Large Igneous Province Research Articles

The physical volcanology of large-scale effusive and explosive silicic eruptions in southeastern Saurashtra, Deccan Traps

AbstractSilicic magmatism, minor overall in the ∼65.5 Ma Deccan Traps continental flood basalt (CFB) province of India, was widespread in the Saurashtra region. We describe the physical volcanology of silicic volcanics and dykes exposed around Rajula–Savarkundla–Gariyadhar–Talaja towns in southeastern Saurashtra. The silicic volcanics conformably overlie basaltic lavas, suggesting rapid subaerial volcanism, and the sequence shows gentle tectonic dips (∼15°) towards the Arabian Sea. Rhyolites and dacites with preserved thicknesses of tens of metres show intense internal rheomorphic deformation, and a dacite shows a well-formed basal autobreccia. The rheomorphic rhyolites, and vitrophyres which often underlie them, lack vitroclasts (glass shards and pumice clasts). They have very similar mineral assemblages (quartz and alkali feldspar phenocrysts, and crystal cargoes dominated by calcic plagioclase and clinopyroxene or orthopyroxene, sometimes with olivine). The rheomorphic units are thus flood rhyolite and dacite lavas, apparently common in the northern-northwestern Deccan, and the vitrophyres their basal chilled parts. Tuffs (including crystal-vitric Plinian fallout ash) and eutaxitic ignimbrites formed from pyroclastic density currents; one tuff contains extraordinary numbers of lithophysae. Ridges of rhyolitic tuff breccias with pervasive secondary silicification and ferruginization represent pyroclastic eruptive fissures. The area thus records large-scale effusive and explosive silicic eruptions. Mafic and silicic dykes intrude the basaltic lavas and rarely the silicic volcanics. Mafic enclaves in several silicic dykes and some volcanics indicate magma mingling as a common phenomenon. The seaward-dipping volcanic units define a regional-scale flexure comparable to coastal flexures in CFB provinces worldwide, suggesting extensive block-faulting of this classical volcanic rifted margin.

Rapid Formation of the Ellice and Osbourn Basins and Ontong Java Nui Breakup Kinematics

AbstractBreakup of the proposed greater Ontong Java Nui large igneous province during the Cretaceous Normal Superchron can be constrained by the opening of the Ellice Basin (EB) separating the Ontong Java and Manihiki Plateaus and the Osbourn Basin separating the Manihiki and Hikurangi Plateaus. Dating of recovered dredged samples using plagioclase 40Ar/39Ar and zircon U/Pb geochronology methods indicates that spreading was well underway in the EB by 118 Ma with full spreading rates up to 3X faster than any observed today of 30–45 cm/yr and spreading likely continued until 112‐108 Ma. Ellice Basin samples show diverse geochemical affinities ranging from mid‐ocean ridge basalt (MORB) or Ontong Java‐like to more enriched OIB‐like. Pb and Nd isotopes from six samples contain varying influences from Pacific MORB and possibly Ontong Java. The geochemistry shows a lack of a clear mantle plume influence despite EB's close temporal and spatial relationship to Ontong Java, while some data resemble the Louisville Seamounts. This compositional diversity complements morphological differences among dredge sites and shows that both in situ MORB and younger overprinted features related to the nearby Tuvalu Seamounts were sampled. 40Ar/39Ar geochronology confirms the age of International Ocean Discovery Program Site U1365 near the Osbourn Trough (OT) to be 102.60 ± 0.26 Ma (2σ, n = 18). This age constrains the timing of a spreading reorientation event observed in the OB to coincide with a global plate reorganization event around 105 Ma and estimates the cessation of spreading at the OT to 96 Ma.

Deciphering the deep Earth heterogeneities from the temperature fluctuation of mantle plumes

Distinguishing deep lower mantle heterogeneities and resolving their physical and chemical properties are challenging due to the difficulty in achieving severe high-temperature and high-pressure conditions simultaneously in mineral experiments. Deep-seated mantle plumes bring these heterogeneities to the Earth's surface, therefore providing a unique insight into the Earth's deep interior. Here, we link the surface temperature fluctuation of mantle plume to the properties of deep-mantle heterogeneities via three-dimensional geodynamic modeling. Our results show that high-viscosity primordial mantle materials significantly increase the surface plume temperature due to their excessive viscous heating, whereas high-density oceanic crust slightly reduces it. Give some estimates for the maximum plume fluctuation temperature through time, the survival of scattered primordial blobs with a high viscosity contrast of ∼10-50 times in the deep-mantle reservoir is required. The temperature variations of the thermochemical mantle plume likely control the observed multiple volcanic episodes of large igneous provinces and periodic changes of volcanic flux along hotspot tracks.

