Crustal Structure Research Articles

Crustal Structure of Northwestern Iran on the Basis of Regional Seismic Tomography Data

Crustal Structure of Northwestern Iran on the Basis of Regional Seismic Tomography Data

Local Seismic Tomography Reveals the Crustal Structure Beneath Arraiolos (Central Portugal): Earthquake of ML 4.9 (15 January 2018)

Local Seismic Tomography Reveals the Crustal Structure Beneath Arraiolos (Central Portugal): Earthquake of ML 4.9 (15 January 2018)

Crustal and Uppermost Mantle Structure of the Iranian Makran Subduction Zone from Ambient Noise and Earthquake Surface Wave Tomography

Crustal and Uppermost Mantle Structure of the Iranian Makran Subduction Zone from Ambient Noise and Earthquake Surface Wave Tomography

New insights into the crustal structures of the Arabian Plate and surrounding plates unveiled from the crustal thickness model and its implications for geodynamics

Geodynamics of the Arabian Peninsula is less understood as crustal structures are not well delineated due to uneven distribution and lack of seismic measurements and ground based geophysical data in the Arabian interior. To address this, we delineated crustal structures in the Arabian Peninsula through the creation of a crustal thickness model from the inversion of GOCO60s gravity field model, which revealed details on the underlying crustal structures. Our model revealed strong crustal thickness variations in the Zagros fold belt that indicates high deformational history and crustal reworked, which also reflect directional collisional events between the Arabian and Eurasian Plates, similar to the Himalaya type as the result of mantle processes. The resolved crustal structures also reveal a relatively slow rifting process on the southern end of the Red Sea than at the northern end, which resulted in the bulging of crust in the southern part of Zagros Fold Mountains. Further, folding and bulging of crustal structures is more intense near the Arabian Platform and slower on the Eurasian side, which could indicate a relatively stable Eurasian plate compared to the geodynamically active Arabian Plate. Overall improvement in our high-resolution crustal thickness model yields an improved representation of crustal structures over previously derived models of the Arabian Peninsula and the Surrounding plates that indicates a variation in the Proterozoic crust in Arabian Plate, possibly indicating a secular variation in the crustal structure.

Thermo‐Tectonic History of the Qilian Shan and Adjacent Areas: New Insights From Apatite U‐Pb and Fission Track Thermochronology

AbstractThe Qilian Shan is situated along the northeastern margin of the Tibetan Plateau, and has undergone multiple episodes of tectonic deformation since the Paleozoic. Strikingly, tectonic deformation in the Qilian Shan and adjacent areas encompasses newly formed and inherited lithospheric and crustal structures. This study presents the Mesozoic‐Cenozoic post‐orogenic thermo‐tectonic histories of these active tectonic units utilizing an integrated approach combining apatite U‐Pb and fission track thermochronometers. The apatite U‐Pb dates are dominantly Paleozoic, mainly recording post‐magmatic cooling of the sampled granitoid rocks. The apatite fission track data record Late Triassic to Cretaceous cooling ages (central ages between ∼223 and ∼88 Ma), and most of the measured mean track lengths are between ∼13 and ∼12 μm. The apatite fission track results and thermal history models revealed that the Qilian Shan, Longshou Shan and Beida Shan experienced various moderate to rapid basement cooling episodes during the Late Triassic to Early Cretaceous, which we interpret as a far‐field response to a series of distant tectonic events including the Paleo‐Asian Ocean's closure, Qiangtang collision, Tethys‐deformation and Lhasa collision. During this period, these fault‐bounded tectonic units displayed different exhumation patterns, with the North Qilian Shan exhibiting a late Early Cretaceous rapid denudation while others preserved uplifted apatite partial annealing zones.

Crustal Structure Beneath the Namche Barwa Massif, Eastern Himalayan Syntaxis: Evidence of Magma Underplating?

