Tectonic Boundary Research Articles

Characteristic of Fault Structures in the south coastal Zone of Taizhou Based on High-Precision Aeromagnetic Data

The south coastal zone of Taizhou lies on the magmatic rock belt of southeast coast of China, which has complex regional geological structure, intense tectonic movement and frequent magmatic activities. Based on the latest high-precision aeromagnetic data, integrated interpretation was completed, combining with regional geology, regional gravity and magnetic susceptibility information. According to the block features in different zones of the RTP aeromagnetic data, the magnetic field characteristics and the relationship with the structure division were described in detail. It showed that the different characteristics of magnetic field is the concentrated reflection such as tectonic movement, magmatic activity and stratigraphic distribution, and the fault structure was inferred and studied, especially the deep and large fault structures. The fault structures were mainly distributed in NE, NNE and NW direction, with approximately equal spacing distribution. The magnetic anomaly is mainly characterized by the boundary, gradient zones and beaded anomalies in different magnetic field. The faults act not only as the important tectonic boundaries in this region, but also the important tectonic belts, controlling the distribution of mineralization. Under the interaction of these faults, they form the basic structural pattern of East-West zone and North-South block; Among them, the NE faults have the largest scale and obviously control over different magnetic field and magmatic activity. The results presented in the paper can provide reference for further study on the distribution and activity characteristics of magmatic rocks in the coastal zone.

A detrital zircon test of large-scale terrane displacement along the Arctic margin of North America

Abstract Detrital zircon U-Pb geochronology is one of the most common methods used to constrain the provenance of ancient sedimentary systems. Yet, its efficacy for precisely constraining paleogeographic reconstructions is often complicated by geological, analytical, and statistical uncertainties. To test the utility of this technique for reconstructing complex, margin-parallel terrane displacements, we compiled new and previously published U-Pb detrital zircon data (n = 7924; 70 samples) from Neoproterozoic–Cambrian marine sandstone-bearing units across the Porcupine shear zone of northern Yukon and Alaska, which separates the North Slope subterrane of Arctic Alaska from northwestern Laurentia (Yukon block). Contrasting tectonic models for the North Slope subterrane indicate it originated either near its current position as an autochthonous continuation of the Yukon block or from a position adjacent to the northeastern Laurentian margin prior to >1000 km of Paleozoic–Mesozoic translation. Our statistical results demonstrate that zircon U-Pb age distributions from the North Slope subterrane are consistently distinct from the Yukon block, thereby supporting a model of continent-scale strike-slip displacement along the Arctic margin of North America. Further examination of this dataset highlights important pitfalls associated with common methodological approaches using small sample sizes and reveals challenges in relying solely on detrital zircon age spectra for testing models of terranes displaced along the same continental margin from which they originated. Nevertheless, large-n detrital zircon datasets interpreted within a robust geologic framework can be effective for evaluating translation across complex tectonic boundaries.

New seismological data from the Calabrian arc reveal arc-orthogonal extension across the subduction zone

The Calabrian Arc subduction-rollback system along the convergent Africa/Eurasia plate boundary is among the most active geological structures in the Mediterranean Sea. However, its seismogenic behaviour is largely unknown, mostly due to the lack of seismological observations. We studied low-to-moderate magnitude earthquakes recorded by the seismic network onshore, integrated by data from a seafloor observatory (NEMO-SN1), to compute a lithospheric velocity model for the western Ionian Sea, and relocate seismic events along major tectonic structures. Spatial changes in the depth distribution of earthquakes highlight a major lithospheric boundary constituted by the Ionian Fault, which separates two sectors where thickness of the seismogenic layer varies over 40 km. This regional tectonic boundary represents the eastern limit of a domain characterized by thinner lithosphere, arc-orthogonal extension, and transtensional tectonic deformation. Occurrence of a few thrust-type earthquakes in the accretionary wedge may suggest a locked subduction interface in a complex tectonic setting, which involves the interplay between arc-orthogonal extension and plate convergence. We finally note that distribution of earthquakes and associated extensional deformation in the Messina Straits region could be explained by right-lateral displacement along the Ionian Fault. This observation could shed new light on proposed mechanisms for the 1908 Messina earthquake.