How to identify pigeonite: A Raman and SEM-EDS study of detrital Ca-poor clinopyroxene from continental flood basalts

How to identify pigeonite: A Raman and SEM-EDS study of detrital Ca-poor clinopyroxene from continental flood basalts

Eruption history of the Columbia River Basalt Group constrained by high-precision U-Pb and 40Ar/39Ar geochronology

Large igneous province volcanism of the Columbia River Basalt Group (CRBG) has been suggested to play a causal role in elevated global temperatures and atmospheric carbon dioxide levels of the Miocene Climate Optimum (MCO). However, assessing the connection between volcanism and warming is dependent upon an accurate and precise chronology for the timing and duration of CRBG emplacement. Building on our previous work (Kasbohm and Schoene, 2018), we present fifteen new high-precision ages, using CA-ID-TIMS U-Pb on zircon and multi-collector 40Ar/39Ar on basaltic groundmass, to provide a detailed dual-chronometer timeline for CRBG eruptions. We use both sets of new ages and precise stratigraphic positions of our samples in an integrated Markov Chain Monte Carlo model to calculate average long-term emplacement rates for main-phase CRBG volcanism of 0.2-0.9 km3/a, with a high likelihood of one prominent hiatus of 60-120 kyr duration occurring after main-phase emplacement. We analyzed trace elements and hafnium isotopes of each dated zircon from CRBG interbeds. The compositions are consistent with both Cascades subduction volcanism and evolved syn-CRBG volcanism proximal to the depositional area. Our age model also yields ages for all magnetic field reversals during the main phase of CRBG emplacement, which can be used to improve calibrations of Miocene paleoclimate records. We find that main-phase CRBG emplacement is coincident with the greatest sustained warmth of the MCO in astronomically-tuned records. Our work shows the power of using both U-Pb and 40Ar/39Ar geochronology in an integrated stratigraphic context to assess data reliability and develop the most robust age model possible for large igneous province emplacement.

Indian carbonatites in the global tectonic context

Chrono-tectonic settings of the carbonatite occurrences of India are reviewed with a focus on the “big picture” of carbonatite emplacements in the Indian plate in relation to global tectonic events associated with the amalgamation and breakup of supercontinents. Four chrono-tectonic domains, namely, Southern domain, Southeastern domain, Northeastern domain and Northwestern domain, are delineated based on the geographical distribution, tectonic settings and temporal relationships amongst the carbonatite complexes. The Southern domain comprises two sub-domains – Paleoproterozic and Neoproterozoic. The Paleoproterozoic sub-domain is related to extension due to relaxation after the Southern Granulite Terrain-Dharwar accretion, while the Neoproterozoic sub-domain is related to rifting related to the fragmentation of Rodinia. The Southeastern domain is related to the Mesoproterozoic fragmentation of Columbia. The Northeastern domain is related to the Mid-Cretaceous breakup of Greater India from Australia-Antarctica driven by the Kerguelen mantle plume that also produced the Rajmahal-Sylhet Large Igneous Province (LIP). The Northwestern domain is related to the Late-Cretaceous Indo-Seychelles-Madagascar split and the passage of Greater India over the reunion hotspot, which also produced the Deccan LIP.

Paleogeography and high-precision geochronology of the Neoarchean Fortescue Group, Pilbara, Western Australia

