Crustal Heterogeneity Research Articles

Subduction Initiation by Plume‐Plateau Interaction: Insights From Numerical Models

AbstractIt has recently been demonstrated that the interaction of a mantle plume with sufficiently old oceanic lithosphere can initiate subduction. However, the existence of large lithospheric heterogeneities, such as a buoyant plateau, in proximity to a rising plume head may potentially hinder the formation of a new subduction zone. Here, we investigate this scenario by means of 3‐D numerical thermomechanical modeling. We explore how plume‐lithosphere interaction is affected by lithospheric age, relative location of plume head and plateau border, and the strength of the oceanic crust. Our numerical experiments suggest four different geodynamic regimes: (a) oceanic trench formation, (b) circular oceanic‐plateau trench formation, (c) plateau trench formation, and (d) no trench formation. We show that regardless of the age and crustal strength of the oceanic lithosphere, subduction can initiate when the plume head is either below the plateau border or at a distance less than the plume radius from the plateau edge. Crustal heterogeneity facilitates subduction initiation of old oceanic lithosphere. High crustal strength hampers the formation of a new subduction zone when the plume head is located below a young lithosphere containing a thick and strong plateau. We suggest that plume‐plateau interaction in the western margin of the Caribbean could have resulted in subduction initiation when the plume head impinged onto the oceanic lithosphere close to the border between plateau and oceanic crust.

The 3D thermal field across the Alpine orogen and its forelands and the relation to seismicity

Abstract Temperature exerts a first order control on rock strength, principally via thermally activated creep deformation and on the distribution at depth of the brittle-ductile transition zone. The latter can be regarded as the lower bound to the seismogenic zone, thereby controlling the spatial distribution of seismicity within a lithospheric plate. As such, models of the crustal thermal field are important to understand the localisation of seismicity. Here we relate results from 3D simulations of the steady state thermal field of the Alpine orogen and its forelands to the distribution of seismicity in this seismically active area of Central Europe. The model takes into account how the crustal heterogeneity of the region effects thermal properties and is validated with a dataset of wellbore temperatures. We find that the Adriatic crust appears more mafic, through its radiogenic heat values (1.30E-06 W/m3) and maximum temperature of seismicity (600 °C), than the European crust (1.3–2.6E-06 W/m3 and 450 °C). We also show that at depths of

Constraints of Crustal Heterogeneity and Q(f) from Regional (<4 Hz) Wave Propagation for the 2009 North Korea Nuclear Test

Constraints of Crustal Heterogeneity and Q(f) from Regional (<4 Hz) Wave Propagation for the 2009 North Korea Nuclear Test

The Prevalence and Significance of Offset Magma Reservoirs at Arc Volcanoes

AbstractDetermining the spatial relations between volcanic edifices and their underlying magma storage zones is fundamental for characterizing long‐term evolution and short‐term unrest. We compile centroid locations of upper crustal magma reservoirs at 56 arc volcanoes inferred from seismic, magnetotelluric, and geodetic studies. We show that magma reservoirs are often horizontally offset from their associated volcanic edifices by multiple kilometers, and the degree of offset broadly scales with reservoir depth. Approximately 20% of inferred magma reservoir centroids occur outside of the overlying volcano's mean radius. Furthermore, reservoir offset is inversely correlated with edifice size. Taking edifice volume as a proxy for long‐term magmatic flux, we suggest that high flux or prolonged magmatism leads to more centralized magma storage beneath arc volcanoes by overprinting upper crustal heterogeneities that would otherwise affect magma ascent. Edifice volumes therefore reflect the spatial distribution of underlying magma storage, which could help guide monitoring strategies at volcanoes.

