Lower Crustal Flow Research Articles

Machine-learning based location of the 2021 MW 7.4 Maduo, Qinghai, China earthquake sequence: Insight into intraplate seismogenesis

On 22 May 2021, an MW 7.4 earthquake occurred in Maduo County, Qinghai Province, China, which is located on the Kunlun Mountain Pass-Jiangcuo fault inside the Bayan Har block, providing a good opportunity to investigate seismogenesis of large intraplate earthquakes. We analyze two years of continuous seismic data from June 2021 to June 2023, which were recorded at 34 portable seismic stations of the MaduoArray deployed in the source zone by our group. The LOC-FLOW workflow of automatic detection and location is applied to construct a complete and high-precision seismic catalog for the region, which includes machine-learning phase picking (PhaseNet), earthquake phase association (REAL), velocity model updating and station correction (VELEST), absolute earthquake location (HypoInverse), and relative location (HypoDD). As a result, 78,832 Maduo aftershocks and other local earthquakes are detected and relocated precisely. Our results show that the length of the Maduo aftershock zone is ∼170 km, which is mainly distributed along the NWW-SEE oriented Kunlun Mountain Pass-Jiangcuo fault, and there is a horsetail bifurcation feature at the eastern end of the aftershock sequence. The seismogenic fault is nearly vertical, and local seismicity occurs on both sides of the fault. Our results also show that there is no seismic gap or aftershock sparse area in the region. Previous studies have revealed a low-velocity and high-conductivity anomaly below the source zone, reflecting fluids ascending from the lower crustal flow. These results provide new insights into the cause of the 2021 Maduo earthquake.

Geochemistry of coastal geothermal systems from southern Baja California peninsula (Mexico): Fluid origins, water-rock interaction and tectonics

The Baja California peninsula forms the western margin of the Gulf of California (GC) rift system, which is an active tectonic setting manifested by seismicity and numerous geothermal sites. The present study examines the geochemistry of thermal fluids (major ions and gas species, δ18O-δD, δ13C, 3He/4He) from the southern tip of the peninsula (Los Cabos Block, LCB). Sampling was mostly focused on the coastal thermal manifestations in the towns of Buenavista and El Sargento, but other sites further inland are also included for broadening the scope of the study. The main objectives include: (i) characterize water-rock interactions and other processes controlling the fluid composition, (ii) constrain solute geothermometry estimates through multi-step geochemical modeling, and (iii) discuss the fluid origins in terms of tectonics and regional shear-wave velocity anomalies in the upper lithosphere. The geothermal systems in the area have a tectonic origin and result from fluid circulation along regional faults that penetrate the upper crust through the granitic basement. Mixing between the thermal fluids and seawater at the coast is clearly illustrated through major ions and δ18O-δD relationships. Reconstruction of the pre-mixing chemical compositions indicates a low-salinity fluid at Buenavista (Cl = 104–109 mg L−1) and a saline fluid at El Sargento (Cl = 7169 mg L−1). The geochemical modeling allows us to validate these endmember compositions and to address a common issue when using solute geothermometry, which is the uncertainty on the state of equilibrium of the thermal fluid with respect to wall-rock minerals. The corresponding results reveal contrasting thermal regimes at depths (Buenavista: 101–122 °C, El Sargento: 212–220 °C), likely associated with differences in geothermal gradient and fluid circulation depth. Our gas samples have among the lowest He isotopic ratios (0.07–0.95 Ra) in the Baja California peninsula, indicating that the origin of helium is mainly crustal. Shear wave tomography demonstrates that the study area is associated with two low velocity regions, one in the crust and the other one in the upper mantle. Our results favor the hypothesis that the heating of fluids is produced by lower-crustal flow driven by strong topographic gradients along the rifted margin. This study provides new insights into the origin of the thermal anomalies in the LCB and the adjacent GC rift system.