While rates of Phanerozoic plate movements and magnetic field reversals have been well studied, little is known about such phenomena on early Earth. The ca. 2.8–2.7 Ga Fortescue Group on the Pilbara craton in Western Australia has been recognized as a well-preserved sequence of Archean rift volcanics thought to derive from a flood basalt province, and may have been moving rapidly across the globe at two different intervals in its depositional history. We present the results of a magnetostratigraphic study integrated with high-precision U-Pb ID-TIMS geochronology aiming to quantify rates of cratonic motion and provide a continuous time series for changes in Pilbara paleogeography during these two rapid intervals, at ∼2.77 and 2.72 Ga. We provide six new or updated high-quality paleomagnetic poles for inclusion in databases tracking Precambrian cratonic motion. During the craton’s largest geographic displacement at ∼2.77 Ga, we resolve a minimum drift rate of 23 ± 20 cm/a if there was substantial rotation of the Pilbara craton along with translational motion, and a more rapid minimum estimate of 64 ± 23 cm/a if the motion was dominated by translation; these estimates exceed both Mesoarchean and most modern rates of plate motion. We provide a new high-precision U-Pb zircon age of 2721.23 ± 0.88/0.88/6.9 Ma for the Tumbiana Formation stromatolite colony, which developed as the Pilbara craton drifted from 51.5 ± 7.0° to 32.1 ± 5.7° paleolatitude. Although the Fortescue Group has been considered an early prototype of large igneous provinces, it was emplaced over a longer duration than its Phanerozoic counterparts and does not fit at least one definition of a large igneous province (LIP). But as a potential prototype of LIP magmatism, the Fortescue succession chronicles eruptive dynamics, rapid paleogeographic changes, and a series of robustly determined magnetic field reversals during the Neoarchean.

A younger and protracted emplacement of the Ontong Java Plateau

The submarine volcanic emplacement of the Ontong Java Plateau (OJP) is the suggested cause of Ocean Anoxic Event 1a (OAE 1a). However, no precise timing and duration exists for the formation of OJP, and its connection to OAE1a relies mainly on proxies in the sedimentary record. We provide high-precision 40 Ar/ 39 Ar data from OJP drill and dredge sites that considerably improve OJP’s eruptive history. The ages determined from this work are as much as 10 million years younger than previous dates and indicate a protracted formation over at least 6 million years. OJP now appears too young to have caused OAE1a, but we suggest that it may have had a role in the later OAE1b. The protracted eruptive sequence has implications for the emplacement dynamics of OJP and other large igneous provinces.

A first high-resolution carbon isotope stratigraphy from the Boreal (NW Germany) for the Berriasian to Coniacian interval—implications for the timing of the Aptian–Albian boundary

High-amplitude changes in sedimentary δ13C characterize the Cretaceous system and have been proven useful for supra-regional chemostratigraphic correlation. In the Cretaceous, these δ13C perturbations indicate large shifts between the global carbon reservoirs that are usually caused by volcanic activity of large igneous provinces, the widespread deposition of thick organic carbon-rich sequences and/or changes in orbital parameters. Here, we present an upper Berriasian to lower Coniacian (c. 142–88 Ma) composite carbon isotope record based on 14 drill cores, 2 outcrops, and almost 5,000 samples. The total record comprises a composite thickness of more than 1,500 m. All cores and successions are located in the larger Hanover area, which represents the depocenter of the North German Lower Saxony Basin in Early to mid-Cretaceous times. In Northern Germany, Boreal Lower Cretaceous sediments are predominantly represented by CaCO3-poor mud and siltstones of up to 2,000 m thickness, which become more carbonate-rich during the Albian–Cenomanian transition and even chalkier in the upper Cenomanian to Coniacian interval. The carbon isotope record reveals a number of global key events, including the Valanginian Weissert Event, the Oceanic Anoxic Events (OAEs) 1a and d, and the Kilian Event (Aptian–Albian boundary, part of OAE 1b). For the early Late Cretaceous, the Mid-Cenomanian Event, the OAE 2 (Cenomanian–Turonian Boundary Event), and the Navigation Event, among others, have been identified. The Kilian Event represents the Aptian–Albian boundary and has been identified herein for the first time in Northern Europe. Based on the evaluation of its relative position to the Vöhrum boundary tuff, we tentatively propose a slightly older age for the Aptian–Albian boundary of c. 113.65 Ma instead of 113.2 Ma. The observed chemostratigraphic events enable a detailed stratigraphic comparison with Tethyan and other Boreal records and associated paleoenvironmental data. Thus, this new detailed chemostratigraphy provides a unique opportunity to potentially overcome many still existing Boreal–Tethyan correlation issues. The presented record can be considered almost complete, albeit a 2-Myr gap during the early Albian is likely, and condensed intervals occur specifically during the lower Aptian.

