiating, circumferential and linear swarms. A series of 5 nearly-concentric rings were identified from unflooded or partially flooded circumferential swarms. The largest of these has a diameter of 1970 km, which almost doubles Heng-o's known size (previously 1060 km based on its topographic annulus). This new diameter also narrows the gap between it and Artemis Corona, the largest on Venus, to only 630 km. Three major radiating graben swarms were identified, extending >700–1000 km from their sources within the central annulus. From cross-cutting relationships, we interpreted that the major radiating swarms formed first, upon initial plume arrival and flattening against the base of the lithosphere. The circumferential swarms followed, starting with the innermost ring, migrating with the leading edge of the plume head as it spread outward against the base of the lithosphere. The distinctive annular ridge/trough system (at about 500 km radius) is associated with the transition between plume head arrival and spreading. The centers of most of the radiating and circumferential swarms are aligned along a NNW-SSE trend that is interpreted to represent an underlying zone of lithospheric weakness. Additional graben-fissure systems in the study area are not associated with Heng-o Corona; some are linked with other magmatic centers nearly up to 1000 km outside the study area.

Shallow Thermal Anomalies and Their Role in the Breakup Evolution Along the Conjugate Margins of the Fram Strait (Svalbard and Eastern North Greenland), Indicated by Low‐Temperature Thermochronology

AbstractWe investigated highly mature sedimentary rocks exposed along both sides of the Fram Strait in the northern North Atlantic using apatite fission track and (U‐Th)/He thermochronology to obtain information on the thermal imprint of rifting and continental breakup processes along a sheared margin. Our data showed that the conjugate margins experienced several heating episodes, which we explain as resulting from heat transfer along segments of the De Geer Fracture Zone, a large continental transform system which connected magmatic centers north and south of the Fram Strait. Heating occurred prior to and during the Eurekan intraplate orogeny, which occupied the position of the present‐day Fram Strait during the Eocene. Heat transfer may have caused or contributed to lithospheric weak zones, which focussed deformation during intraplate orogeny. Movements along the transform fault system continued during the Oligocene, after the end of the Eurekan Orogeny, causing further structural weakening of pre‐existing fault zones. These were exploited during the final continental breakup leading to the opening of the Fram Strait. No unambiguous thermal signature associated with this latest stage of breakup was detected. Our data underline recent studies on the importance of structural inheritance and continental transform faults for the prolonged and complex processes of continental rifting and breakup.

Long‐Lived Aftershocks in the New Madrid seismic Zone and the Rest of Stable North America

AbstractSeismicity in the New Madrid seismic zone and the rest of stable North America remains controversial. Some workers view present‐day earthquakes there as long‐lived aftershocks; others view them as background seismicity concentrated in lithospheric weak zones. Separating long‐lived aftershocks from background seismicity, however, is challenging. Here, we used the nearest‐neighbor (NN) method to identify long‐lived aftershocks. The NN method calculates the distances between pairs of earthquakes in a space‐time‐magnitude domain. If the distances are too close to be expected for independent background events that follow the Gutenberg‐Richter law and Poisson distribution, the pairs are taken as clustered events (i.e., the later one in a pair is an aftershock of the earlier one). Our results suggest that, depending on the size and location of the 1811–1812 New Madrid mainshocks, 10.7%–65.0% of the M ≥ 2.5 earthquakes in the New Marid region between 1980 and 2016 may be long‐lived aftershocks. Similarly, considerable present‐day earthquakes in South Carolina are long‐lived aftershocks of the 1886 Charleston earthquake, but the contemporary seismicity in Charlevoix, Québec is predominately background seismicity. These results suggest that present‐day seismicity in stable North America, and perhaps other stable continents, includes both background seismicity and long‐lived aftershocks. Distinguishing aftershocks from background seismicity can be important for hazard assessment.