Unraveling the tectonic evolution of the Andean hinterland (Argentina and Chile, 30°S) using multi-sample thermal history models

The Andean hinterland of Chile and Argentina defines a region of voluminous magmatism, polyphase deformation, and high topography above a long-lived subduction zone. Construction of this high-elevation domain has been variably attributed to isostatic uplift during crustal thickening induced by internal hinterland shortening, underthrusting during growth of the external thrust belt, or lower crustal flow. Alternatively, uplift may be related to dynamic processes associated with lithospheric foundering or flat-slab subduction. This study integrates geo/thermochronological, structural, magmatic, and sedimentological datasets to reconstruct hinterland evolution and evaluate potential drivers of exhumation. Our thermal history modeling approach implements a new time-depth extension to HeFTy 2 software, which enables simultaneous inversion of multiple samples along a structural or topographic profile. This extension addresses transient effects such as isotherm deflection and the transition from geothermal to atmospheric gradients and permits changes in the relative position among samples (folding, tilting) within user-defined constraints. Single- and multi-sample modeling results based on published apatite (U-Th)/He and fission track data (AHe, AFT) along the western Andean hinterland confirm late Eocene (∼30–40 Ma) cooling below at least ∼120°C, whereas thermal histories derived from new AHe, AFT, and zircon He analyses from the eastern hinterland resolve Mesozoic cooling below ∼80–160°C, followed by protracted residence at ∼60–80°C and rapid exhumational cooling in the early Miocene. Multi-sample models further require ∼10° eastward tilting of eastern sample locations, compatible with hinterland uplift via underthrusting and development of a crustal-scale fault-bend fold that was geometrically and kinematically linked to shortening in the external (eastern) thrust belt. Results underscore the enhanced resolution of multi-sample thermal history models in testing structural hypotheses and deciphering the timing, magnitude, and mechanisms of exhumational cooling during changing geodynamic conditions.

Crustal Structure and Anisotropy Measured by CHINArray and Implications for Complicated Deformation Mechanisms Beneath the Eastern Tibetan Margin

AbstractWe investigated velocity and anisotropic structure of the crust beneath the eastern margin of the Tibetan Plateau to better understand its deformation and evolution mechanism. We performed H‐κ and Pms anisotropy analyses to obtain crustal thickness, Vp/Vs ratio, fast polarization direction, and splitting time from 711 stations, and further conducted quality control using slowness, harmonic and statistical analyses. The Songpan‐Ganzi Block has a large splitting time and a fast polarization direction roughly parallel to the GPS motion and SKS fast direction. It also shows an overall high but complex distribution of Vp/Vs ratio, and large variations in crustal thickness, indicating that crustal deformation is likely caused by crustal shortening and lower crustal flow. The northern Sichuan‐Yunnan Rhombic Block (SYRB) is featured by a thick crust and high Vp/Vs ratio, suggesting that the crust is likely inflated by partial melting lower crustal rocks. The subblock also exhibits a strong azimuthal anisotropy with a splitting time greater than 0.6 s. The fast polarization direction aligns with the nearly N‐S extended direction and rotates clockwise in front of the Emeishan Large Igneous Province (ELIP). The observed anisotropy agrees with aligned amphibole minerals under a simple shear condition, supporting a southward lower crust flow being diverted by the ELIP. Anisotropy measurements on the southern SYRB are less robust and widely scattered, suggesting a deformation mechanism different from the northern SYRB. In addition, the southeastern margin of the Sichuan Basin shows a systematic pattern of crustal anisotropy consistent with a pure shear deformation mechanism.