Bore-hole Well-cores Provide Evidence for Crustal Heterogeneity Beneath the Koyna-Warna Reservoir Site, Deccan Volcanic Province, India

Abstract Extensive geophysical investigations are ongoing to address the causative mechanisms, which could explain the sustained seismicity in the Koyna-Warna region of the Deccan Volcanic Province (DVP), western India. Towards this effort, several exploratory bore-holes ranging up to 1500 m in depth were drilled by CSIR-NGRI around the Koyna-Warna seismic zone for investigation of shallow crustal properties and long term monitoring of seismicity. The focus of this study is the examination of lithology from four bore-well-cores around the Warna area: from west to east, KBH05 (Phansavale), KBH09 (Khadi Kolvan), KBH10 (Udgiri) and KBH08 (Paneri). High-resolution lithologs of these bore-well-cores have been generated, which reveal subtrappean heterogeneity, especially given the limited areal extent of about three hundred square kilometers enclosing them. In this contribution, an account of lithological characteristics is provided, which were not available in the literature prior to this study. The discovery of relict greenstone volcanic rocks concealed beneath the Deccan Volcanic Province is reported, from one of the bore-well-cores as well as a thin layer of sediments in another. Fossil imprinted ash beds of substantial thickness are observed in two of the well-cores. Intrusive sheets of aplite/granite within the gneisses, seen in one of the well-cores, provides compelling evidence of a reworked basement beneath the Deccan Volcanic Province. The heterogeneity reported here is consistent with patterns of selective locales of earthquake occurrences in the Koyna-Warna zone; definitive roles for reservoir triggered seismicity needs further examination.

The role of crustal models in the dynamics of the India–Eurasia collision zone

SUMMARY We investigate how different crustal models can affect the stress field, velocities and associated deformation in the India–Eurasia collision zone. We calculate deviatoric stresses, which act as deformation indicators, from topographic load distribution and crustal heterogeneities coupled with density driven mantle convection constrained by tomography models. We use three different crustal models, CRUST2.0, CRUST1.0 and LITHO1.0 and observe that these models have different crustal thickness and densities. As a result, gravitational potential energy (GPE) calculated based on these densities and crustal thicknesses differ between these models and so do the associated deviatoric stresses. For GPE only models, LITHO1.0 provides a better constraint on deformation as it yields the least misfit (both orientation and relative magnitude) with the surface observations of strain rates, lithospheric stress, plate motions and earthquake moment tensors. However, when the stresses from GPE are added to those associated with mantle tractions arising from density-driven mantle convection, the coupled models in all cases provide a better fit to surface observations. The N–S tensional stresses predicted by CRUST2.0 in this area get reduced significantly due to addition of large N–S compressional stresses predicted by the tomography models S40RTS and SAW642AN leading to an overall strike-slip regime. On the other hand, the hybrid models, SINGH_S40RTS and SINGH_SAW that are obtained by embedding a regional P-wave model, Singh et al., in global models of S40RTS and SAW642AN, predict much lower compression within this area. These hybrid models provide a better constraint on surface observations when coupled with CRUST1.0 in central Tibet, whereas the combined LITHO1.0 plus mantle traction model provides a better fit in some other areas, but with a degradation of fit in central Tibet.

The role of inherited crustal and lithospheric architecture during the evolution of the Red Sea: Insights from three dimensional analogue experiments

The Red Sea represents an incipient ocean basin that is in transition from continental rifting to ocean initiation. In this paper we assess how pre-existing mantle and crustal heterogeneities within the African and Arabian lithospheres influenced rift development and propagation using three-dimensional (3D) brittle-ductile analogue laboratory experiments with a rotational boundary condition. Here we investigate the effect of three different configurations: (i) pre-existing rheological heterogeneities in the crust, (ii) in the lithospheric mantle, and (iii) in both. The experimental outcomes show that a linear weak zone in the lithospheric mantle layer exerts a first-order control on the overall mechanical behaviour of extending lithosphere, rift architecture and style of rifting. Crustal heterogeneities control second-order features, such as the trends of the main boundary escarpments along the rift axis. Our experiments reproduce the geological history of the Red Sea and provide key insights on its deformation history in space and time. We provide, for the first time, 3D dynamic constraints on how pre-existing weak zones in the lithosphere have controlled the temporal evolution of the Red Sea and determined its distinctive present-day morphology. The results find that certain morphological elements of the Red Sea can only be explained by the combined effects of crustal and mantle lithosphere heterogeneities. When applied to other natural examples, our findings provide significant new insights on the role of crustal and lithospheric heterogeneities during continental rifting.