Pangaea's breakup: the roles of mantle plumes, orogens and subduction retreat

Abstract What drives the breakup of a supercontinent remains contentious. Previously proposed mechanisms include mantle plumes, subduction retreat and basal traction from mantle convection. Here we review the geological record of plumes, orogens and subduction zones during the breakup of Pangaea and investigate the potential roles played by these factors through 4D spherical geodynamic modelling. We found that mantle plumes provided the dominant force that drove the breakup of Pangaea, particularly in triggering the initial breakup. Young orogens as continental lithospheric weak zones generally guided the development of continental rifts, consistent with the geological record that rifting within Pangaea commonly developed along pre-existing orogens. However, the marginal drag force produced by subduction retreat, and basal traction associated with subduction-related mantle flow, likely also played a role in the breakup of Pangaea. In addition, the weakening effect of plume-induced melts can sometimes help to break the continental lithosphere away from orogens, as exemplified by the breakup between Antarctica and Australia. Furthermore, geodynamic modelling suggests that subduction is responsible for generating mantle plumes. A particular such example is the formation of the Kerguelen plume, triggered by subduction along the northern margin of Australia, which facilitated the breakup between East Antarctica and Australia.

Linkage between the India–Asia collision and far-field reactivation of the Altai mountains

The present northwest-trending Altai mountains, as far as ∼2000 km north of the Indian–Asian collision front, are dominated by a transpressional strike-slip fault system, and are generally considered as one of distant tectonic effects of the ongoing India–Asia collision. However, the Cenozoic reactivation of the Altai and its linkage with the tectonic evolution of the Tibet plateau remain mechanically challenging. Here, we constructed large-scale 2D numerical models with inherited lithospheric heterogeneities from India to Altai representing the onset of the India–Asia collision. The sensitivity of model results to variable thermo-mechanical and geometric properties of the orogen-like Tibet and Altai as well as variable thermal state of the craton-like India and Junggar reveals that both the particular lithospheric architecture of western Tibet and basement structural inheritance of central Asia regulate the long-distance deformation propagation from the India–Asia plate boundary to the Altai–Sayan region. The model results indicate that the direct collision of the underthrusting Indian lithosphere with the rigid Tarim block beneath western Tibet plays a dominant role in the ultra-long-distance deformation propagation to central Asia. As less-deformed rigid blocks, the Tarim and Junggar basins with Precambrian basements serve as secondary indenters that transfer the compressive stresses extensively to the Altai–Sayan region. Constrained by substantial surface geology and geophysical observations, this numerical study recognizes the linkage between the evolving lithospheric structure of western Tibet with the northward younging trend of reactivation of the orogenic belts in central Asia. By means of the rigid Indian–Tarim lithospheric mantle collision, the plate-convergence stress can be effectively transmitted through Tarim to Tian Shan and subsequently through Junggar to Altai, leading to successive reactivation of the Tian Shan and Altai mountains. The locations of reactivation are determined by the inherited structural zones of lithospheric weakness that formed during the Paleozoic assembly of the Central Asian Orogenic Belt.

Compression at Strike‐Slip Fault Is a Favorable Condition for Subduction Initiation

AbstractThe recent statistics suggests that over 60% of active Cenozoic subduction initiation (SI) cases are related to the strike‐slip fault. A number of previous studies have shown that the lithospheric weak zone is a necessary condition for the SI. However, the direct effect of strike‐slip motion on lithospheric weakening and SI has rarely been investigated in numerical models due to the challenge of complex 3D boundary conditions. In this study, a new 3D model has been built with both strike‐slip and compression boundary conditions. The model results indicate that the compression at a strike‐slip boundary provides a favorable condition for the SI, with producing and maintaining a lithospheric‐scale weak zone that facilitates strain localization and SI. In addition, the high strike‐slip velocity and buoyant overriding plate contributes to the SI of young oceanic plate. This mechanism satisfies a large number of natural SI cases in the Cenozoic.