The contribution of Rob Westaway to the study of fluvial archives

Robert Westaway was a structural and hard-rock geologist who turned his attention to the study of Late Cenozoic fluvial archives, believing that the preservation of staircases of river terraces, particularly representing the Middle and Late Pleistocene, could only be explained in terms of crustal activity in response to surface processes, the latter affected by climatic change. His entry into this research area coincided with the realisation that such terrace sequences required surface or crustal uplift to have taken place over the time interval represented. Workers were unable to explain the observed amounts of uplift in terms of erosional isostasy, a process that could potentially have explained such one-way crustal movement. Westaway envisaged a mechanism by which mobile lower crust migrated to beneath uplifting areas, maintaining and reinforcing their uplift. The mechanism requires complex mathematics to explain it, as well as lending itself to mathematical modelling of the process, based on varying crustal properties and changes in the rates of surface processes in response to climatic fluctuation. Essentially the lower-crustal effect can be envisaged as a positive-feedback enhancement of erosional isostasy.It became apparent that Westaway's theories could explain geomorphological and sedimentary fluvial archives that were otherwise difficult to elucidate. Mantle-based erosional isostasy could not explain terrace staircases, for example. Many of these occur in regions that are tectonically inactive, and so cannot be attributed to neotectonic activity. A game-changer in terms of persuading the wider community came from the recognition of crustally ultrastable regions in which progressive long-timescale uplift has not occurred: Archaean cratons. Westaway's envisaged lower-crustal flow would not be expected in such regions, which have cold, brittle and immobile crust to its full depth. Ancient fluvial deposits are found close to modern valley-floor levels in such areas. Regions of younger Precambrian crust (Proterozoic) showing intermediate situations were subsequently identified. Other dilemmas could be resolved, such as the ‘back-tilting’ of the early-Middle Pleistocene Bytham River in the English Midlands, caused by its drainage crossing crustal blocks with different properties that have accordingly experienced differential uplift. Although glacio-isostasy, mostly seen in the effects of rebound since the Last Glacial Maximum (LGM), is largely accommodated in the mantle, and thus is reversed as a response to glacial loading and unloading, in areas of suitable crustal type there is evidently a small lower-crustal component that is less readily reversible.Westaway's important contribution has yet to be fully integrated into received wisdom in geomorphological and Quaternary circles, although much of it is now widely accepted and more will be explored and published in due course.

Middle Miocene Onset of the Litang Fault System Records Kinematic Change in Eastern Tibet

AbstractThe ∼400‐km‐long Litang fault system (LTFS) is a major intracontinental strike‐slip fault inside the Chuandian block, eastern Tibet, but its evolution and role in accommodating the India‐Asia convergence remain poorly known. Structural analysis shows that the LTFS splits into 5 strands as a left‐lateral, right‐stepping en‐echelon pattern formed under NW‐directed compression, subsequently reactivated by transtensive faults under NNE‐directed extension. Displaced geological and morphological markers yield a cumulative left‐lateral offset of 28.9–42.8 km. Inverse thermal‐history modeling of thermochronological data of the faulted rocks reveal accelerated cooling at 38–35 Ma, 16–13 Ma, and 7–5 Ma. The late Eocene rapid cooling is ascribed to the reactivation of the Garze‐Litang suture. Rapid cooling events at 16–13 Ma and 7–5 Ma record the onset of transpression and transtension of the LTFS, respectively, yielding a geologic slip rate of 2.6 ± 0.7 mm/yr. Both bifurcated geometry and slow slip rate of the LTFS since 16–13 Ma indicate diffuse deformation inside the Chuandian block, contrasting with strain localized on fast‐slip strike‐slip faults on the block margins. This implies a significant kinematic transition in the middle Miocene, such that the extrusion of the segmented mega‐blocks has been accommodated by both localized and distributed deformation in eastern Tibet. This tectonic transition could be explained by a change in lithospheric rheology from an earlier rigid state to a viscous state underneath the Chuandian block due to thermal weakening of the lower crust. We thus reconcile the end‐member geodynamic models of block extrusion and lower crustal flow in late Cenozoic times.

Hydrothermal activity and metallogenic environment revealed by seismic wave tomography in Yunnan Province, China

S-wave velocity structures are revealed using double-difference tomography, which shows large-scale low-velocity anomalies around the Xiaojiang and Lijiang−Xiaojinhe fault zones at 10−25 km depth. In contrast, an obvioushigh-speed anomaly exists in the center of Emeishan area, which is consistent with the EmeishanLarge Igneousprovince. Combined with the geological and geophysical data, the low-velocity anomaly might relate to the lower crustal flowof central Tibetan Plateau. The high-velocity anomaly associated with mantle plume in Emeishan would obstruct the crustal flow and limit it around the large deep faults. Meanwhile, the results also show that many mineral resources are mainly concentrated on the major faults and low-velocity zones, indicating that the lower crustal flow and the large deep faults provide conditions for mineral fluids and minerals deposit.