Structural signatures of the Amazonian Craton in eastern Colombia from gravity and magnetometry data interpretation

Geophysical interpretation of potential field data plays an important role in the integration of geological data. Estimation of density and magnetic susceptibility variations within the upper crust helps evaluating the continuity of geological structures in the field. In the present study we use gravity and magnetic data in NW Amazonian Craton in Colombia. Total horizontal gradient of the reduction to magnetic pole were used to delineate magnetic lineaments and domains showing four zones, each with its own features. Multiscale edge detection (worming) of the data help delineate upper crustal structures that we interpret as tectonic boundaries that correlate with the four zones identified. 3D density and magnetic susceptibility inversion showed high density and/or high magnetic susceptibility sources correlated with these crustal structures. Zone (1) is located south of the Guaviare River, with predominant NW-SE and NE-SW magnetic lineaments; zone (2), located from south of the Guaviare River to the north, present nearly E-W magnetic lineaments and a deep E-W edge interpreted as a possible shear zone parallel to Guaviare, Orinoco and Ventuari rivers; zone (3) from south of the Vichada River to the north, with NE-SW and NW-SE lineaments; N-S zone (4) cuts the zones (2) and (3), characterized by high density/magnetic susceptibility source bounded by N-S deep edges. A more complete tectonic evolution interpretation requires further work, but we speculate that the zone (4) could indicate an aborted rift/collision suture and that the zone (2) is indicative of a younger deformation event. Shear direction at (2) is not clear: geological maps show NEE-SWW right-lateral faulting, but geophysical anomalies suggest left-lateral displacement, highlighted by left dislocation of the Orinoco River. We also speculate that a N-S edge located at the SE of the area can be related with the Atabapo Belt and the limit of Ventuari-Tapajós and Rionegro geochronological provinces.

Recent Macaronesian kinematics from GNSS ground displacement analysis

Macaronesia is a complex oceanic region spanning three tectonic plates in the northeast Atlantic ocean. It is composed of four archipelagos, widely distributed and limited to the east by the Iberian Peninsula and north-western coast of Africa. This study aims to clarify recent Macaronesian kinematics from 19 GNSS stations located on the four archipelagos and the Iberian and African coastlines. The analysis is based on nearly 15 years of common data acquisition and aimed to detect new effects of intraplate tectonics or similar local/regional events consistent with calculated ground displacements. Evaluating the GNSS stations residual velocities relative to those expected from the NNR-MORVEL56 model, higher residuals were found at continental coastal stations (Africa) than at oceanic ones (Canaries and Madeira). From the computed strain rate map, the possible existence of a shear zone connecting the Gloria and Transmoroccan fault systems, already mentioned by other authors, was depicted. Cluster statistical analysis of the horizontal residual velocities helped to identify tectonic boundaries in Macaronesia and four groups of analogous intraplate residual velocities within this region. Three of four groups were identified in the Azores, highlighting the African-Nubian-Eurasian diffuse plate boundary in this region. Furthermore, in the Canary Islands, two distinct kinematic behaviours were detected, possibly due to the activity along a previously detected tectonic fault between Tenerife and Gran Canaria, where some stations have similar intraplate residuals to those at Madeira and Cape Verde stations, while others have similar intraplate residuals to those of continental stations. Finally, all stations on oceanic crust, except Cape Verde, present recent ground subsidence which may be attributed to isostatic adjustment.