Spatiotemporal distribution of seamount volume along the Kyushu-Palau Ridge: Implications for rejuvenated volcanism

• Accurate quantification of seamount volcanism reveals multiphase volcanism along the Kyushu–Palau Ridge. • The proximity of Pre-Oligocene lithospheric weak zones to high concentrations of seamount volume indicate tectonic inheritance facilitated magma migration. • Rejuvenated volcanism related to the propagated back-arc spreading during Oligocene contributed to the development of the Kyushu-Palau Ridge. The Kyushu-Palau Ridge (KPR) is a remnant arc that initially formed with the subduction of the Pacific plate beneath the West Philippine Sea. The KPR split from the proto-Izu-Bonin-Mariana (IBM) arc with the back-arc spreading at ∼ 30 Ma. Even though most of the seamounts have been dated, the factors that drive the volcanic arc evolution along the KPR remain unknown. In this study, we quantify the spatiotemporal distribution of the seamount volume along the KPR, representing a quantitative indicator of erupted volcanism. A spatial filtering method based on the White Top-Hat Transform is used to identify seamounts from the bathymetry. Combining seismic data interpretation and gravity data inversion, we estimate the seamount volume distribution along the KPR. Results show that high concentrations of the seamount volume are in proximity to pre-Oligocene lithospheric weak zones, indicating that tectonic inheritance has facilitated magma migration during the early-stage volcanic arc generation. Subsequently, the younger than 30 Ma age of the majority of the seamounts suggests a late-stage rejuvenated volcanism in the Oligocene. In addition, the spatiotemporal variations of the seamount volume in the northern and central segments are synchronous with the propagated spreading of the Perce Vela and Shikoku basins, further indicating that the rejuvenated volcanism was driven by the propagated back-arc spreading. Rejuvenated volcanism can thus be an important contributive factor to remnant volcanic arc formation.

Fingerprinting secondary mantle plumes

Many vertical seismic velocity anomalies observed below different parts of the Eurasian plate are rooted in the transition zone between the upper and lower mantle (410–660 km), forming so-called secondary plumes. These anomalies are interpreted as the result of thermal effects of large-scale thermal upwelling (primary plume) in the lower mantle or deep dehydration of fluid-rich subducting oceanic plates. We present the results of thermo-mechanical numerical modelling to investigate the dynamics of such small-scale thermal and chemical (hydrous) anomalies rising from the lower part of the Earth's upper mantle. Our objective is to determine the conditions that allow thermo-chemical secondary plumes of moderate size (initial radius of 50 km) to penetrate the continental lithosphere, as often detected in seismo-tomographic studies. To this end, we examine the effect of the following parameters: (1) the compositional deficit of the plume density due to the presence of water and hydrous silicate melts, (2) the width of the weak zone in the overlying lithosphere formed because of plume-induced magmatic weakening and/or previous tectonic events, and (3) a tectonic regime varied from neutral to extensional. In our models, secondary plumes of purely thermal origin do not penetrate the overlying plate, but flatten at its base, forming “mushroom”-shaped structures at the level of the lithosphere-asthenosphere boundary. On the contrary, plumes with enhanced density contrast due to a chemical (hydrous) component are shown to be able to pass upwards through the lithospheric mantle to shallow depths near the Moho when (1) the compositional density contrast is ≥ 100 kg m−3 and (2) the width of the lithospheric weakness zone above the plume is ≥ 100 km. An extensional tectonic regime facilitates plume penetration into the lithosphere but is not mandatory. Our findings can explain observations that have long remained enigmatic, such as the “arrow”-shaped zone of low seismic velocities below the Tengchong volcano in south-western China and the columnar (“finger”-shaped) anomaly within the lithospheric mantle discovered more than two decades ago beneath the Eifel volcanic fields in north-western Germany. It appears that a chemical component is a characteristic feature not only of conventional hydrous plumes located over presently downgoing oceanic slabs, but also of upper mantle plumes in other tectonic settings.