AbstractThe Namche Barwa massif (NBM) in the Eastern Himalayan Syntaxis (EHS) hosts some of the most active Cenozoic crustal deformation, magmatism, metamorphism, and extreme topographic relief on earth. The crust‐mantle dynamics responsible for the tectono‐magmatism in the NBM remains debated. We image the detailed crustal structures under the NBM using the recordings of a dense seismic array of 400 3‐component node geophones and 4,290 P‐wave receiver functions (PRFs). Our results show a wide range of negative seismic phases in the middle and lower crust and weak Moho signal in the northern part of the NBM, under the Yarlung‐Zangbo River (YZR). The upper crustal discontinuities show “hut” shape structures, and these features were validated with the forward simulation and PRFs from three broadband seismic stations in the region. These crustal structures indicate the presence of magma underplating in the upper mantle under the NBM.

Crustal geoelectric structure of the Indian plate at the western extremity of the Bundelkhand craton and Sharda Depression in the Ganga Basin

Crustal geoelectric structure of the Indian plate at the western extremity of the Bundelkhand craton and Sharda Depression in the Ganga Basin

Crustal magnetic structure and implications for the Eastern Himalayan Syntaxis revealed by EMAG2-v3

Crustal magnetic structure and implications for the Eastern Himalayan Syntaxis revealed by EMAG2-v3

Exploring Data Set Bias and Decision Support with Predictive Uncertainty Through Bayesian Approximations and Convolutional Neural Networks

Abstract Individual seismic catalogs can contain multiscale observations from fault level to global scales and associated waveforms from discrete events reflect crustal structure across many different scales and locations. Seismic network aperture, geographic location, and observation distance may not provide informative guidance or intuition on how different catalogs will behave across models trained under different conditions. We rely on uncertainty to provide guardrails for when to trust model decisions, but understanding when our uncertainty is trustworthy is an open challenge. Here, we explore Bayesian approximation methods for assigning predictive uncertainty in seismic event classification problems. We find that computationally expensive Bayesian approximations do not outperform simple ensemble methods. We also find that when exploiting multiple seismic event catalogs, joint training with data from all the catalogs combined with Bayesian approximations and supervised training for classification can obscure bias and result in less robust uncertainty while also not providing substantial performance benefits compared to training individual models for each catalog.

Crustal and Upper Mantle Structure of the Assam Valley Region, NE India: A Review of Geophysical Findings

The northeastern region of India is one of the six most seismically active convergent plate tectonic areas in the world. The north–south convergence along the Indo-Tibetan Himalayan Ranges and the east–west subduction within the Indo-Burma Ranges create a complex stress regime, resulting in significant seismic activity and a history of great/large earthquakes. The region’s intricate strain patterns, active faults, and potential seismic gaps underscore the need for detailed subsurface studies to effectively assess seismic hazards and impending seismicity. Geophysical research is essential for understanding the region’s geodynamic evolution, seismotectonics, and mineral resources. This manuscript reviews the geological and tectonic settings of the region and summarizes recent geophysical studies, including seismic, gravity, magnetic, and magnetotelluric surveys conducted in the Assam Valley and adjacent areas (within latitudes 24.5–28.5° N and longitudes 89–97.5° E). The review highlights key findings on hydrocarbon-bearing sediments, the configuration of the crystalline basement, the heterogeneous structures of the crust and upper mantle, and seismic discontinuities. By synthesizing these results, the review aims to enhance the understanding of seismic hazards in Northeast India, guide mitigation strategies, and identify key knowledge gaps to direct future research efforts.

The Role of Orogenic Collapse on Tectonic Inheritance of Passive Continental Margins