Late Ediacaran lateral-escape tectonics as recorded by the Patos shear zone (Borborema Province, NE Brazil)

Lateral escape tectonics mediated by strike-slip fault zones are an efficient mechanism of rock deformation in the continental crust of collision zones. The Borborema shear zone system, which shows dimensions comparable to the Cenozoic extrusion of Indochina, defines a lateral escape setting of the Brasilano orogeny in Northeastern Brazil. In this paper, the timing of high-grade metamorphism and compositions of the terranes involved in the shear deformation were investigated. The Patos shear corridor deforms the Siderian to Neoarchean rocks of the Granjeiro Complex that, in turn, form the basement of Seridó-Lavras da Mangabeira metapelites. U-Pb zircon ages and Sm-Nd whole-rock isotopic compositions indicate that the 2.80 – 2.35 Ga basement sequences mainly include juvenile material, whereas zircons from synkinematic migmatites indicate that the partial melting occurred in the Late Ediacaran (ca. 565 Ma). Cooling rates provided by 40Ar/39Ar range from 12 to 17°C/Ma, indicating a differential shear zone exhumation, in agreement with a transpressive setting. The presence of allochthonous Siderian sequences dismembered along the shear zone suggests that the Seridó-Granjeiro corridor defines a major tectonic boundary connected to the collisional front defined by the convergence of São Francisco-Congo and Amazonian cratons.

Morphotectonic Studies of Ghaggar Basin in the Northwestern Frontal Part of the Himalaya Based on Remote Sensing and GIS Techniques

ABSTRACT The sub-Himalayan zone of the Himalaya is bounded by the Main Boundary Thrust (MBT) in the northern side and by the Himalayan Frontal Thrust (HFT) in the southern side. HFT is the youngest major tectonic boundary of the Himalaya, separating the Siwalik Hills from the Indo-Gangetic plains. The HFT and Piedmont fault make the densely populated foothill region vulnerable to seismicity. A morphotectonic study of active faults and frontal anticlines was carried out near the Ghaggar basin along the NW part of the Himalaya. To investigate morphotectonic setup during tectonic deformation, spatial imagery and morphometric parameters extracted from DEM were used along with field investigation. The analysis divulges an important active tectonic setting of the Siwalik foothills, which governs the morphology of the Ghaggar basin and fluvial processes in the area. Variations in the topographic pattern and the interaction of the river drainage system with faults and folds were measured to describe the evolution of tectonic landforms. Drainage pattern development, drainage basin asymmetry young stage of hypsometric curve, active mountain fronts and vigorous down cutting of river indicate active anticline growth. This active fold growth is advanced by the movements along the blind thrust system of the HFT. Morphotectonic evidence such as fracture, fault scarp, river terraces, and active anticlines during field investigation further confirms the recent tectonic activity due to fault-propagated fold growth.

Victoria Land, Antarctica: An Improved Geodynamic Interpretation Based on the Strain Rate Field of the Current Crustal Motion and Moho Depth Model

In Antarctica, the severe climatic conditions and the thick ice sheet that covers the largest and most internal part of the continent make it particularly difficult to systematically carry out geophysical and geodetic observations on a continental scale. It prevents the comprehensive understanding of both the onshore and offshore geology as well as the relationship between the inner part of East Antarctica (EA) and the coastal sector of Victoria Land (VL). With the aim to reduce this gap, in this paper multiple geophysical dataset collected since the 1980s in Antarctica by Programma Nazionale di Ricerche in Antartide (PNRA) were integrated with geodetic observations. In particular, the analyzed data includes: (i) Geodetic time series from Trans Antarctic Mountains DEFormation (TAMDEF), and Victoria Land Network for DEFormation control (VLNDEF) GNSS stations installed in Victoria Land; (ii) the integration of on-shore (ground points data and airborne) gravity measurements in Victoria Land and marine gravity surveys performed in the Ross Sea and the narrow strip of Southern Ocean facing the coasts of northern Victoria Land. Gravity data modelling has improved the knowledge of the Moho depth of VL and surrounding the offshore areas. By the integration of geodetic and gravitational (or gravity) potential results it was possible to better constrain/identify four geodynamic blocks characterized by homogeneous geophysical signature: the Southern Ocean to the N, the Ross Sea to the E, the Wilkes Basin to the W, and VL in between. The last block is characterized by a small but significant clockwise rotation relative to East Antarctica. The presence of a N-S to NNW-SSE 1-km step in the Moho in correspondence of the Rennick Geodynamic Belt confirms the existence of this crustal scale discontinuity, possibly representing the tectonic boundary between East Antarctica and the northern part of VL block, as previously proposed by some geological studies.