Ancient structural inheritance explains gold deposit clustering in northern Perú

Abstract The punctuated distribution of major gold deposits along orogenic belts is poorly understood. In northern Perú, gold deposits cluster along trends that transect the subparallel Phanerozoic belts that accreted onto the western margin of the Archean–Proterozoic Amazonian craton. This suggests the influence of as yet unrecognized pre-Andean basement structures in controlling the localization of Phanerozoic deposits. We report the newly discovered Tomac ophiolite, which represents a missing link in tracing a cryptic basement collisional suture striking obliquely to the Peruvian Andes. The Tomac ophiolite signals the opening of a former Neoproterozoic ocean basin between the Amazonian craton and a continental ribbon of dispersed microterranes. Re-accretion of the continental ribbon resulted in the formation of a collisional belt between ca. 465 and 440 Ma. Rifting and re-accretion occurred parallel to the strike of the ca. 1400–1000 Ma Grenvillian-Sunsás orogen, indicating that structural corridors that transect the Andes at an oblique strike angle reflect inheritance of preexisting lithospheric weak zones. Our study demonstrates that Phanerozoic gold deposits in northern Perú cluster at the intersection between the newly defined basement suture and the superimposed Andean belt and associated well-documented cross faults. Gold mineralization was localized at ancient structural intersections, where enhanced permeability facilitated anomalous energy and mass transfer, promoting major ore accumulation during episodes of accretion.

Ongoing Lithospheric Alteration of the North China Craton Revealed by Surface‐Wave Tomography and Geodetic Observations

AbstractThe North China Craton (NCC) is renowned for cratonic reactivation and dramatic destruction. Detailed lithospheric structure is crucial for understanding the geological evolution and seismogenesis of the NCC. We imaged the lithospheric structure of the central and eastern NCC with ambient noise and teleseismic surface wave tomography. Our new model unveils remarkable N‐S variation in the lithospheric structure. An E‐W elongated lithospheric weak zone, coinciding with contemporary left‐lateral shearing revealed by geodetic observation, occupies the northern central and eastern NCC. Our model suggests that radical destruction of the eastern NCC in the late Mesozoic facilitated the late‐Cenozoic shearing induced by the continuous India‐Eurasia convergence. The late‐Cenozoic shearing progressively altered the pre‐thinned lithosphere accompanied by asthenospheric upwelling and strong seismic activity. Our results suggest a transition from the Pacific subduction dominating tectonic regime to the India‐Asia collision dominating lithospheric deformation framework in the Cenozoic and ongoing lithospheric alteration of the NCC.

Probing the Southern African Lithosphere With Magnetotellurics: 2. Linking Electrical Conductivity, Composition, and Tectonomagmatic Evolution

AbstractThe tectonic history of southern Africa includes Archean craton formation, multiple episodes of subduction and rifting and some of the world's most significant magmatic events. These processes left behind a compositional trail that can be observed in xenoliths and measured by geophysical methods. The abundance of kimberlites in southern Africa makes it an ideal place to test and calibrate mantle geophysical interpretations that can then be applied to less well‐constrained regions. Magnetotellurics (MT) is a particularly useful tool for understanding tectonic history because electrical conductivity is sensitive to temperature, bulk composition, accessory minerals, and rock fabric. We produced three‐dimensional MT models of the southern African mantle taken from the SAMTEX MT data set, mapped the properties of ∼36,000 garnet xenocrysts from Group I kimberlites, and compared the results. We found that depleted regions of the mantle are uniformly associated with high electrical resistivities. The conductivity of fertile regions is more complex and depends on the specific tectonic and metasomatic history of the region, including the compositions of metasomatic fluids or melts and the emplacement of metasomatic minerals. The mantle beneath the ∼2.05 Ga Bushveld Complex is highly conductive, probably caused by magmas flowing along a lithospheric weakness zone and precipitating interconnected, conductive accessory minerals such as graphite and sulfides. Kimberlites tend to be emplaced near the edges of the cratons where the mantle below 100 km depth is not highly resistive. Kimberlites avoid strong mantle conductors, suggesting a systematic relationship between their emplacement and mantle composition.

Plume-Induced Subduction Initiation: Revisiting Models and Observations

Subduction initiation induced by a hot and buoyant mantle plume head is unique among proposed subduction initiation mechanisms because it does not require pre-existing weak zones or other forces for lithospheric collapse. Since recognition of the first evidence of subduction nucleation induced by a mantle plume in the Late Cretaceous Caribbean realm, the number of studies focusing on other natural examples has grown. Here, we review numerical and physical modeling and geological-geochemical studies which have been carried out thus far to investigate onset of a new subduction zone caused by impingement of a mantle plume head. As geological-geochemical data suggests that plume-lithosphere interactions have long been important - spanning from the Archean to the present - modeling studies provide valuable information on the spatial and temporal variations in lithospheric deformation induced by these interactions. Numerical and physical modeling studies, ranging from regional to global scales, illustrate the key role of plume buoyancy, lithospheric strength and magmatic weakening above the plume head on plume-lithosphere interactions. Lithospheric/crustal heterogeneities, pre-existing lithospheric weak zones and external compressional/extensional forces may also change the deformation regime caused by plume-lithosphere interaction.