Revised chronology of the middle−upper Cenozoic succession in the Tuotuohe Basin, central-northern Tibetan Plateau, and its paleoelevation implications

Currently, the climatic implications associated with the Cenozoic tectonic history and growth mechanisms of the Tibetan Plateau lack consensus and remain controversial. This is due in part to chronological uncertainties and few paleoelevation records distributed in the central to northern Tibetan Plateau, which we address here with the development of a robust chronology (using magnetostratigraphy, biostratigraphy, detrital zircons, and regional radiochronologic dating) and a paleoelevation reconstruction for the Tuotuohe Basin (central-northern Tibet). We refined the age of the Tuotuohe Formation (37−33 Ma), Yaxicuo Formation (33−23.6 Ma), and Wudaoliang Formation (23.6−19.7 Ma). We estimated early Oligocene (ca. 29 Ma) paleotemperatures of the Tuotuohe Basin from 11 °C to 16.1 °C, which correspond to paleoelevations of 2.9 km (±0.4 km) when the relative humidity is 64% and 2.5 km (±0.5 km) when this value is 75%, using various methods including ostracod assemblages, gastropods, charophytes, branched glycerol dialkyl glycerol tetraether analysis, regional empirical formulas, and climate model simulation. Paleoelevation data and existing geological evidence in the vicinity indicate that late Eocene to late Oligocene uplift was associated with upper-crustal shortening. Since the middle Miocene, uplift has been associated with convective removal of lithospheric mantle and/or lower-crustal flow beneath the Hoh Xil Basin.

Syn‐ and post‐rift lower crustal flow under the Sunda Shelf, southern Vietnam: A role for climatically modulated erosion

AbstractTectonic subsidence on rifted, passive continental margins are largely controlled by patterns of extension and the nature of strain partitioning in the lithosphere. The Sunda Shelf, adjacent to the SW South China Sea, is characterized by deep basins linked to regional Cenozoic extension associated with propagating seafloor spreading caused by slab pull from the south. Analysis of seismic reflection profiles and drilled sections crossing the Nam Con Son and Cuu Long basins highlight Oligocene extension, with most of the thinning concentrated in the ductile mid‐lower crust. Upper crustal extension was modest and ductile flow is inferred to be directed northwestwards, towards the oceanic crust. Basin inversion occurred in the Mid Miocene, associated with the collision of the Dangerous Grounds Block and Borneo. Subsequent accelerated tectonic subsidence exceeded predictions from uniform extension models assumed to relate to extensional collapse after inversion. We correlate this to a period of faster erosion onshore driven by strong monsoon rains in Indochina and Peninsular Thailand at that time. Erosion of the onshore basement, inducing rock uplift and coupled with loading of the basins offshore, drives ductile mid‐lower crustal flow, likely to the northeast under Indochina, and/or to the west where Plio‐Pleistocene subsidence of the shelf is very slow. Significant sediment delivery from the Mekong River into the Cuu Long Basin began in the Late Miocene and migrated seawards as the basin filled. Mass balancing suggests that the basins of this part of the Sunda Shelf are filled through erosion of bedrock sources around the Gulf of Thailand. There is no need for sediment delivery from a major river draining the Tibetan Plateau to account for the deposited volumes.

Crustal flow driving twin domes exhumation and low-angle normal faulting in the Menderes Massif of western Anatolia

Lower crustal flow in regions of post-orogenic extension has been inferred to explain the exhumation of metamorphic core complexes and associated low-angle normal (detachment) fault systems. However, the origin of detachment faults, whether initially formed as high-angle or low-angle shear zones, and the extension is symmetric or asymmetric remains enigmatic. Here, we use numerical modeling constrained by geophysical and geological data to show that symmetric extension in the central Menderes Massif of western Anatolia is accommodated by the crustal flow. Our geodynamic model explains how opposite dipping Gediz and Büyük Menderes detachment faults are formed by ∼40° footwall rotation. Model predictions agree with seismic tomography data that suggests updoming of lower crust beneath the exhumed massifs, represented as “twin domes” and a flat Moho. Our work helps to account for the genetic relation between the exhumation of metamorphic core complexes and low-angle normal faulting in both Cordillera and Aegean orogenic regions and has important implications on crustal dynamics in extensional provinces.