AbstractThe break‐up and drift of continents is governed by the reactivation of structures in the lithosphere that occur in preference to formation of new structures during rifting. Numerical analyses indicate that mantle heterogeneities should be a first‐order control on tectonic inheritance; however, the importance of mantle relative to crustal structures has not been tested by observation in the geological record. Seismic anisotropy analysis using shear wave splitting of seismic waves are a useful tool to constrain past episodes of lithospheric deformation in the upper mantle. Analysis of published SKS directions colinear to orogenic trends, geometry of intracratonic faults and rift basins, and prolongation of basement structures into oceanic lithosphere all suggest a role of the mantle in controlling tectonic inheritance and crustal orogenesis. The geometry of the continental margin and temporal constraints point to a process involving lithosphere‐scale structures mimicked in the oceanic lithosphere due to orogenic collapse. The occurrence of fracture zones of the Labrador Sea that correspond with major structures in the Grenville, Appalachian, and older orogens suggest that deformation of the lithosphere and propagation of oceanic transforms are influenced by rheology in the upper mantle that formed during previous episodes of continental collision. If inheritance occurs at a lithospheric scale and is rooted in the mantle, it implies that orogenic collapse is an important mechanism for ocean opening along rifted continental margins. The geometry and morphology of the modern Atlantic Ocean may have been predetermined by Archean plate tectonic processes.

Crustal Structure of the Hikurangi Subduction Zone Revealed by Four Decades of Onshore‐Offshore Seismic Data: Implications for the Dimensions and Slip Behavior of the Seismogenic Zone

AbstractFour decades of seismic reflection, onshore‐offshore and ocean‐bottom seismic data are integrated to constrain a high‐resolution 3‐D P‐wave velocity model of the Hikurangi subduction zone. Our model shows wavespeeds in the offshore forearc to be 0.5–1 km/s higher in south Hikurangi than in the central and northern segments (VP ≤ 4.5 km/s). Correlation with onshore geology and seismic reflection data sets suggest wavespeed variability in the overthrusting plate reflects the spatial distribution of Late Jurassic basement terranes. The crustal backstop is 25–35 km from the deformation front in south Hikurangi, but this distance abruptly increases to ∼105 km near Cape Turnagain. This change in backstop position coincides with the southern extent of shallow slow‐slip, most of which occurs updip of the backstop along the central and northern margin. These relationships suggest the crustal backstop may impact the down‐dip extent of shallow conditional stability on the megathrust and imply a high likelihood of near/trench‐breaching rupture in south Hikurangi. North of Cape Turnagain, the more landward position of the backstop, in conjunction with a possible reduction in the depth of the brittle ductile transition, reduces the down‐dip width of frictional locking between the southern (∼100 km) and central Hikurangi margin by up‐to 50%. Abrupt transitions in overthrusting plate structure are resolved near Cook Strait, Gisborne and across the northern Raukumara Peninsula, and appear related to tectonic inheritance and the evolution of the Hikurangi margin. Extremely low forearc wavespeeds resolved north of Gisborne played a key role in producing long durations of long‐period earthquake ground motions.

Late Cretaceous Ophiolite Emplacement and Cenozoic Collisional Tectonics in the Northeastern Arabian Plate: Insights From New Broadband Magnetotelluric Data

AbstractThe Late Cretaceous obduction of the Semail Ophiolite onto the rifted passive Arabian margin and the Cenozoic collisional tectonics with the final closure of the Neo‐Tethys Ocean, are major contributors to the present‐day crustal architecture of the northern United Arab Emirates (UAE). We acquired the first 3D grid magnetotelluric observations from 73 stations in the northern UAE in order to image deep crustal electrical structures associated with the two significant compressional episodes. Inversion of the broadband magnetotelluric data reveals the subsurface geometry of the high‐resistivity ophiolite blocks, the underlying conductive thrust sheet (Haybi‐Hawasina nappe), a “wedge‐shaped” conductive foreland basin flanking the Hajar mountain ranges, and at depth, a high‐resistivity structure associated with a fold‐thrust belt adjacent to the allochthonous units, or Proterozoic crystalline basement. The ophiolite along the eastern coast (from cities Khor Fakkan to Fujairah) is more than 20 km thick and dips eastward. Across the Wadi Ham fault, the resistive ophiolite lies against less resistive materials that may represent Bani Hamid metamorphic rocks. The thin‐skinned thrust sheets of the proximal‐distal Tethyan sedimentary units (Haybi‐Hawasina complexes) exhibit low resistivity in the Dibba zone. In contrast to its gently east‐dipping geometry in the Dibba zone, the conductive Haybi‐Hawasina structure in the southern portion of the study area appears nearly vertically (>20 km) beneath the dense Khor Fakkan and Aswad ophiolite blocks. This result suggests extensive deformation of the thrust sheets beneath the southerly dense ophiolite blocks.