Connection Between a Subcontinental Plume and the Mid‐Lithospheric Discontinuity Leads to Fast and Intense Craton Lithospheric Thinning

AbstractRemoval and thinning of cratonic lithosphere is believed to have occurred under different tectonic settings, for example, near subduction zones and above mantle plumes. Subduction‐induced cratonic modification has been widely discussed; however, the mechanisms and dynamic processes of plume‐induced lithospheric removal remain elusive and require further systematic investigation. In this study, we use a series of 2‐D thermo‐mechanical models to explore the dynamics of the removal and thinning of cratonic lithosphere due to the interaction between a mantle plume and a weak mid‐lithosphere discontinuity (MLD) layer. Our modeling results suggest that the interaction between a mantle plume and weak MLD layer can lead to a large‐scale removal of the cratonic lithosphere once a connection between the hot upwelling and weak MLD layer is established. The presence of a vertical lithospheric weak zone and its closeness to the plume center play critical roles in creating a connection between the weak MLD and hot plume/asthenosphere. Furthermore, delamination of cratonic lithosphere is favored by a larger plume radius/volume, a higher plume temperature anomaly, and a lower viscosity of the MLD layer. A systematic comparison between subduction‐induced and plume‐induced lithospheric thinning patterns was also conducted. We summarize their significant differences on the origin and migration of melt generation, the water content in melts, and topographic evolution. The combination of numerical models and geological/geophysical observations indicates that mantle plume‐MLD interactions may have played a crucial role in lithospheric removal beneath the South Indian, South American, and North Siberian Cratons.

Lithosphere structure and its implications for the metallogenesis of the Nanling Range, South China: Constraints from 3-D magnetotelluric imaging

We have obtained a lithospheric electrical resistivity model of the Nanling Range through 3-D inversion of magnetotelluric data from 118 sites comprising an array covering this important metallogenic belt. Approximately divided by longitude 114°E, the western and eastern parts of this area show significantly different lithospheric electrical features, spatially coincident with its west-east differential Jurassic metallogeny. The lithosphere beneath western Nanling is mainly characterized by high-resistivity, but with an approximately north–south trending, lithosphere-scale, nearly upright, high-conductivity zone that occurs beneath the Qin-Hang magmatic belt. We interpret this high-conductively zone to be a long-lived lithospheric weak zone being located on an ancient plate-boundary that experienced multiple reactivations. The zone controlled Jurassic mafic magmatic events, e.g., 174–176 Ma Daoxian and 150–154 Ma Ningyuan, and also acted as a narrow pathway for the rapid upwelling of hot and volatile-rich asthenosphere-derived materials during the Middle-Late Jurassic Cu-Mo/Pb-Zn mineralization. In contrast, to the east of longitude 114° E, a nearly 80 km deep, clear and continuous electrical lithosphere-asthenosphere boundary is imaged, which could result from a major lithospheric delamination event, probably caused by the early Paleozoic Wuyi-Yunkai orogeny and subsequent post-orogenic collapse, or the subduction and rollback of the Pacific slab after the Triassic. The eastern Nanling region with its thinned lithosphere is suggested as allowing broader zones of upwelling asthenospheric during the intensive Jurassic W-Sn metallogenesis, causing slow upwelling and lateral spreading of asthenosphere-derived materials and subsequent strong extension of the crust and also the mixing and assimilation with melts. Consequently, metallic elements were extensively extracted from the crust, eventually giving rise to the world-class W-Sn deposits in the eastern Nanling region.