Magnetotelluric study of an intraplate seismic zone in the Palghar region of the Deccan Volcanic Province, India

The Deccan Volcanic Province in the western part of the peninsular India consists of a thick pile of flood basalts resting mainly on the Archaean and Proterozoic rocks forming the basement. This intraplate region experiences moderate seismic activity, the most recent one being a swarm-type activity in the Palghar region, about 120 km north of Mumbai, that started in November 2018 and has produced a few thousand micro-earthquakes and a 4.3 magnitude earthquake since then. We have carried out a magnetotelluric (MT) study along a 35-km long profile across the seismic zone to delineate the subsurface structure to understand the possible cause for the seismic activity. Broadband MT data were acquired at 18 sites with average station spacing of 2 km. Impedance tensors were analyzed for distortion and dimensionality, decomposed into TE- and TM-mode, and inverted by a 2D inversion algorithm. The geoelectric structure yields an assemblage of highly resistive and moderately conductive blocks in the uppermost crust resting on a major listric-type fault, that possibly reaches the surface at the West Coast Fault from a depth of about 15 km beneath the Panvel flexure zone. In conjunction with the regional Bouguer gravity anomaly and the seismicity distribution, we infer that the upper crustal heterogeneities coupled with the basement fault and low rheological strength of the fractured upper-to-middle crust might be leading to triggering of the seismicity in the region.

A Comprehensive Analysis of Attenuation Characteristics Using Strong Ground Motion Records for the Central Seismic Gap Himalayan Region, India

ABSTRACT This study aims to estimate attenuation characteristics of the central Himalayan region of India concerning various strong-motion parameters such as Kappa value (κ) and site effects. We have tried to elaborate on the regional structural heterogeneities and their implications towards the seismic hazard assessment of the study region. A total of 81 earthquakes recorded at 50 stations situated in the central Himalayan region of India are used for the purpose. The particular focus is kept on Kappa value, which shows variability from 0.03 s to 0.095 s, inferring the higher values obtained in plains with deep sediment accumulations proving high-frequency energy dissipation and stiff-soil/rocky sites exhibit comparatively limited attenuation accordingly. To substantiate these results various attenuation parameters such as coda wave quality factor (Qc), intrinsic attenuation parameter (Qi), and scattering attenuation parameter (Qs), have been estimated for two regions in the central seismic gap Himalayan region of India employing the single backscattering model and Wennerberg formulation. The estimated values of Qc, Qi, and Qs are found to be highly dependent on frequency in the frequency range 1.5–24 Hz for both the regions. The average frequency-dependent relationships () estimated for both regions are and , respectively. The low value of Q0 shows that the region is highly heterogeneous while the higher value of η indicates higher seismicity in the area. It is also found that intrinsic attenuation is predominant over the scattering attenuation, envisaging the behavior of the wave attenuation through the absorption within the granitic layer at shallow depths. At lower frequencies, Qc values are found close to Qs values, which is in agreement with the theoretical measurements suggesting the presence of complex crustal heterogeneities beneath the region affecting the propagation of seismic waves experiencing considerable decay of energy through scattering. To confirm the aggregate attenuation on the stations, the site characteristics are also determined for examining the behavior of the amplification as the ground motion is comprised of the combined effect of the source, path, and site. The sites are amplified at a predominant frequency ( fpeak ) in between 1.5 to 10 Hz for the central Himalayan region. The different attenuation and amplification parameters like kappa, Q, and site effects can be utilized for detailed seismic hazard analysis (based on ground motion prediction equations) of the area as this region is of great importance from a socio-economic point of view.