Upper crustal structure of the Xinfengjiang reservoir from ambient noise double beamforming tomography and its implications for induced seismicity

Upper crustal structure of the Xinfengjiang reservoir from ambient noise double beamforming tomography and its implications for induced seismicity

Lineament fabric, crustal structure and isostatic state of the Rub’ Al-Khali Basin from gravity data

Lineament fabric, crustal structure and isostatic state of the Rub’ Al-Khali Basin from gravity data

The Collaborative Seismic Earth Model: Generation 2

AbstractGeological interpretations, earthquake source inversions and ground motion modeling, among other applications, require models that jointly resolve crustal and mantle structure. With the second generation of the Collaborative Seismic Earth Model (CSEM2), we present a global multi‐resolution tomographic Earth model that serves this purpose. The model evolves through successive regional‐ and global‐scale refinements. While the first generation aggregated regional models, with this study, we ensure consistency between all individual submodels, resulting in a model that accurately explains wave propagation across scales. Recent regional tomographic models were incorporated, comprising continental‐scale inversions for Asia and Africa, as well as regional inversions for the Western US, Central Andes, Iran, and Southeast Asia. Across all regional refinements, over 793,000 source‐receiver pairs contributed. Moreover, the long‐wavelength Earth model (LOWE) introduces large‐scale structures outside of pre‐existing local refinements. A full‐waveform inversion for global anisotropic P‐and S‐wave speed structure over a total of 194 iterations with a minimum period of 50 s on a large data set of 1 hr of waveform data from 2,423 earthquakes and over 6 million source‐receiver pairs ensures that regional updates in the crust and uppermost mantle translate into updates of deeper, global‐scale structure. To test the performance of CSEM2, we evaluate waveform fits between observed and synthetic seismograms at 50 s for an independent data set on the global scale, and on the regional scale for lower periods. We accurately simulate waveforms within and across regional refinements, maintaining the original resolution of the submodels embedded in the global framework.

Seismicity and Present‐Day Crustal Deformation in the Southern Puna Plateau

AbstractThe Southern Puna plateau in the central Andes has a complicated tectonic history that includes episodes of distributed shortening and extension, lithospheric delamination, uplift and Quaternary backarc volcanism. In this study, the upper crustal structure and present‐day deformation in this area is investigated using a new regional earthquake catalog derived with a deep‐learning‐based phase picker. Results show abundant strike‐slip seismicity at shallow depths in the eastern Southern Puna plateau that reveals active fault systems in the area and indicates N‐S extension/E‐W compression that changes orientation and relative magnitude from north to south. A broad zone of seismic quiescence in the western plateau may indicate a zone of upper crustal decoupling from large‐scale deformation. The region separating the western and eastern plateau exhibits a complex stress field that can be related to the boundary of east/west oriented middle‐to‐lower crustal flow in the main volcanic arc. Southeast of the plateau in the Sierras Pampeanas, crustal seismicity deepens and is dominated by conjugate reverse faulting structures associated with the direction of plate convergence. Vp and Vs seismic velocity models of the upper crust obtained through local earthquake tomography with the improved seismic catalog show low‐velocity anomalies near intermontane basins, except in the Antofagasta basin where a high‐velocity anomaly possibly represents shallow intrusive component of Quaternary basaltic volcanism. Below the Cerro Galan caldera, an upper crustal 10‐day long earthquake swarm is observed which may indicate local stress perturbations from fluids at the top of the crustal magmatic system that feeds this volcano.