A top-down control on upper crustal inheritance on the south-eastern Tibetan Plateau

Mantle tectonics, such as asthenospheric upwelling and lithospheric decoupling, usually controls subsequent fluid formation and migration from deep to shallow levels in the lower lithosphere and promotes rock failure and deformation. Based on the concept whereby the fluid migration location defines the structural inheritance, we aim to relate crustal processes to mantle tectonics using fluids revealed by a new magnetotelluric (MT) array across the Ailao Shan-Red River belt. As imaged via a 3D resistivity model, two lower-lithospheric conductive anomalies are determined as consistent with the Dian-Qiong (DQ) suture and Song Da (SD) belt and interpreted to contain interconnected melt. Because potassic magmatism was enabled by partial melting in the lower lithosphere, we could infer that the DQ and SD have reworked as the major fluid migration channels. Fluid migration is considered to drive lithospheric decoupling at a low-viscosity conductive layer, which is inferred feeding by aqueous and melt fluids originating from the two channels and diffusing at depths from 15 to 20 km. Constrained by the geochronology results, this weak layer could have provided convenience to induce the entirely upper crustal translation-rotation within the time interval between potassic magmatism and strike-slip of the Red River fault. This translation-rotation process is inherited from the underlying mantle processes and may further be remotely affected by the upper crustal movement of the Tibetan Plateau, conforming with the crustal rotation observed to the north of our study region. This work provides a compelling example of the tectonic control of the mantle on inherited responses in the crust.Plain Language Summary Crustal deformation is usually controlled by mantle tectonic processes, especially the southeastern Tibetan Plateau. Some models have proposed that lithospheric shearing and lower-crustal flow have controlled the crustal deformation in the Cenozoic. However, crustal rotation models show less possibility of a channel flow and relate the crustal deformation to the remote effect of India-Asia collision. Our study investigates above hypotheses by imaging the electrical resistivity of rocks beneath the major Red River Fault area in southeastern Tibet, using magnetotelluric data with high quality in a dense array. The lithosphere of the study area is interpreted to have been divided into two horizontal systems by a fluid diffusion layer at the bottom of upper crust. Resistivity low in the lower lithosphere is inferred due to partial melting of deep lithosphere and reworking of the paleo sutures in the Late Cenozoic, which have spread at the bottom of upper crust. Rather than the channel flow, this fluid migration process sensitively reflected in our model relates the mantle tectonics to crustal rotation by providing rheological conditions. We, hence, propose an inherited structure model featured by lithospheric decoupling and upper-crustal translation-rotation that may occur before the major strike-slip event.

Relocation of the 8 January 2022 Menyuan (Ms 6.9), China, earthquake sequence: A conjugated type of rupturing event with a seismic gap in the Qilian-Haiyuan fault in northeast Tibet

On 8 January 2022, an earthquake with Ms 6.9 struck Menyuan County, Qinghai Province, China, which provides an opportunity to investigate the seismotectonics of the Qilian-Haiyuan fault in northeast Tibet. We collect seismic observation data from China Seismic Network Center from 8 January to 6 February 2022 around the 2022 Menyuan mainshock. The double-difference relocation algorithm is applied to relocate 904 earthquakes, and 820 high-precision hypocenter locations are obtained. The mainshock epicenter was located at 10.2 km depth in the Tuolaishan fault. The aftershock sequence extended towards nearly E-W along the Tuolaishan fault and NW-SE along the Lenglongling fault, parts of the Qilian-Haiyuan fault, but there were fewer aftershocks at the beginning of the rupture near the intersection between the Lenglongling and Tuolaishan faults, suggesting a conjugated type of rupturing with a seismic gap. Integrating with focal mechanism solutions of mainshock and one large aftershock, we suggest that the rupture is a left-lateral strike-slip event with a high dip. The stress change around the faults intersection. The 1986 Ms 6.4 earthquake may trigger the 2016 Ms 6.4 earthquake, and then the extrusion stress could transfer to around the 2022 Ms 6.9 earthquake source area due to the Indo-Eurasian collision. Previous tomographic models revealed low-velocity anomalies under the Menyuan earthquakes, reflecting that fluids could decrease friction coefficient and normal stress on the fault planes and further promote the generation of these earthquakes. These fluids could be contained in the lower crustal flow and mantle upwelling flow under northeast Tibet.