Rejuvenation of ‘pop-up’ tectonics for Shillong Plateau in NE Himalayan region

Seismicity variation along with crustal heterogeneity using natural source is of prime interest in advance research nowadays. In the global scenario, North-East India is one of the prominent seismicity active zone characterized by its tectonically complex set-up. The seismicity study area encompasses between 18°–32°N latitude and 88°–102°E longitude, which basically locates the Shillong plateau and its surrounding in north-east part of India. Here, we have attempted to forecast the seismicity pattern as well as the crustal heterogeneity for Shillong plateau in north-east part of India. In this regard, we have used the most basic seismicity parameter like b value and fractal correlation dimension (Dc) along with focal mechanism characteristic over the entire region of study. The correlation experiment between the lower Dc value and b value indicates the possibility of an earthquake of large magnitude in the south central Burma basin and Shillong plateau region. Various dynamic faults and associated seismicity spread over all around the North-East India supports the possible rejuvenation of pop-up structure for Shillong plateau. We have tested our proposed method on real dataset, presented and analysed our experimental results and compared it with other methods and the overall results are found to be satisfactory.

Diagnosing an ancient shallow-angle subduction event from Cenozoic depositional and deformational records in the central Andes of southern Peru

The spectrum of responses exhibited by upper plates to changes in the subduction zone geometry remains poorly constrained. In the Paleogene, the central Andes in southern Peru potentially experienced an episode of flat or shallow-angle subduction. Presented here are new geochronologic, thermochronologic, basin subsidence backstripping analysis, and 2D flexural modeling results. These are synthesized with existing records on the temporal variation of the volcanic arc position, basin fill chronostratigraphy, deformation style and location, and geophysical constraints on lithospheric architecture to investigate how this segment of the Andes developed during a proposed ancient modification to the subduction zone dip. Basin chronostratigraphy confirms persistent contribution from volcanic arc products during the Eocene–Miocene, arguing against complete cessation of arc magmatism. Sediment accumulation was pulsed and generally more rapid during the late Eocene–Oligocene event. The subsidence curve is generally concave-up, atypical for a foreland basin situated within the compressional central Andes. The tectonic subsidence magnitude observed from backstripping analysis is at least 1 km less than the magnitude of subsidence predicted by 2D flexural model results. This suggests a component of basin uplift, potentially as a dynamic response to subduction shallowing, may have opposed flexural subsidence that was driven by crustal thickening and topographic loading. New geo- and thermochronologic results constrain the timing of Late Triassic pluton emplacement, and initial unroofing that predates the proposed timing of flat or shallow-angle subduction. This new timing suggests initial deformation, with mixed thin- and thick-skinned deformation style, in the Eastern Cordillera may be linked to reactivation of inherited structures rather than inboard deformation advance as a response to subduction flattening. The lack of basement-cored uplifts, preservation of mantle lithosphere beneath this segment of the central Andes, and volcanic arc patterns argue against the occurrence of a flat slab event. Instead, these results point to a shallow-angle subduction event and highlight the diverse behaviors of Cordilleran margins affected by variable subduction zone geometry and upper plates with inherited crustal heterogeneities.

Spatiotemporal Variations in Upper Crustal Extension Across the Different Basement Terranes of the Lake Tanganyika Rift, East Africa

AbstractPreexisting crustal heterogeneities are shown to influence rift process at a variety of scales. However, our understanding of how crustal inheritance influences rift‐scale spatiotemporal kinematics of faulting in magma‐poor rift environments is still very limited. Studies of active continental rifts can provide high‐fidelity assessments of extensional processes and structures that are not possible through examination of ancient rifts that have undergone subsequent deformation events or are buried deeply beneath passive margins. We examine the influence of crustal inheritance on active rifting through balancing and restoring a series of regional cross sections across the Lake Tanganyika Rift in the Western Branch of the East African Rift System. The cross sections are produced using legacy seismic reflection data, reprocessed through prestack depth migration. This type example of a young, magma‐poor continental rift transects several different basement terranes, including an Archean/Paleoproterozoic craton, and Proterozoic mobile belts. The Lake Tanganyika Rift exhibits two classic bell‐shaped profiles of extension along strike, reaching a maximum of 7.15 km. A spatiotemporal integration of the extension data, and comparison with the various basement terranes the rift transects, reveals that extension in cratonic blocks is more widely distributed compared to mobile belt terranes, where strain rapidly localizes onto border faults by later rift stages. These results reveal how crustal inheritance exerts a fundamental control on the evolution of extension localization, ultimately impacting the geometry and structural architecture of rift basins.

