Geophysical Transect Research Articles

Managing the remediation strategy of contaminated megasites using field-scale calibration of geo-electrical imaging with chemical monitoring

Groundwater contamination is a threat to drinking water resources and ecosystems. Remediation by injection of chemical reagents into the aquifer may be preferred to excavation to reduce cost and environmental footprint. Yet, successful remediation requires complete contact between contamination and reagents. Subsurface heterogeneities are often responsible for diffusion into low-permeable zones, which may inhibit this contact. Monitoring the spatial distribution of injected reagents over time is crucial to achieve complete interaction. Source zone contamination at megasites is particularly challenging to remediate and monitor due to the massive scale and mixture of contaminants. Source zone remediation at Kærgård Plantation megasite (Denmark) is monitored here, with a new methodology, using high-resolution cross-borehole electrical resistivity tomography (XB-ERT) imaging calibrated by chemical analyses on groundwater samples. At this site, high levels of toxic non-aqueous phase liquids (NAPL) are targeted by in-situ chemical oxidation using activated persulfate. It may take numerous injection points with extensive injection campaigns to distribute reagents, which requires an understanding of how reagent may transport within the aquifer. A geophysical (XB-ERT) monitoring network of unprecedented size was installed to identify untreated zones and help manage the remediation strategy. The combination of spatially continuous geophysical information with discrete but precise chemical information, allowed detailed monitoring of sulfate distribution, produced during persulfate activation. Untreated zones identified in the first remediation campaign were resolved in the second campaign. The monitoring allowed adjusting the number of injection screens and the injection strategy from one campaign to the next, which resulted in better persulfate distribution and contaminant degradation in the second campaign. Furthermore, geophysical transects repeated over the timespan of a remediation campaign allowed high-resolution time-lapse imaging of reagent transport, which could in the future improve the predictability of transport models, compared to only using on a-priori assumptions of the hydraulic conductivity field.

Geophysical transects in the Abitibi greenstone belt of Canada from the mineral-exploration-oriented Metal Earth project

The Metal Earth project integrates geophysics, geology, geochemistry, and geochronology to improve the understanding of metal endowment in Precambrian terranes. Magnetics (airborne), gravity, magnetotellurics, and reflection seismic methods are the primary geophysical tools employed. Data were collected along 13 transects in the initial phase of the project. All geophysical tools are crucial for understanding the structure of the shallow, middle, and deeper crust and identifying pathways along which the constituents of critical minerals might have migrated from a source to a deposit. The magnetic data are used predominantly to help map the geology away from the transects, and the gravity data are useful for extending the near-surface geology to depths up to 8 km. The magnetotelluric data show the upper Archean crust to about 10 km as highly resistive, except for some conductive subvertical zones that correspond to major deformation zones, many of which are known to be metalliferous. This suggests that these conductive zones could have been hydrothermal fluid pathways feeding the mineral deposits. These zones can be traced to larger horizontal conductive zones in the midcrust. The seismic reflection data are consistent with and complement this: the upper crust is primarily nonreflective; however, the midcrust shows many horizontal reflectors, usually with a consistent dip to the north. Processing crooked-line seismic data is problematic, and techniques have been developed to improve the imaging, including multifocusing, 3D processing, full-waveform inversion, and cross-dip moveout methods. Passive seismic data have also been collected. Ambient-noise surface-wave tomography can be used to infer broad zones of similar seismic velocity between major reflectors, while receiver function analysis has been used to identify deeper structures such as horizontal features at or below the Moho and a dipping structure evident to about 70 km depth.

Thermo‐Kinematic Evolution of the Eastern European Alps Along the TRANSALP Transect