Remnants of magma underplating by the Emeishan mantle plume within the lithosphere beneath southeastern margin of the Tibetan Plateau

The Emeishan Large Igneous Province (ELIP) is located along southeastern margin of the Tibetan Plateau, where long-wavelength crustal thickening and surface uplift have been attributed to the influx of weak crustal material from central Tibet into the lower crust beneath the Yangtze Craton. We have developed a new shear-wave velocity model for the lithosphere beneath eastern Tibet, based on a two-step linearized joint inversion technique of P-wave receiver functions (PRFs) with Rayleigh wave dispersion (10–60 s period) and Love wave dispersions (8–30 s period) for three dense seismic arrays. We applied bootstrap resampling technique to calculate the results of inversion. The resulting shear velocity model revealed two major low-velocity features in the middle–lower crust beneath southeastern Tibet. The horizontally extensive low-velocity zone (LVZ) beneath the northern Sichuan–Yunnan diamond-shaped block (SYDSB) can be interpreted as possible lower crustal flow from central Tibet, whereas the low-velocity anomaly observed beneath the Xiaojiang Fault is consistent with the presence of a partially molten layer enhanced by shear deformation along large faults. These two LVZs are separated by a lithospheric-scale high-velocity zone (HVZ) that is situated beneath the geologically inferred core of the ELIP. Based on other geophysical and geological evidence, we interpret this high-velocity anomaly body as remnants of the Emeishan plume-strengthened lithosphere that originally formed during the Permian volcanism, and we speculate that it has acted as a barrier to the hypothesized flow of lower crustal material from central Tibet since the late Cenozoic.

Late Miocene or older canyon incision in the northern U.S. Cordillera shown by erosion rates, incision models, and basalt flow ages

Deep canyons along the Salmon, Snake, and Clearwater rivers in central Idaho, USA suggest long-lasting transient incision, but the timing and drivers of this incision are not well understood. The perturbation of the Yellowstone hotspot, eruption of flood basalts, and drainage of Lake Idaho all occurred within or near to this region, but the relationship among these events and incision is unclear. Here, we utilized in situ 10Be cosmogenic radionuclide concentrations for 46 samples (17 new) of fluvial sediment across the region to quantify erosion rates, calibrate stream power models, and estimate incision timing. We estimate that transient incision along the Salmon River began prior to ca. 10 Ma. However, canyon age decreases to ca. 5 Ma or earlier farther to the north. For a group of tributaries underlain by basalt, we use the age of the basalt to estimate that local transient incision began between ca. 11.5 and 5 Ma. Based on these timing constraints, the canyons along the Salmon and Clearwater rivers predate the drainage of Lake Idaho. We argue that canyon incision was triggered by events related to the Yellowstone hotspot (e.g., basalt lava damming, subsidence of the Columbia Basin, reactivation of faults, and/or lower crustal flow). Furthermore, our models suggest basalt may be more erodible than the other rock types we study. We show that lithology has a significant influence on fluvial erosion and assumptions regarding river incision model parameters significantly influence results. Finally, this study highlights how geodynamic processes can exert a significant influence on landscape evolution.