Eikonal Equation-Based Seismic Tomography of the Source Areas of the 2008 Mw 7.9 Wenchuan Earthquake and the 2013 Mw 6.6 Lushan Earthquake

ABSTRACT We use the eikonal equation-based seismic travel-time tomography method to image the source areas of the 2008 Wenchuan earthquake and the 2013 Lushan earthquake in the Longmenshan fault zone. High-resolution VP and VS models are obtained by inverting 75,686 P-wave and 74,552 S-wave travel times of local earthquakes during the period from 2009 to 2018. The tomographic models reveal strong crustal velocity heterogeneities in the study area. A significant velocity contrast exists across the Longmenshan fault zone: The western Songpan–Ganzi block is a high-velocity body, whereas the eastern Sichuan basin is a low-velocity anomaly. The hypocenter of the 2008 Wenchuan earthquake is between a high-velocity and a low-velocity anomaly. Beneath the Wenchuan mainshock, there is a significant low-velocity structure in the lower crust. The 2013 Lushan earthquake occurred in rocks associated with a high-velocity anomaly. A distinct low-velocity zone with low seismicity is imaged between the 2008 Wenchuan earthquake and the 2013 Lushan earthquake, where the crustal ductile deformation is likely to occur. The Baoxing complex to the northwest of the Lushan hypocenter exhibits as a high-velocity anomaly, which may be a carrier of stress accumulation and more prone to seismic activities in the future.

Spectral decomposition of the Engineering Strong Motion (ESM) flat file: regional attenuation, source scaling and Arias stress drop

We perform a spectral decomposition of the Fourier amplitude spectra disseminated along with the Engineering Strong Motion (ESM) flat file for Europe and Middle East. We apply a non-parametric inversion schema to isolate source, propagation and site effects, introducing a regionalization for the attenuation model into three domains. The obtained propagation and source components of the model are parametrized in terms of geometrical spreading, quality factor, seismic moment, and corner frequency assuming a ω2 source model. The non-parametric spectral attenuation values show a faster decay for earthquakes in Italy than in the other regions. Once described in terms of geometrical spreading and frequency-dependent quality factor, slopes and breakpoint locations of the piece-wise linear model for the geometrical spreading show regional variations, confirming that the non-parametric models capture the effects of crustal heterogeneities and differences in the anelastic attenuation. Since they are derived in the framework of a single inversion, the source spectra of the largest events which have occurred in Europe in the last decades can be directly compared and the scaling of the extracted source parameters evaluated. The Brune stress drop varies over about 2 orders of magnitude (the 5th, 50th and 95th percentiles of the ∆σ distribution are 0.76, 2.94, and 13.07 MPa, respectively), with large events having larger stress drops. In particular, the 5th, 50th and 95th percentiles for M > 5.5 are 2.87, 6.02, and 23.5 MPa, respectively whereas, for M < 5.5, the same percentiles are 0.73, 2.84, and 12.43 MPa. If compared to the residual distributions associated to a ground motion prediction equation previously derived using the same Fourier amplitude spectra, the source parameter and the empirical site amplification effects correlate well with the inter-event and inter-station residuals, respectively. Finally, we calibrated both non-parametric and parametric attenuation models for estimating the stress drop from the ratio between Arias intensity and significant duration. The results confirm that computing the Arias stress drop is a suitable approach for complementing the seismic moment with information controlling the source radiation at high frequencies for rapid response applications.