Mineralization proximal to the final Nuna suture in northeastern Australia

Mineralization along continental suture zones is facilitated through the frequent presence of pathways from fertile mantle source regions to crustal repositories. Due to their inherent rheological weakness, these suture zones are often concealed, which hinders surface-based observations. Here, we use zircon U-Pb and sericite 40Ar/39Ar dating, and whole-rock geochemical data, to investigate the crystallization and mineralization history from a sequence of granites (sensu lato), volcanic rocks and sedimentary rocks from the Au–Ag–Pb–Zn Empress Springs Project in northeast Australia, which are under >50 m of Phanerozoic cover and located near the interpreted ca. 1.6 Ga tectonic boundary between the North Australia Craton (Mount Isa Inlier) and Laurentia (Georgetown Inlier). Zircon U-Pb dating indicates that granite emplacement, volcanic eruptions and dolerite intrusions occurred between 1564 ± 6 and 1546 ± 13 Ma (2σ), corresponding to the 1560–1550 Ma Esmeralda Supersuite and the Croydon Volcanic Group exposed in the western Georgetown Inlier. U-Pb ages from detrital zircon grains in a sedimentary rock revealed a near unimodal ca. 1560 Ma population, likely sourced from the surrounding granitic and volcanic rocks. Sericite 40Ar/39Ar dating yielded disturbed spectra with evidence for incorporation of excess radiogenic Ar, but with a probable Carboniferous to Permian age, potentially dating the timing of gold mineralization. Geochemical similarities point towards an epithermal origin for the Empress Springs Project, consistent with post-orogenic mineralization at either ca. 1560–1550 Ma or 330–250 Ma. It is likely that a series of west-dipping lithospheric-scale faults below the Empress Springs Project, here termed the Empress Suture Zone, demarcate the final suture zone associated with the assembly of Nuna. Reactivation of the Empress Suture Zone ~1.3 b.y. after crystallization with potential contemporaneous gold mineralization attests to its longevity for fluid mobility.

Evolution of Meso-Cenozoic subduction zones in the ocean-continent connection zone of the eastern South China Block: Insights from gravity and magnetic anomalies

The evolution of unified East Asian continental margin was accompanied by large-scale magmatic and tectonic events. Here we investigate the tectonic evolution history of the ocean-continent connection zone in the eastern South China Block (SCB) during the Mesozoic-Cenozoic subduction of the (Paleo-) Pacific plates based on gravity and magnetic methods that provide a window to the deep crustal structure. The decomposed multi-scale gravity wavelets reveal subduction-related intra-crustal structures and the deep structure of the Mesozoic Greater South China Block (GSCB). The depth of Moho discontinuity was inverted by gravity data, constrained by the latest global crustal model of CRUST 1.0, and the inverted depth is closely consistent with and slightly larger than the depth on seismic profiles. The undulation of the Moho discontinuity beneath the South China Block and the deep supra-crustal faults along the continental margin are considered to be associated with the subduction of the (Paleo-) Pacific plates. In general, the Moho depth in the eastern SCB gets shallower from west to east. The oblique subduction-related strike-slip faults can be traced on the 7.4 km-upward continued field of the magnetic anomaly. The geologically-indistinct Jurassic magmatic arc along the SCB continental margin is verified by residual crustal field of the Bouguer gravity anomaly. The southern Ryukyu-Taroko line is speculated as the inland segment of the Early Jurassic paleo-subduction zone, and represents the southeastern paleo-plate boundary of the GSCB in the Jurassic. As the backarc-arc-forearc paleo-system migrated eastwards since the Early Cretaceous, the subduction zone retreated to the current Taiwan-Okinawa Trough, and the Jurassic magmatic arc-forearc was transformed to a backarc, indicating an eastward migration or jumping of the Yanshanian subduction along the continental margin of the SCB. The tectonic boundary of the Mesozoic GSCB was also identified through the magnetic anomaly field and the high-order gravity field.