AbstractThe eastern European Alps are shaped by the indentation of Adria into Europe. Recent tomography, depicting detached slab fragments, has been interpreted as evidence of continuous southward subduction of European lithosphere, contrary to an often‐invoked subduction polarity reversal. Orogen‐scale exhumation, driven by rock displacement along active faults, may reflect subduction polarity within the framework of doubly‐vergent Coulomb wedge theory, provided the absence of rheological contrasts across the colliding plates. Low‐temperature thermochronology can evaluate crustal cooling in response to changes in tectonic and erosional boundary conditions. This study investigates the consistency of observed crustal re‐organization, exhumation, and mantle processes in the Eastern Alps. Thermo‐kinematic forward models driven by reconstructions of crustal shortening along the TRANSALP geophysical transect were subjected to variations in shortening rates, thermophysical parameters, and topographic evolution, supplemented by new fission‐track data. The thermo‐kinematic models reproduce: (a) the orogen‐scale structural geometry, (b) the distribution of thermochronometer ages, (c) observed time‐temperature paths, and (f) the present‐day surface heat flux. Results suggest that exhumation is driven by rock displacement along active faults without the need to involve mantle‐driven buoyancy forces. Taken together, the results identify two possible scenarios: if the Tauern Ramp is a retro‐thrust and the southward shift of deformation in the Southern Alps is a response to new Coulomb‐wedge conditions, then our results support a Mid‐Miocene reversal of the subduction polarity. Alternatively, crustal deformation does not reflect mantle processes entailing a high degree of inter‐plate decoupling.

Geophysical data reveal a tectonic indentation and constrain the transition zone between the São Francisco Craton and the Borborema Paleoplates across the Neoproterozoic Sergipano Orogen, beneath the Cretaceous Tucano Basin, NE Brazil

Geophysical techniques can improve knowledge about the deep structures of collisional orogens. Therefore, magnetotelluric and gravimetric data were combined in this study to enhance the poorly understood subsurface structure of the transition zone from the São Francisco Craton to the Neoproterozoic Sergipano Orogen in northeastern Brazil. The results along a 240-km-long, NE–SW trending geophysical transect revealed two main geoelectric domains beneath the Cretaceous Tucano Basin, which can be linked to two distinct tectonic blocks on the surface. The less resistive SW domain corresponds to the rocks of the Serrinha Block within the São Francisco Craton. In contrast, the more resistive NE domain matches the basement rocks of the northernmost units of the Sergipano Orogen. The results are discussed considering Paleoproterozoic and Neoproterozoic scenarios with different interpretations of the NE domain. Based on the surface geology and additional gravimetric and magnetotelluric data, the best scenario is to consider the NE domain as representing the geoelectric signature of the three northernmost geological domains of the Sergipano Orogen (Macururé, Poço Redondo–Marancó, and Canindé) and that of the Pernambuco–Alagoas Block (PEAL). PEAL collided with the São Francisco paleolithosphere forming the Sergipano Orogen. The NE geoelectric domain can be interpreted as an indenter of the tectonically intruded PEAL beneath the São Francisco paleolithosphere. The new data properly define the northern limit of the São Francisco Craton and explain both indentation structures and contrasting sediment provenance observed in the Sergipano Orogen.

Geological and geophysical transect of the Middle Tien Shan across the Naryn and Atbashi depressions

The article presents the results of integrated geological and geophysical research works dealt with detailed magnetotelluric (MT) sounding and the study of the morphology and spatial position of the sedimentary cover and basement structures along the key transect of the Middle Tien Shan crossing the Naryn and Atbashi depressions. The data on the distribution of deep electrical conductivity of the crust and upper mantle were compared with the seismic profiling data. The compilation of the results of structural-geological and geophysical studies provided the opportunity to draw 2D upper-crust geological structure, consistent with the structure of electrical conductivity of the crust to depths of about 10 km. The detailed geological cross-sections and the structural and geological data allow us to characterize the deformations of the Cenozoic sedimentary complex and Paleozoic basement surface associated with the alpine activation of the key segment of the Tien Shan. It is shown that the Cenozoic structural parageneses emerged during a relatively short-term phase of deformation and orogeny under conditions of horizontal compression and transpression, which did not manifest themselves during sagging that occurred previously.

Jurassic-Cretaceous magmatic arcs in the Eastern Black Sea: Evidence from geophysical studies and 2D modeling