Earthquakes evoked by lower crustal flow: Evidence from hot spring geochemistry in Lijiang-Xiaojinhe fault

Geothermal fluid is considered to be one of the important methods of earthquake forecasting, and some achievements have been made in previous studies. However, there is still a lot of work to be done before geothermal fluid can actually be used for earthquake forecasting. Lijiang-Xiaojinhe fault (LXF) which is a quake-prone region that lies on the border of Tibetan Plateau and Yangtze block is a good natural experimental field for earthquake research. In this contribution, we performed a systematic hydrogeochemistry and isotopic analysis of the geothermal waters and gases in the LXF zone. High Cl- (0.69–529.44 mg/L) concentrations in hot springs indicate that they are contaminated by deep fluid obviously. Further continuous observation (2020–2022) indicates that there are at least four times correspondence between ion concentration in two hot springs (Jinhe (94) and Xiaoshuiping (331)). Two earthquakes (ML5.2 earthquake on January 02, 2022 and ML4.6 earthquake on April 16, 2022) in Ninglang are only 50 km away from 331 hot spring, and obvious abnormal concentrations of Cl-, Na+ and SO42- were observed before the earthquakes. Combined with geological background and previous studies, we propose that lower crustal flow could be the reason why earthquake-prone in LXF. The lower crust flow through the LXF from northwest to southeast, causes internal deformation of the LXF zone. Then, a large amount of groundwater infiltrates deep underground along the fault, strengthens the pore fluid pressure, promotes the water–rock reaction, weakens fault zone, and trigger earthquakes. Ion concentrations in hot springs are sensitive to seismic activity in LXF. Unusually high or low ion concentrations in hot springs could represent the process of earthquake development, monitoring proper geothermal waters and gases in LXF will help us to study the pre-seismic hydrochemical precursors, and have a potential to provide important bases for earthquake forecasting.

An approach to determine brittle upper crustal thinning: Insights into crustal extension discrepancy in the central part of Qiongdongnan Basin

Summing fault heaves is the most commonly used method to evaluate upper crustal thinning. However, since fault deformation width (W) is often assumed as a constant in the range of 75–150 km, the stretching factor estimated from fault geometry (βf) accompanies significant uncertainty. Here we propose a new approach to determine brittle stretching factors on the foundation of numerical analyses of W and further compare our results to previous methods, with specific reference to the central part of Qiongdongnan Basin, South China Sea (SCS). Our results suggest that the value of W is generally less than 80 km and mostly less than 50 km in the northern SCS margin. We confirm that applying an overestimated value of W can lead to an underestimated amount of fault-related extension and overstatement of extension discrepancy in the rifted margin. Results also indicate an inverse discrepancy with our new method in the southeast of the basin. The difference in syn-rift sediment thickness across the rifted margin likely drove the lower crust flow causing a transition between inverse and positive extension discrepancies.

Viscous relaxation as a probe of heat flux and crustal plateau composition on Venus

It has recently been suggested that deformed crustal plateaus on Venus may be composed of felsic (silica-rich) rocks, possibly supporting the idea of an ancient ocean there. However, these plateaus have a tendency to collapse owing to flow of the viscous lower crust. Felsic minerals, especially water-bearing ones, are much weaker and thus lead to more rapid collapse, than more mafic minerals. We model plateau topographic evolution using a non-Newtonian viscous relaxation code. Despite uncertainties in the likely crustal thickness and surface heat flux, we find that quartz-dominated rheologies relax too rapidly to be plausible plateau-forming material. For plateaus dominated by a dry anorthite rheology, survival is possible only if the background crustal thickness is less than 29 km, unless the heat flux on Venus is less than the radiogenic lower bound of 34 [Formula: see text]. Future spacecraft determinations of plateau crustal thickness and mineralogy will place firmer constraints on Venus's heat flux.