Carbon release from Large Igneous Province magmas estimated from trace element-gas correlations

Large Igneous Provinces (LIPs) facilitate massive transfers of CO2 and other volatiles from the mantle to atmosphere, contributing to past global warming and environmental disruption. However, the scale and evolution of magmatic CO2 fluxes during these events remain uncertain due to the tendency of CO2 to degas deep in magmatic systems. Here we estimate LIP CO2 using an approach based on an observed correlation between gas CO2/S ratios and trace elements in volcanic rocks. We apply this method to a compilation of published geochemical data for tholeiitic LIP lavas and to a new major and trace element dataset for alkaline rocks from the Siberian Traps. Our results indicate that CO2/S and therefore CO2 in tholeiitic and alkaline magma suites from LIPs span 1–2 orders of magnitude, emphasizing that changing CO2 concentrations can combine with magma flux to drive strong variations in CO2 release through the evolution of LIP magmatism.

The Zunyi manganese deposit, South China: A consequence of climatic-oceanic changes triggered by the eruption of Emeishan large Igneous Province?

South China hosts a large number of Middle Permian marine sedimentary Mn deposits (MSMDs) coeval to the eruption of the Emeishan Large Igneous Province (ELIP). The linkage between these MSMDs and ELIP eruption remains unclear. Here, we observed anomalously high Hg concentrations and Hg/TOC ratios, and a shift of Δ199Hg to near-zero at the base of the Middle Permian Zunyi MSMD. These results suggest ELIP eruption as a possible driving force for the deposition of the Zunyi MSMD. Principal component analysis (PCA) of trace element concentrations of the Zunyi MSMD samples revealed three clusters of trace elements. Cluster A elements (Cr, Zn, Mo, Sb, and Tl) are anomalously high at the basal of the Zunyi MSMD, confirming ELIP as a driver for the formation of this deposit. Cluster B elements (Ba, Rb, and Cu) are high in the entire deposit, supporting elevated oceanic productivity. A stepwise increase in Cluster C elements (Li, Ga, Sn, Zr, Nb, Hf, Ta, and W) concentrations in the Zunyi MSMD suggests enhanced terrestrial weathering. We infer that the ELIP eruption triggered an increased input of terrestrial nutrients to the ocean, causing increased oceanic productivity and rising dissolved O2 levels in the surface water of the middle Permian ocean in South China, favoring Mn deposition and MSMDs formation.

Valanginian climate cooling and environmental change driven by Paraná-Etendeka basalt erosion

Abstract The Valanginian Weissert Event (ca. 134 Ma) has long been linked to emplacement of the Paraná-Etendeka large igneous province (LIP). Although several Mesozoic crises were triggered by volcanic CO2 emissions and global warming, causing oceanic oxygen depletion, the Weissert Event featured climate cooling and limited marine anoxia. Here, the impact of silicate weathering on environmental change during the Weissert Event is investigated by presenting the first osmium isotope (187Os/188Os) data sets for the late Berriasian–Valanginian interval. These records document a possible rise in weathering and nutrient runoff during the latest Berriasian, followed by a switch to Paraná-Etendeka basalt erosion during the Valanginian. The high weatherability of LIP basalts enhanced global silicate weathering despite limited coeval climate warming. This weathering triggered the documented cooling, with consequent micronutrient runoff potentially aiding ocean fertilization, highlighting a mechanism linking LIP formation with environmental change that was uniquely different than that of other Mesozoic crises.

Contribution from ancient subducted slab to the Emeishan Large Igneous Province: Constraints from the petrogenesis of mafic intrusions in the western Guangxi area, South China

Contribution from ancient subducted slab to the Emeishan Large Igneous Province: Constraints from the petrogenesis of mafic intrusions in the western Guangxi area, South China

Timing and Provenance of Volcanic Fluxes Around the Permian‐Triassic Boundary Mass Extinction in South China: U‐Pb Zircon Geochronology, Volcanic Ash Geochemistry and Mercury Isotopes