Structural and tectonic interpretation of EGM2008 gravity data around the Laccadive ridge in the Western Indian Ocean: an implication to continental crust

The present study attempts to structural and tectonic interpretation around the Laccadive ridge in the Western Indian Ocean using EGM2008 Bouguer gravity data. The different derivatives of EGM2008 Bouguer gravity data indicate that the tectonic boundaries are predominantly confined in the N-S and NNW-SSE directions followed by E-W and NE-SW trends. The Euler deconvolution result suggests that the deformities in the region are of both crustal- and lithospheric-scales. The gravity highs/lows in the study area are analyzed based on sediment/crustal thickness and Moho depth estimated using radially averaged power spectrum technique. The study highlights the Moho undulation followed by crustal thickening/thinning underneath the Laccadive ridge and its surroundings for their anomalous gravity signatures. The affinity of the structural pattern observed in the Laccadive ridge region with continent trends, moderate crustal thickness and shallower Moho observed below the Laccadive ridge axis, suggest thinned rifted continental crust with shallow basement.

Updated tectonic terrane boundaries of Botswana determined from gravity and aeromagnetic data

We used existing high-resolution gravity and aeromagnetic data to map the crustal units underneath Botswana. The thick sedimentary cover in this region has always been a challenge to understand the geological and tectonic configuration of unexposed crustal terranes using traditional geological methods. We utilized a standard physical mapping technique to convert previously obtained gravity and aeromagnetic data into apparent density and magnetic susceptibility maps, respectively, of the crustal tectonic terranes. Then, the derived maps were fused into a single apparent physical map to facilitate its geotectonic interpretation. The results showed that most of the tectonic boundaries were inaccurately mapped due to limited availability of high-resolution geophysical information. Moreover, we found that some terranes were missing from the current tectonic map. We used the apparent physical property map and the basement rocks information from literature to significantly update the tectonic map of Botswana, which shows the spatial extent of the geological units beneath the Kalahari sedimentary cover. The new map gives an insight into the geodynamics of Botswana crust.

SKS Splitting and Upper Mantle Anisotropy Beneath the Southern New England Appalachians: Constraints From the Dense SEISConn Array

AbstractThe geology of southern New England reflects subduction and terrane accretion during the Appalachian Orogeny and rifting during the breakup of Pangea. The presence of a low‐velocity seismic anomaly in the upper mantle beneath New England suggests the possible presence of vertical mantle flow (upwelling). It remains poorly understood how the lithosphere beneath southern New England was deformed by past tectonic events; furthermore, the details of the present‐day mantle flow field remain elusive. Observations of seismic anisotropy have the potential to constrain both past and present upper mantle deformation. Here, we present SK(K)S splitting observations at stations of the Seismic Experiment for Imaging Structure Beneath Connecticut array, a deployment of 15 broadband seismic stations across northern Connecticut. This linear array crosses a number of major terrane boundaries and traverses the Mesozoic Hartford Rift Basin in its central portion; its dense station spacing affords an opportunity to probe anisotropic structure on length scales that are relevant for the complex geology of southern New England. We find evidence for average fast splitting directions that are generally parallel to the absolute motion of the North American Plate, but in a few specific regions they are aligned with local tectonic boundaries. We document a striking decrease in splitting delay times (measured at low frequencies) from 0.9 s at the western end of the array to 0.2 s at the eastern end. We discuss several scenarios that might explain this observation, and explore the implications of our measurements for both present‐day mantle flow and past lithospheric deformation beneath southern New England.