Pre-existing lithospheric weak zone and its impact on continental rifting – The Mid-Polish Trough, Central European Basin System

The large portion of Central and NW Europe was subjected to widespread continental rifting since the Carboniferous-Permian transition. This process led to nucleation of several large depocentres, coalescing from the late Permian onwards into the extensive Central European Basin System (CEBS), that grew and evolved until the regional-scale Late Cretaceous inversion. Development of deep grabens from Germany to the Netherlands and the North Sea was connected with volcanism and crustal thinning. The exception makes the Mid-Polish Trough (MPT), trending NW-SE across Poland along the Teisseyre-Tornquist Zone (TTZ) at the margin of the East European Craton (EEC). The MPT is underlain by relatively thick lithosphere and its formation was not associated with volcanic activity in the direct neighbourhood. To investigate this anomaly, we compiled grids for key horizons defining the crustal architecture, separately for NW European and Polish sectors of the CEBS. Seismic reflection and well data were used to build the base Permian grid, whereas deep seismic refraction and reflection data along with constrained gravity inversion were employed to construct the top basement and Moho grids. Based on a dense net of seismic refraction profiles, the additional top Ediacaran grid was created solely for the Polish part of the CEBS. Our data show distinct crystalline crust thinning SW of the TTZ (β factor = 3.68) expressed in both shallowing of Moho and deepening of the crystalline basement. We interpret this feature as a fossilised crustal neck inherited after an Ediacaran rifting preceding the break-up of Rodinia. The early Permian rifting moderately contributed to the crustal thinning (β factor = 1.4), overprinting the primary rifted margin geometry. The 8 km deep MPT developed in front of a pre-existing crustal neck that obstructed further propagation of extensional deformation. Lithospheric mantle beneath a rift axis was probably too strong to accommodate extension but the thinner lithosphere farther west was weak enough to be stretched. Therefore, the location of the pre-existing weakness zone relative to a change in lithosphere thickness may localise a rift graben and shift the magmatism away from the surface expression of the rift. Thus, our study demonstrates that fossil crustal architecture was a key factor controlling the amount, distribution, and style of continental rifting.

An immediate response to the Indian-Eurasian collision along the northeastern Tibetan Plateau: Evidence from apatite fission track analysis in the Kuantan Shan-Hei Shan

The Tibetan Plateau offers an ideal region for understanding the processes of continental collision and related intracontinental deformation. However, when the Cenozoic deformation was initiated in the present northeastern margin of the plateau, >2000 km north to the Indian-Eurasian collision front, remains an issue in debate. Two competing end-member models have been proposed, involving immediate deformation response and progressive deformation propagation. Solving this problem partly relies on determining whether there was contemporaneous deformation with initial Indian-Eurasian collision along the northeastern margin of the Tibetan Plateau. In this study, we employ apatite fission track (AFT) analysis to shed new light on the exhumation history of the Kuantan Shan-Hei Shan along the northeastern margin of the Tibetan Plateau, with a motivation to determine whether deformation-related exhumation during initial collision stage occurred in the region. The 22 granite samples produce AFT ages ranging from 22.3 ± 2.6 Ma to 175 ± 18 Ma and mean track lengths from 11.17 ± 2.26 μm to 13.63 ± 1.93 μm. Thermal history modeling indicates five episodes of rapid exhumation-related cooling, which took place in the early Jurassic, early Cretaceous and late Cretaceous, during/since the Eocene and since the middle Miocene. The exhumation events prior to the Cenozoic are attributed to far-field responses of successive closure of fossil oceans and assembly of blocks along the southern margin of the Eurasian continent. The Eocene exhumation along the Kuantan Shan-Hei Shan is speculated to represent an immediate response to the initial Indian-Eurasian collision. These results lend support to the immediate deformation response model along the northeastern plateau. The episodic exhumation observed in this study, combined with previous geophysical findings, implies that the Kuantan Shan-Hei Shan represents the southern margin of the Alashan Block, serving as a significant and long-lived lithospheric weak zone susceptible to intracontinental deformation during episodes of far-field tectonic event.