AbstractAnomalous mercury (Hg) contents recorded near the Permian‐Triassic boundary (PTB) are often linked to Siberian Traps Large Igneous Province (STLIP) volcanism and the Permian‐Triassic boundary mass extinction (PTBME). However, mounting evidence indicates that the relation between STLIP volcanism and Hg “anomalies” is not straightforward. This study focuses on the timing and provenance of volcanic fluxes around the PTBME in South China. We constrain carbon isotope (δ13C) and Hg concentration and isotope records by utilizing high‐precision U‐Pb zircon ages from two expanded deep‐water marine sections spanning the Late Permian to Early Triassic in the Nanpanjiang Basin. Results reveal two episodes of Hg enrichment. The oldest episode predates the onset of a large negative δ13C excursion, which is documented to be older than 252.07 ± 0.130 Ma. The second episode occurred between 251.822 ± 0.060 and 251.589 ± 0.062 Ma, coinciding with the nadir of the δ13C excursion. Volcanic ash geochemistry and Hg isotope compositions suggest that mercury was mainly sourced from subduction‐related volcanic arc magmatism in the Tethys region, which peaked between 251.668 ± 0.079 and 251.589 ± 0.052 Ma. These results are compatible with suggestions that regional arc volcanism contributed to the causes of the PTBME in South China and provide evidence that Hg anomalies close to the PTB are not a reliable stratigraphic marker for the PTB extinction event. This study demonstrates that relations between volcanism, environmental perturbations and mass extinction during the Permian‐Triassic transition are better resolved with the aid of high‐precision U‐Pb zircon ages.

GEOCHEMICAL AND Sr-Nd ISOTOPE SYSTEMATICS OF THE LATE PERMIAN–EARLY TRIASSIC TRAPS FROM THE KUZNETSK BASIN: MAGMA SOURCES AND CORRELATION WITH THE NORIL’SK REGION VOLCANICS

The study provides whole-rock geochemical and Rb-Sr and Sm-Nd isotope examinations of Permian-Triassic volcanics (basalts, basaltic andesites) and sills (trachydolerites, monzodiorites) from the Kuznetsk Basin, which is part of the Siberian Large Igneous Province. It is shown that the volcanic section of the Kuznetsk Basin consists of the Lower (I and II units: TiO2 = 1.6–1.9 wt.%, Gd/Yb = 2.0–2.1) and Upper (III–VIII units: TiO2 = 1.4–1.8 wt.%, Gd/Yb = 1.7–1.9) Sequences. The Mayzass sill is geochemically correlated with the Unit II basaltic andesite of the Lower Sequence, and the Syrkashev sill is geochemically akin to the Upper Sequence. The Kuznetsk traps originated from the partial melting of a Sr-Nd isotopically heterogeneous subduction-modified lithospheric mantle. They are geochemically comparable to low-Ti basalts of the low–middle Nadezhdinsky suite from the Noril’sk region, but differ in Sr-Nd isotope composition. The Sr-Nd isotope signatures of the uncontaminated lavas and Syrkashev sill rocks (εNd(T) = (+4.6)–(+2.4), (87Sr/86Sr)T = 0.7047–0.7054) are inherited from the ancient lithospheric mantle and do not represent the contribution of the Siberian plume. The Mayzass sill rocks (εNd(T) = (+2.2)–(+2.1); (87Sr/86Sr)T = 0.7055–0.7063)are derived from a parental melt contaminated with crustal carbonates.

Syn-Bushveld “granite sheets” associated with the Molopo Farms Complex intruding into Transvaal Supergroup strata in southern Botswana

Abstract LA-ICPMS U-Pb isotope analyses are presented for zircons from a thin granite sheet intersected in a borehole drilled into the upper Transvaal Supergroup wall rocks to the Molopo Farms Complex in southern Botswana. Many of the zircons have irregular or angular grain margins, and some have rounded cores. Approximately half of the analysed grains yielded concordant 207Pb/206Pb ages ranging between 2 282 ± 29 and 2 113 ± 16 Ma. Assuming that these grains were inherited from the surrounding upper Transvaal Supergroup sedimentary strata, the youngest zircon age provides a maximum depositional age for these sediments. Importantly, these zircon ages coincide with dates from detrital zircons in upper Transvaal Supergroup strata in the eastern Kaapvaal Craton and in the Magondi Supergroup in western Zimbabwe. Hf isotope analyses of the rounded zircons suggest that these grains were sourced from a variable mixture of an old basement and more juvenile material. A younger age of 2 060 ± 12 Ma was obtained from a single subhedral zircon grain from the granite sheet. This is interpreted as the granite’s emplacement age, making it coeval with the adjacent Molopo Farms Complex that forms part of the Bushveld Large Igneous Province.