The origin, tectonic development, and lithosphere structure of the East Black Sea Basin (EBSB) are governed by the evolution of the northern branch of the Tethys ocean. The most spectacular features of its evolution could retain their imprints in geophysical fields and models, which we used to constrain a geophysical transect for the crust and upper mantle crossing the EBSB and the Shatsky Ridge (SR) from the Eastern Pontides to the Northern Caucasus. 2D gravity and magnetic modeling, constrained by wide-angle seismic data, revealed thin high-density and high-velocity sub-oceanic crust of the EBSB with the Moho shallowing up ~20 km depth. A spectacular feature of the Black Sea magnetic field is the Alushta-Batumi anomaly (ABA) above the SR that could be an imprint of subduction-related Middle Jurassic magmatic arc, whereas the Cretaceous (in Eastern Pontides) magmatic arc manifests itself by a chain of magnetic anomalies on the southern shoreline of the Black Sea. The high-velocity heterogeneity, revealed by seismic tomography, could be an image of a slab due to Mesozoic (Middle Jurassic and Cretaceous) subduction of the northern branch of Neotethys ocean. It shows rather a flat subduction slab that plunges northwards from subcrustal depths south of Eastern Pontide to the depth of > 70–80 km below the SR. Middle Jurassic and Cretaceous subduction fronts are located closely in the region of Eastern Pontides, whereas the related magmatic arcs are spaced differently – over the SR for the Middle Jurassic arc and along the southern coastline for Cretaceous Eastern Pontide magmatic arc correspondingly. The latter could be caused by the opening of the EBSB in the Cretaceous that separated the eastern segment of the BS onto the Eastern Pontides – Arkhangelsky Ridge and the SR – Northern Caucasus domains.

Changing environments and human interaction during the Pleistocene–Early Holocene from the shallow coastal area of Dor, Israel

AbstractThe protected Tel-Dor coastal embayment in the eastern Mediterranean preserves an unusually complete stratigraphic record that reveals human–environmental interactions throughout the Holocene. Interpretation of new seismic profiles collected from shallow marine geophysical transects across the bay show five seismic units were correlated with stratigraphy and age dates obtained from coastal and shallow-marine sediment cores. This stratigraphic framework permits a detailed reconstruction of the coastal system over the last ca. 77 ka as well as an assessment of environmental factors that influenced some dimensions of past coastal societies. The base of the boreholes records lowstand aeolian deposits overlain by wetland sediments that were subsequently flooded by the mid-Holocene transgression. The earliest human settlements are submerged Pottery Neolithic (8.25–7 ka) structures and tools, found immediately above the wetland deposits landward of a submerged aeolianite ridge at the mouth of the bay. The wetland deposits and Pottery Neolithic settlement remains are buried by coastal sand that records a middle Holocene sea-level rise ca. 7.6–6.5 ka. Stratigraphic and geographic relationships suggest that these coastal communities were displaced by sea-level transgression. These findings demonstrate how robust integration of different data sets can be used to reconstruct the geomorphic evolution of coastal settings as well as provide an important addition to the nature of human–landscape interaction and cultural development.

Small atoll fresh groundwater lenses respond to a combination of natural climatic cycles and human modified geology

Freshwater lenses underlying small ocean islands exhibit spatial variability and temporal fluctuations in volume, influencing ecologic management. For example, The Palmyra Atoll National Wildlife Refuge harbors one of the few surviving native stands of Pisonia grandis in the central Pacific Ocean, yet these trees face pressure from groundwater salinization, with little basic groundwater data to guide decision making. Adding to natural complexity, the geology of Palmyra was heavily altered by dredge and fill activities. Our study based at this atoll combines geophysical and hydrological field measurements from 2008 to 2019 with groundwater modeling to study the drivers of observed freshwater lens dynamics. Electromagnetic induction (EMI) field data were collected on the main atoll islands over repeat transects in 2008 following ‘strong’ La Niña conditions (wet) and in 2016 during ‘very strong’ El Niño conditions (dry). Shallow monitoring wells were installed adjacent to the geophysical transects in 2013 and screened within the fresh/saline groundwater transition zone. Temporal EMI and monitoring well data showed a strong contraction of the freshwater lens in response to El Niño conditions, and indicated a thicker lens toward the ocean side, an opposite spatial pattern to that observed for many other Pacific islands. On an outer islet where a stand of mature Pisonia trees exist, EMI surveys revealed only a thin (<3 m from land surface) layer of brackish groundwater during El Niño. Numerical groundwater simulations were performed for a range of permeability distributions and climate conditions at Palmyra. Results revealed that the observed atypical lens asymmetry is likely due to more efficient submarine groundwater discharge on the lagoon side as a result of lagoon dredging and filling with high-permeability material. Simulations also predict large decreases (40%) in freshwater lens volume during dry cycles and highlight threats to the Pisonia trees, yielding insight for atoll ecosystem management worldwide.