The Catalan magnetic anomaly: Its significance for the crustal structure of the Gulf of Lion passive margin and relationship to the Catalan transfer zone

Abstract The western part of the Gulf of Lion (GoL) passive margin, at the foot of the Pyrenees mountain chain, hosts the Catalan magnetic anomaly (CMA), a significant NW-SE-trending linear magnetic anomaly 50 km long with no associated gravimetric signal. The CMA is located on the continental slope near the NW-SE-trending Catalan transfer zone (CTZ), and it extends from the shelfbreak to the abyssal plain across thinned to hyper-thinned continental domain. This study interprets an updated geophysical dataset to better understand the Pyrenees–GoL transition and proposes a new interpretation of the CMA's source. Gravimetric and magnetic modelling of the lithosphere along two perpendicular cross-sections indicates that the CMA is generated by a magnetic body (80 × 20 × 12 km), called here the Catalan body, within the thinned continental domain. We interpret it as a mass of mafic material underplating and intruding the continental crust along a crustal fault belonging to the CTZ system. From the new map of Moho depths, a wide NW-SE-trending zone is highlighted on the western extremity of the GoL margin at the foot of the eastern Pyrenees Chain. It is characterized by (i) a heterogeneous crustal structure and mantle rise underneath the Tertiary Catalan sedimentary basin in the continental shelf area, (ii) a necking zone in a more proximal location compared to the eastern GoL margin, and (iii) the presence of magmatism (Catalan body) under and in the thinned continental crust, in the necking zone domain. These characteristics may be induced by the transtensional motion along the CTZ during the Tertiary opening of the Liguro-Provencal Basin, contemporary with the extension of the GoL margin. However, the role of inherited structures from the Hercynian orogeny, Mesozoic rifting and Pyrenean orogeny cannot be excluded when explaining the crustal heterogeneity, transfer zones and Moho morphology in this region.

The influence of topography and crustal heterogeneity on coseismic deformation of the 2008 Mw7.9 Wenchuan Earthquake, China

The Longmen Shan Thrust Belt, which lies at the convergence of the eastern Tibetan Plateau and Sichuan Basin, is characterized by a topographic relief of 4000 m over a distance of 100 km. While the rock underlying the Sichuan Basin is strong, the crust underlying the Tibetan Plateau is weak. Four models that simulate deformation caused by distributions of dislocations along a geometrically complex fault system are developed to investigate how the contrast in elevational and crustal heterogeneity influences coseismic deformation of the 2008 Mw7.9 Wenchuan earthquake. The models are configured with an assembly precise ramp-décollement structures and planar fault geometries. The coseismic slip distribution is estimated by inverting three component GPS observations with Green's functions calculated for each model configuration. By comparing the slip distribution and residuals among the displacement predictions from these four models and measurements of GPS, the crustal heterogeneity model robustly explained most of the observed GPS horizontal and vertical coseismic displacements. Results revealed four composite dislocation asperities distributed in the shallow crust and a slip of 4–8 m on the sub-surface décollement fault with a depth of ~20 km. The resulting geodetic moment is 8.10 × 1020 Nm, which is in agreement with the seismological estimates. The deep slip predictions obtained with the ramp-décollement fault geometry indicate that the eastern Tibetan Plateau was thrust over the Sichuan Basin rather than the plateau being uplifted owing to the underlying weak crust.

Crustal growth and reworking: A case study from the Erguna Massif, eastern Central Asian Orogenic Belt

Despite being the largest accretionary orogen on Earth, the record of crustal growth and reworking of individual microcontinental massifs within the Central Asian Orogenic Belt (CAOB) remain poorly constrained. Here, we focus on zircon records from granitoids in the Erguna Massif to discuss its crustal evolution through time. Proterozoic–Mesozoic granitoids are widespread in the Erguna Massif, and spatiotemporal variations in their zircon εHf(t) values and TDM2(Hf) ages reveal the crustal heterogeneity of the massif. Crustal growth curve demonstrates that the initial crust formed in the Mesoarchean, and shows a step-like pattern with three growth periods: 2.9–2.7, 2.1–1.9, and 1.7–0.5 Ga. This suggests that microcontinental massifs in the eastern CAOB have Precambrian basement, contradicting the hypothesis of significant crustal growth during the Phanerozoic. Phases of growth are constrained by multiple tectonic settings related to supercontinent development. Calculated reworked crustal proportions and the reworking curve indicate four reworking periods at 1.86–1.78 Ga, 860–720 Ma, 500–440 Ma, and 300–120 Ma, which limited the growth rate. These periods of reworking account for the crustal heterogeneity of the Erguna Massif.