Emplacement age of the Tshibwe kimberlite, Democratic Republic of Congo, by in-situ LAM-ICPMS U/Pb dating of groundmass perovskite

Kimberlitic perovskites have been investigated in order to constrain the emplacement age of the Tshibwe kimberlite. U–Pb isotopic analyses of the perovskite grains from the Tshibwe kimberlite samples yielded an age of 85 ± 10 Ma, which is in agreement with stratigraphic and other circumstantial geological evidence. This Late Cretaceous age coincides with a sharp decrease of spreading rates at the southern Mid-Atlantic Ridge at about 84 Ma. This probably coincided with a change in the direction and velocity of African plate motion. Previous studies have proposed that such changes may provide a trigger for kimberlite magmatism. The Kasai craton appears to have been affected by two main pulses of kimberlite magmatism, an earlier pulse at 130-113 Ma resulting in kimberlites of the NE-SW-oriented Lucapa Corridor in Angola, and a later pulse of roughly 85-70 Ma responsible for the Tshibwe, Mbuji Mayi and possibly the kimberlites of the Kabinda field in the DRC, all of which are in clusters oriented roughly E-W. Mbuji-Mayi lithospheric mantle appears to be warm as the one beneath Kabinda kimberlites, whereas Tshibwe appears to have a cool lithospheric mantle. We suggest that the Tshibwe kimberlite was emplaced before a regional lithospheric heating event occurred that affected the mantle beneath Mbuji Mayi and (most likely) Kasendou. Therefore, the most consistent interpretation is that Tshibwe is the oldest of the three and all three kimberlite clusters are likely of Late Cretaceous age. Their apparently similar age and orientation suggests that they were emplaced along an east-west oriented zone of lithospheric weakness in the northeastern Kasai craton, tentatively termed the “Kasai corridor”.

Geophysical reassessment of the Azimuth 125° Lineament: emplacement model and propagation of its dikes

This study analyzes the area along the Azimuth 125° Lineament (Az 125°) supposed to occur from Rondonia to Rio de Janeiro, in Brazil. The lineament is an important structural feature in association with great carbonatite and kimberlite complexes in Brazil (e.g., Goias and Alto Paranaiba Alkaline Provinces), consisting of extensive NW-SE oriented faults. The origin of the Az 125° Lineament and its dikes emplacement are widely discussed. The main intention of this work is to propose a model of emplacement of the Az 125° dikes. Also, some important magnetic characteristics of the lineament are presented. The magnetic data clearly define the features of the Az 125° in the southern portion of the study area, but we found no magnetic geophysical evidence that this lineament occurs from the Mato Grosso state to the Rondonia state. The dikes that compose the great Az 125° feature have distinct magnetic attributes, as their polarities (normal and reversed). It is suggested that the alternating polarity is due to local re-magnetization sometime after their intrusion. It is proposed that the dikes were positioned along a zone of lithospheric weakness and their propagation was facilitated due to the occurrence of mid-crustal axial feeder chambers.

Role of strain-dependent weakening memory on the style of mantle convection and plate boundary stability

How plate tectonic surface motions relate to the convecting mantle remains one of the major problems in geosciences. In particular, the cause and consequence of strain localization at plate boundaries remains debated, even though strain memory, i.e. the ability to preserve and reactivate tectonic inheritance over geological time, appears to be a critical feature in plate tectonics. Here, we analyze how a parameterized damage weakening rheology, strain-dependent weakening, affects the time-dependence of plate boundary formation, the transition between mobile and stagnant-lid, and the reorganization of plates in 2-D convection models. The strain-dependent weakening within our models allows for a self-consistent formation and preservation of lithospheric weak zones, which are formed as remnants of subduction zones due to large-scale compressional deformation in the trench region. Such inherited weak zones can be reactivated as intra-plate subduction zones, ridge adjacent subduction, or as spreading centers themselves. Due to the weakening along plate boundaries, the inherited weak zones, and partly the accumulated strain along spreading centers, which weakens the shallow parts of the lithosphere, the longevity of mobile-lid convection increases. Strain-dependent weakening also enhances strain localization along convergent plate boundaries which increases their stability and longevity. As a consequence, tectonic inheritance is an important contribution to understanding the time-dependence of plate reorganization. Strain-dependent weakening results in a shift of the mobile-stagnant lid transition to higher effective yield stresses, if the weak zones fully penetrate the lithosphere and are relatively weakened by at least 20 %.