Uncovering implicit Seismogenic associated regions towards promoting urban resilience

Earthquakes pose a significant threat to urban environments, highlighting the need for enhanced seismic resilience. To improve understanding of earthquake dynamics and the interplay of seismic activity across space, this study introduces a novel approach for identifying associated regions that exhibit interdependence seismic behavior, revealing a network structure of earthquake interplays. This model was applied to earthquakes exceeding 3.0 Mw in Iran (1976–2023), using a 1° × 1° grid. Monthly and seasonal timespans were evaluated to capture potential short-term and long-term interactions. The model revealed a network of interdependent seismic regions in southern and southwestern Iran, predominantly located within the Zagros belt. Notably, the strongest associations were observed between spatial units 45 and 36, located approximately 6° apart in southern Iran. These units exhibited significant association in both monthly and seasonal scenarios, with support values of 0.28 and 0.65, and average confidence values of 0.58 and 0.84, respectively. The second significant bilateral relation was detected between neighboring spatial units 22 and 36, with support values of 0.26 and 0.59, and average confidence values of 0.57 and 0.80, respectively. The recognized structure was compared to the established seismotectonic zoning. This network aligns with established seismotectonic provinces, particularly in the seasonal scenario. The model also identified potential interactions between distinct zones in the monthly scenario, highlighting areas where urban development strategies might need reevaluation. Additionally, the analysis revealed implicit causal relationships between spatial units, pinpointing areas susceptible to or influencing seismic activities elsewhere. These results contribute to a deeper understanding of crustal structure, earthquake propagation, and the potential for seismic activity to trigger earthquakes in nearby or distant areas. This knowledge is crucial for developing effective strategies to build earthquake-resilient cities.

The Inherited Crustal Structure and Lithospheric Thermal Field Beneath the Sea of Marmara (NW Türkiye): Observations From 3D Gravity Modeling and Seismic Tomography Analysis

AbstractThe North Anatolian Fault (NAF) extends for over 1,000 km across Türkiye and poses significant seismic hazard in the region. The Main Marmara Fault (MMF) segment of the NAF in the Sea of Marmara (NW Türkiye), exhibits along‐strike segmentation in its interseismic strain accumulation. Constraining the lithospheric configuration below the MMF is critical to understand its segmentation and assessing seismic hazard in the area. We present a new 3D lithospheric‐scale density of the Sea of Marmara, that combines gravity modeling and seismic tomography analysis. Using forward and inverse gravity modeling with free‐air gravity data and available constraints of geological units we derived the intra‐crustal density structure. Shear‐wave velocity tomography models provided insights into the temperature and density configuration of the uppermost mantle, and the geometry of the 1330°C isotherm. Our results highlight significant crustal density variations: lower‐density crust in the Sakarya Zone and Strandja Massif, and denser crust below the Istanbul Zone, which overlies a relatively hotter lithospheric mantle. This lithospheric configuration reflects both ongoing tectonic processes and inheritance from past geological events, including the drifting of the Istanbul Zone crustal block and the signature of past subduction events. The extent of the Istanbul Zone denser crust spatially correlates with the locked segment of the MMF. The bimaterial nature of the fault segment likely influences its interseismic and coseismic behavior. The denser, stiffer Istanbul Zone crust would promote interseismically locked conditions in contrast to the adjacent, more compliant crustal block and could result in asymmetric rupture with a preferred directivity.

The effect of the geotectonic settings of volcano-plutonic belts on the metallogeny of the porphyry copper ore regions and clusters

The effect has been appreciated of the geotectonic settings of formation of volcano-plutonic belts on the ore-formation zonality of their related complex porphyry copper ore-magmatic systems (equivalent to ore regions and clusters). It is shown that combinations of different types of ore deposits, spatially and genetically associated with porphyry copper deposits within the volume of the unified systems, depend on the earth’s crust structure, on the position and nature of the ore-bearing magmatic chambers, as well as on the metallogeny of the belt basements. The identified specific characteristics of the complex multiformational metallogeny of the ore-magmatic systems should be taken into account in the forecast and prospecting for both porphyry copper deposits and their associated ore deposits of other ore-formation types.