Lower Crustal Heterogeneity Beneath the Northern Tibetan Plateau Constrained by GPS Measurements Following the 2001 Mw7.8 Kokoxili Earthquake

AbstractWe use GPS measurements across the Xidatan segment of the Kunlun fault in the Tibetan Plateau to investigate the surface deformation following the 2001 Mw 7.8 Kokoxili earthquake. We find significant postseismic deformation during the period 2007–2015, characterized by an asymmetric shear across the 2001 rupture, with average velocities reaching ~10 mm/year about 30–45 km south of the coseismic rupture. We also find that the postseismic transients diffused away from the coseismic rupture through time, with a shift of the maximum transient rates from ~20–30 km south of the rupture in 2001–2002 to ~30–45 km in 2007–2015. Viscoelastic relaxation is the dominant physical process during the period 2007–2015. The estimated effective viscosity of the lower crust beneath the Songpan‐Ganzi terrane is 2 × 1018–3 × 1018 Pa s from the 2001–2002 data, and it has increased to ~2 × 1019 Pa s for the period 6 to 14 years after the event. The large asymmetry in the postseismic deformation field indicates a laterally heterogeneous lower crust beneath the northern Tibetan Plateau. Viscoelastic relaxation models show that the viscosity of the lower crust beneath the Qaidam basin is ~2 and ~4 times larger than the viscosity of the lower crust beneath the Songpan‐Ganzi terrane in 2001–2002 and 2007–2015, respectively. Based on these data and results from previous studies, we postulate that the Kunlun fault itself is not the unique rheological boundary and that additional domains with viscosity increasing from the Qiangtang terrane to the Qaidam basin appear to be required.

Geodynamics of the Broad Triangle Area (active seismic zone) in Asia: Stress field modeling

The Broad Triangle Area (BTA) is a significant active seismic zone located mainly in East Asia. The Pamir-Baikal tectonic belt, North-South tectonic belt and Himalayan tectonic belt compose the three boundaries of the BTA. Previous studies suggested that the geodynamics of the BTA may be related to coupling between the Indian and Eurasian plates, yet quantitative analysis is lacking. In this study, we use the elastic finite element method in the ANSYS software to establish a spherical shell model and analyze the stress field of the model. We obtain a best-fit model that matches the geological evidence and seismicity by considering the real geological background; a series of models with different initial settings are also discussed to analyze the boundary influence on the different models. Our modeling results indicate that the India-Eurasia collision, the stable Siberia craton and the blocking effects of the Yangtze and North China cratons play the most important roles in the formation of the BTA. Crustal heterogeneity has little influence on the distribution of seismicity within the BTA. Further, the effects of subduction of the Pacific and Philippine plates may not reach the BTA area, which is different from the results of other studies.

Interactions between propagating rifts and linear weaknesses in the lower crust

AbstractPre-existing structures in the crust such as shear zones, faults, and mobile belts are known to exert a significant control on the structural evolution of continental rifts. However, the influence of such features when the extension direction progressively changes over time remains uncertain. Here we present new results from three-dimensional lithospheric-scale laboratory experiments of rotational extension that provide key insights into the temporal evolution of propagating rifts. We specifically test and characterize how rifts propagate and interact with linear crustal rheological heterogeneities oriented at variable angles with respect to the extension direction. Results show that approximately rift-parallel pre-existing heterogeneities favor the formation of long, linear faults that reach near-final lengths at early stages. Low angles between the heterogeneities and the propagating rift axis may result in strong strike-slip reactivation of the pre-existing structures if they are suitably oriented with respect to the stretching direction. When the linear heterogeneities are oriented at intermediate to high angles rift branches become laterally offset as they propagate, resulting in complex rhombic fault patterns. Rift-perpendicular crustal heterogeneities do not affect fault trends during rift propagation, but cause stalling and deepening of laterally growing rift basins. Similarities between the analogue experimental results and selected natural examples provide insights on how nature finds the preferential pathway to breakup in heterogeneous continental lithosphere.