Enigmatic crustal and upper mantle structure in the NE Sino-Korean Craton based on nuclear explosion seismic data

We interpret the crustal and upper mantle structure along ∼2500 km long seismic profiles in the northeastern part of the Sino-Korean Craton (SKC). The seismic data with high signal-to-noise ratio were acquired with a nuclear explosion in North Korea as source. Seismic sections show several phases including Moho reflections (PmP) and their surface multiple (PmPPmP), upper mantle refractions (P), primary reflections (PxP, PL, P410), exceptionally strong multiple reflections from the Moho (PmPPxP), and upper mantle scattering phases, which we model by ray-tracing and synthetic seismograms for a 1-D fine-scale velocity model. The observations require a thin crust (30 km) with a very low average crustal velocity (ca. 6.15 km/s) and exceptionally strong velocity contrast at the Moho discontinuity, which can be explained by a thin Moho transition zone (< 5 km thick) with strong horizontal anisotropy. We speculate that this anisotropy was induced by lower crustal flow during delamination dripping. An intra-lithospheric discontinuity (ILD) at ∼75 km depth with positive velocity contrast is probably caused by the phase transformation from spinel to garnet. Delayed first arrivals followed by a long wave train of scattered phases of up to 4 s duration are observed in the 800–1300 km offset range, which are modelled by continuous stochastic velocity fluctuations in a low-velocity zone (LVZ) below the Mid-Lithospheric Discontinuity (MLD) between 120 and 190 km depth. The average velocity of this LVZ is about 8.05 km/s, which is much lower than the IASP91 standard model. This LVZ is most likely caused by rocks which are either partially molten or close to the solidus, which explains both low velocity and the heterogeneous structure.

Paleocene–Miocene topographic and tectonic evolution of the northern central Andean plateau, southern Peru

The Paleocene–Miocene sedimentary successions along the Western Cordillera – Altiplano Plateau margin in southern Peru record the growth history of the northern Altiplano Plateau and the development of the foreland basin system. We evaluate the paleoelevation history of the northern Altiplano Plateau for the period of primary upper crustal shortening (i.e., Paleocene–Miocene) to determine the amount of shortening-related surface uplift. We provide new apatite fission-track and tuff zircon U–Pb ages to add to the existing age models of the strata. We further calculate the first-order approximation of paleoelevations of these formations from paleo-meteoric water δ18Omw values reconstructed from authigenic carbonate δ18Oc. The calculations show that surface elevations increased by more than 1 km along with the basin development from backbulge and foredeep to wedge-top and hinterland basins. The calculated paleoelevation of 1.8–2.4 km at the end of contractional deformation during the late Miocene indicates that a rapid surface uplift of more than 1.5 km is required to attain the modern high elevations. It is further inferred that this post-late Miocene surface uplift was probably associated with the lower crustal flow and ensuing convective removal of lower lithosphere materials.

Testing the cenozoic lower crustal flow beneath the Qinling Orogen, northeastern Tibetan Plateau

The E–W-trending Qinling Orogen lies in the transitional zone between the northeastern extent of the Tibetan Plateau and the lower-elevation portion of the continent. This region is suggested to have accommodated flow of the low-viscosity lower crust from the Tibetan Plateau northward around the Sichuan Basin during the Cenozoic Himalayan–Tibetan orogeny. To test this model, we combined field geologic mapping, balanced cross-section construction, thermochronology, and geophysical interpretation to constrain the history of crustal thickening from the Mesozoic to the present. In this study, we obtained new thermochronological data from four samples, which yielded 40Ar/39Ar ages of ∼194 Ma for hornblende and 208-151 Ma for biotite and apatite fission track ages of 69-42 Ma. The new thermochronological data and field mapping demonstrate that most shortening structures initially formed in the Mesozoic and underwent minor structural overprinting in the Cenozoic. Systematic structural analyses and restoration of balanced cross-sections demonstrate that the Qinling Orogen was shortened by a minimum of 35% strain (80 km shortening magnitude) after the Mesozoic Paleotethys Ocean closure and continental collision. The crust thickened substantially to >60 km, and the thermochronological data demonstrate that erosion and denudation were minor, such that the crust would have remained thick into the Cenozoic. An additional balanced cross-section across Cenozoic strata suggests >11% Cenozoic shortening, and the observed shortening alone is enough to thicken the crust to the presently observed ∼40–45 km. Therefore, we argue that lower crustal flow is not needed to account for the crustal thickness of the Qinling Orogen and that the data do not support the occurrence of processes associated with lower crustal flow beneath the Qinling Orogen.