The missing craton edge: Crustal structure of the East European Craton beneath the Carpathian Orogen revealed by double-difference tomography

The Trans-European Suture Zone (TESZ) is the most important and extensive continental suture in Europe, marking the edge of the East European Craton (EEC), from the North Sea to the Black Sea. It corresponds to significant changes in surface geology and deep crustal structure, evident in seismic, gravitational and magnetic studies. However, the TESZ disappears beneath the Eastern Carpathians accretionary nappes and Neotethys ophiolites, thrusted over the subducted EEC passive margin in Romania that may have experienced progressive southward break-off generating post-collisional volcanism and anomalous seismicity in the Vrancea region of the South-East Carpathians. To illuminate the missing TESZ section and investigate the change in crustal properties from the Precambrian EEC across the collisional orogen and the impact of related volcanism, we determined a 3D seismic model of P and S wave velocities of the Eastern Carpathians in Romania. With the advent of new permanent broadband and short-period seismic stations of the Romanian Seismic Network, we were able to lower the earthquake magnitude detection threshold to 0.5 ML, largely expanding the earthquake database and create seismic images of the crust across the Carpathians. Using double-difference tomography and waveform cross-correlation differential times, we relocated local crustal earthquakes between 2010 and 2017 and jointly inverted for the 3D P and S -wave velocity structure down to the Moho discontinuity. Our study provides the highest resolution 3D crustal seismic model of this area to date and emphasizes the manifestation of surface tectonic boundaries at lower crustal depths. The TESZ is highlighted at mid-to lower-crustal depths beneath the Carpathian nappes, east of the post-collisional volcanic belt, as a gently-dipping transition from high Vp in the eastern footwall, the dipping Precambrian basement, to low Vp beneath the Carpathian Orogen to the west and a downward decrease in Vp/Vs. In parts of the volcanic region, Vp/Vs ratios increase suggesting the presence of fluids, mafic material and partial melts that may have altered the seismic structure of the TESZ. Relocated hypocenters are vertically distributed in the velocity transition zone from low to high especially beneath the youngest volcanoes in the south, consistent with a hypothetically active magmatic plumbing system. In the northern segment, where volcanoes are older, relatively high velocities are estimated and seismicity tends to cluster in the top 10 km, likely connected to a recently reactivated fault system. Seismicity in the foreland of the East Carpathians aligns along the NW-SE trending major crustal faults and continues beneath the thrust belt, suggesting the East European Craton basement extends beneath the orogen as far as the volcanic belt.

ANALISA PERBANDINGAN STRUKTUR BAJA 3 LANTAI MENGGUNAKAN SYSTEM CBF DAN EBF DENGAN METODE PUSH OVER ANALYSIS

Indonesia merupakan termasuk negara yang berada di ring of fire dan di beberapa batas lempeng tektonik, sehingga tingkat risiko terjadinya gempa bumi sangatlah tinggi.Analisa struktur yang digunakan yaitu perbandingan sistem concentrically braced frames (CBF) dan eccentrically braced frames (EBF) berdasarkan pada peraturan struktur tahan gempa. Hasil analisis menunjukkan Daktilitas struktur dengan sistem CBF lebih kecil daripada sistem EBF. Struktur dengan sistem CBF lebih kaku dan kokoh dibandingkan dengan sistem EBF. Daktilitas yang bagus adalah yang bernilai lebih dari 1, sehingga struktur tidak mudah runtuh. Pada kondisi linier, sistem CBF memiliki perilaku atau kinerja yang lebih baik ditunjukkan dengan penggunaan material, displacement maksimum dan drift ratio yang lebih kecil. Pada kondisi nonlinier, sistem ini (EBF) memiliki perilaku maupun kinerja yang lebih baik terbukti dengan penyerapan energi dan daktilitas yang lebih baik.

2D Hydro‐Mechanical‐Chemical Modeling of (De)hydration Reactions in Deforming Heterogeneous Rock: The Periclase‐Brucite Model Reaction