Magma plumbing systems in the Parnaíba Basin: Geochemistry, geochronology, and regional correlations with Mesozoic large igneous provinces

Magma plumbing systems in the Parnaíba Basin: Geochemistry, geochronology, and regional correlations with Mesozoic large igneous provinces

High-resolution calcareous nannofossil biostratigraphy and facies of Nimar Sandstone Formation (Bagh Group), central India and its bearing on Oceanic Anoxic Event 2

It is proposed to consider Narmada Valley Late Cretaceous sequences in three sectors with proper selection of stratotype in each sector: Eastern Sector: comprising of Lameta group of rocks with type area in Jabalpur area, essentially laid down in lacustrine-paedogenic milieu and containing a variety of fossils including dinosaur remains of Late Maastrichtian age. No evidence of sea incursion is available in the Eastern Sector either via the Narmada lineament or the K-G Basin. Central Sector: comprising of the Bagh Group in ascending order of: Nimar Sandstone Formation–Nodular Limestone Formation–Coralline-Bryozoan Limestone Formation, laid down in estuarine complex entirely within Turonian without any evidence of Cenomanian and Coniacian age calcareous nannofossils or other fossils being present. In the western part of Central Sector: Siliciclastic rocks and limestones referred to as Nimar Sandstone Formation curiously lack Late Cenomanian marker species, hence could be of Early Turonian age. Western Sector: comprises of Lacustrine sediments associated with Deccan Flood Basalt. The absence of the Cenomanian/Turonian Boundary and Black shales negates the presence of global OAE2 events in the Narmada lineament, but several OAE events are recognised in the Cauvery Basin, southeastern India. Uranium- and Thorium-rich Lower and Upper Nimar Sandstone Formation overlying crystalline basement are interpreted to have been essentially laid down in a fully marine estuarine milieu with reworked plant, and palynofossils including dinosaur remains. Calcareous nannofossils recovered from Upper Nimar Sandstone Formation over four decades ago and assigned Late Turonian age is revised based on modern data incorporating calcareous nannofossil and planktonic foraminifera datum and now suggest an older age of Early Turonian matching a zone with FAD of Quadrum gartneri at the base and FAD of H. helvetica at the top. It must be emphasised that except for the solitary and unique incursion of Early Turonian (ca. 93.9 Ma) sea along the Narmada lineament reaching eastward (>300 km) up to the present town of Indore, no other marine intrusion ever took place on Indian shield except along collision facing Subathu–Dogadda lineament during Late Maastrichtian. It would be erroneous to suggest that the global eustatic rise of sea level caused the entry of the Turonian sea along the Narmada lineament; instead, it was earlier speculated by me that Large Igneous Province activity of Mahajanga Flood Basalt of Madagascar (ca. 93.9 Ma) could have most probably caused a short-lived Turonian sea to enter Narmada lineament from the west around C/T boundary.

Links between large igneous province volcanism and subducted iron formations

Links between large igneous province volcanism and subducted iron formations

Crustal structure and the seismogenic environment in Yunnan imaged by double-difference tomography

The large-scale faulting and earthquake activities that developed extensively in the Yunnan area are associated with the collision of India and Eurasia. The fine crustal structure can provide a better understanding of the crustal deformation, seismogenic environment, and rupture processes. We performed a new 3-dimensional (3D) P wave velocity structure and seismic relocation using double-difference tomography based on seismic observations. The tomography images show that large-scale low-velocity anomalies spread around the margin of the south Chuan–Dian Block, Xiaojiang fault (XJF), and the Lijiang–Xiaojinhe fault (LJ-XJHF) in the middle and lower crust. There is an obvious high-speed anomaly in the Emeishan large igneous province (ELIP). We infer that the low-velocity anomaly under the LJ-XJHF zone may be derived from the lower crustal flow extruded from the central Tibetan plateau and obstructed by the ELIP, while the velocity anomalies around the XJF might be caused by shear heating, which is associated with the large-deep strike–slip fault and the transmission of stress in the southeast direction. The inversion results also show that the Yangbi earthquake occurred at the NW–SE boundary of high and low velocity from the upper crust to the lower crust, which coincides well with the location of the Yangbi earthquake sequence and the Weixi–Qiaohou fault. Meanwhile, the earthquake relocations show that the aftershocks are mainly distributed at low velocities. All the aforementioned research results indicate that the Yangbi earthquake might be attributed to the intrusion of the soft material flow along the Weixi–Qiaohou fault in the NW–SE direction. These low-viscosity crustal materials would cause brittle fractures and result in NW–SE sinistral strike–slip faults.