AbstractDeformation at tectonic plate boundaries involves coupling between rock deformation, fluid flow, and metamorphic reactions, but quantifying this coupling is still elusive. We present a new two‐dimensional hydro‐mechanical‐chemical numerical model and investigate the coupling between heterogeneous rock deformation and metamorphic (de)hydration reactions. We consider linear viscous compressible and power‐law viscous shear deformation. Fluid flow follows Darcy's law with a Kozeny‐Carman type permeability. We consider a closed isothermal system and the reversible (de)hydration reaction: periclase and water yields brucite. Fluid pressure within a circular or elliptical inclusion is initially below the periclase‐brucite reaction pressure and above in the surrounding. Inclusions exhibit a shear viscosity thousand times smaller than for the surrounding. For circular inclusions, solid deformation has a minor impact on the evolution of fluid pressure, porosity, and reaction front. For elliptical inclusions and far‐field shortening, rock pressure inside the inclusion is higher compared to circular inclusions, and reaction‐front propagation is faster. The reaction generates a large change in porosity (∼0.1%–55%) and in solid density (∼2,300–3,500 kg m−3), and the reaction front exhibits steep gradients of fluid pressure and porosity. Reaction‐front propagating increases the weak inclusion's surface and causes an effective, reaction‐induced weakening of the heterogeneous rock. Weakening evolves nonlinear with progressive strain. Distributions of fluid and rock pressure as well as magnitudes and directions of fluid and solid velocities are significantly different. The models mimic basic features of shear zones and show the importance of coupling deformation and metamorphism. The applied MATLAB algorithm is provided.

Macro- and microstructural analysis of the Zhujiafang ductile shear zone, Hengshan Complex: Tectonic nature and geodynamic implications of the evolution of Trans–North China orogen

AbstractIn worldwide orogenic belts, crustal-scale ductile shear zones are important tectonic channels along which the orogenic root (i.e., high-grade metamorphic lower-crustal rocks) commonly experienced a relatively quick exhumation or uplift process. However, their tectonic nature and geodynamic processes are poorly constrained. In the Trans–North China orogen, the crustal-scale Zhujiafang ductile shear zone represents a major tectonic boundary separating the upper and lower crusts of the orogen. Its tectonic nature, structural features, and timing provide vital information into understanding this issue. Detailed field observations showed that the Zhujiafang ductile shear zone experienced polyphase deformation. Variable macro- and microscopic kinematic indicators are extensively preserved in the highly sheared tonalite-trondhjemite-granodiorite (TTG) and supracrustal rock assemblages and indicate an obvious dextral strike-slip and dip-slip sense of shear. Electron backscattered diffraction (EBSD) was utilized to further determine the crystallographic preferred orientation (CPO) of typical rock-forming minerals, including hornblende, quartz, and feldspar. EBSD results indicate that the hornblendes are characterized by (100) <001> and (110) <001> slip systems, whereas quartz grains are dominated by prism <a> and prism <c> slip systems, suggesting an approximate shear condition of 650–700 °C. This result is consistent with traditional thermobarometry pressure-temperature calculations implemented on the same mineral assemblages. Combined with previously reported metamorphic data in the Trans–North China orogen, we suggest that the Zhujiafang supracrustal rocks were initially buried down to ∼30 km depth, where high differential stress triggered the large-scale ductile shear between the upper and lower crusts. The high-grade lower-crustal rocks were consequently exhumed upwards along the shear zone, synchronous with extensive isothermal decompression metamorphism. The timing of peak collision-related crustal thickening was further constrained by the ca. 1930 Ma metamorphic zircon ages, whereas a subsequent exhumation event was manifested by ca. 1860 Ma syntectonic granitic veins and the available Ar-Ar ages of the region. The Zhujiafang ductile shear zone thus essentially record an integrated geodynamic process of initial collision, crustal thickening, and exhumation involved in formation of the Trans–North China orogen at 1.9–1.8 Ga.

Geomorphic assessment of the factors contributing to the evolution of landforms in Ukhaldhunga area, Kosi River valley, Kumaun Himalaya, Uttarakhand

This paper documents the various factors contributing to the evolution of landforms in the Kosi River valley, Ukhaldhunga area of the Kumaun Himalaya, Uttarakhand. The area falls in the major tectonic boundary between the Lesser Himalaya and the sub-Himalaya. Passing through the Parewa-Ukhaldhunga-Dabara area, this tectonic contact is known as the Main Boundary Thrust (MBT), which is characterized by presence of varied landforms. Study of the development of geomorphology, particularly those evolved in the MBT zone, suggests that the morphological features are intimately associated with the tectonic activities related to the movement of rocks along the MBT and the surface processes such as fluvial and mass movement. The various landforms observed in the MBT zone include strath terraces, fill terraces, paleochannels and V-shaped valley that pertain to fluvial genesis; gravity induced landforms such as landslide debris and colluvial fans; and tectonic landforms such as linear fault traces, swerving/swing of river course and linear depressions. Signature of recent tectonic activity along the MBT is observed between Dabara and Khakrakot. The activity is evident in the form of 8-km-long active fault trace trending NW-SE. Normal fault related landforms are observed in the hanging wall block of the MBT. Neotectonic strath terraces and fill terraces are well developed in the MBT zone. Multiple events of recurrence of landslides are evident from exposed section of landslide debris fans. The bedrocks are highly sheared, suggesting successive tectonic movements along the MBT. As a consequence, tectonic wedges have developed in the bedrocks. These wedges have facilitated for the recurrence of a number of landslides. The anthropogenic activities have also aggravated the slope stability. Truncation of older debris fan surfaces by an erosional/fault scarp is observed and the scarp separates the older fans from the youngest debris fan deposit. The scarp runs parallel to the MBT and Kosi River. Swerving of the Kosi River is as a result of the tectonic forcing along active lineaments/faults. The trend of lineaments of the maximum and high density is NE-SW, which is transverse to the trend of the NW-SE trending Himalayan fold-thrust belt.

Constraining the timing and evolution of a long-lived tectonic boundary: an example from the Early Paleozoic, Argentina

The Guacha Corral shear zone (GCSZ) is one of the most prominent crustal-scale shear zones in the Eastern Sierras Pampeanas of Argentina, juxtaposing two different metamorphic terranes: the eastern hanging wall, which was metamorphosed in the 550-510 Ma Pampean Orogen, and the western footwall, metamorphosed in the 500-440 Ma Famatinian Orogen. In this study, the tectono-thermal evolution of the footwall was constrained using U–Pb dating of zircon, monazite and apatite, and results were compared to the broader evolution of the shear zone. The main episode of migmatization was recorded in the footwall at ~475 Ma by zircon and monazite, indicating anatexis during the Famatinian Orogeny at amphibolite facies conditions (~700 °C – Ti-in-zircon thermometer). It was at this time that the major thrusting event was taking place due to the docking of the Precordillera/Cuyania terrane to the proto-Andean margin. Anatexis in the footwall was followed by cooling reaching upper greenschist facies conditions recorded by apatite U/Pb ages at ~400 Ma. The cooling rates during retrogression of footwall migmatites from ~690 °C to ~500 °C were of ~3–4 °C/Ma. The cooling processes after peak metamorphism were temporally related to the waning of the Famatinian Orogeny during the Early Devonian. After crossing apatite closure temperature, rocks entered greenschist facies conditions and continued motion on the shear zone favored the influx of fluids to retrogress the footwall. This late motion is temporally related to the collision of the Chilenia terrane to the proto-Andean margin at ~390 Ma, marking the start of the Chanic/Achalian Orogeny.

Constraints of nonseismic geophysical data on the deep geological structure of the Benxi iron-ore district, Liaoning, China

Abstract The Benxi area in Liaoning Province is one of the most important iron-ore districts in China. This study uses nonseismic geophysical data (in the form of gravity–magnetic–magnetotelluric data) and based on the section modeling method to model the deep, three-dimensional geological structure of the Benxi area. Based on the modeling results and deep geological structure characteristics, the Benxi area can be divided into three first-order deep geological tectonic units. A close relationship is between tectonic unit and iron-ore concentrations. First, high-quality iron-ore deposits occur within the tectonic boundary and sedimentary boundary zone of the Jiao–Liao–Ji Belt, reflects the protective effect of sedimentary cover on the iron-bearing formation. Second, enriched iron-ore deposits are mainly developed in Mesozoic granitic intrusion zone, reflects the hydrothermal leaching of silicon in host iron-bearing formations during magma intrusion. Thus, the findings of this study have important implications for future prospecting in the Benxi iron-ore district.