Analysis of neotectonic structures in the piedmont region of Pir Panjal Range NW Himalaya by integrating geomorphic indicators coupled with geophysical transects (GPR)

Ground-penetrating radar (GPR) survey was performed at ten potential sites to delineate the blind surface ruptures, structural discontinuities/shallow subsurface active faults and palaeoseismic deformation in the dynamic range front environment of Pir Panjal. A total of 20 GPR profiles were recorded in four districts with the unshielded 100 MHz rough terrain antenna system and 250 MHz shielded antenna, and out of them, 10 profiles were processed and correlated with detailed structural and lithological map of the area. Several tectonic landforms such as strath surfaces, uplifted fluvial terraces, fault scarps, structural discontinuities, offset ridges, meanders, linear valleys and liquefaction structures mapped during field exercises were meticulously examined by GPR prospecting. Steeply dipping hyperbolas, warped and inclined radar reflections that sharply cut or offset stratigraphic reflections correspond to thrust fault strands. The processed data unearth normal and reverse faults, as well as liquefaction structures covered by recent deposits. However, their correlation with geological and structural data gave a more precise insight into the structural settings of the studied area. The different discontinuity relations specified in the Karewa deposits are mainly due to the regional compressional stress regimes and gradual anisotropic lithology prevailing in the area. The GPR data in agreement with the exposed geological units and structural models suggest that bulk of the landscapes have been evolved/sculptured under the influence of compressional and extensional tectonics along the concomitant active faults. The structural discontinuities (Faults) and the mechanical properties of Karewa deposits will be useful to infer the neotectonic activity prevailing in the area. The whole study would be helpful for the seismic hazard assessment of parts of the Kashmir Valley, NW Himalaya.

Magnetotelluric transect across the São Luís cratonic fragment, the Gurupi belt and the Parnaíba basin, N-NE Brazil

The study area is located in the N-NE region of Brazil where Precambrian rocks of the São Luís Cratonic Fragment and the Gurupi belt are overlain by Phanerozoic sediments of the Parnaíba and São Luís basins, making the assessment of the extents of these geotectonic domains difficult. One of the main objectives of the present study was to elaborate a NNW-SSE-trending, 180-km long magnetotelluric transect, aiming to identify the geotectonic domains of the study area by means of their geoelectric characteristics. The magnetotelluric (MT) technique is a geophysical method that detects the natural variations of the terrestrial electric and magnetic fields so as to investigate the geoelectric characteristics of the subsurface. By means of a MT survey for field data acquisition, calculation of impedance tensors and 2D inversion modeling, the geophysical transect and additionally an interpretive geological model were elaborated for the study area. The combined transverse electric (TE) and transverse magnetic (TM) geophysical model revealed superficial conductive (Parnaíba and São Luís basins and sedimentary coverings), resistive (São Luís Cratonic Fragment) and very resistive (Gurupi mobile belt) portions up to depths of the order of 15 km. These results attested for correlations between the São Luís Craton (South American Plate) and the West African Craton (African Plate), especially regarding the Gurupi Belt (in Brazil) and the Rokelide Belt (in Liberia), which are considered to be the boundaries of a triple junction related to the supercontinent Rodinia taphrogenesis. Besides presenting an unprecedented geophysical investigation along the São Luís Cratonic Fragment and the Gurupi Belt, this study promotes the discussion on the correlations between the Brazilian Gurupi and the African Rokelide belts for metallogenetic purposes.

Shallow groundwater flow and inverted fresh/saline-water interface in a hypersaline endorheic basin (Great Basin, USA)

Pilot Valley is an 828-km2 arid-region endorheic basin in western USA. Bounding mountain ranges rise as much as 1,900 m above the nearly flat 379-km2 playa floor. Up to 3.8 m of Pleistocene Lake Bonneville mud and thin oolitic sand layers form the surface layer of the basin floor. Groundwater conditions were evaluated using data from shallow monitoring wells and borings, springs, infiltrometer measurements, slug and dilution tests, geophysical transects, and precision elevation surveys. Alluvial fan groundwater discharges at fan/playa interface springs and underflows to the shallow basin sediments along the western side of the basin; the groundwater only underflows along the eastern side. Precision surveying established a Lake Bonneville shore-line break in slope as the cause of the spring discharges. Tectonic tilting causes groundwater to flow from east to west and to the topographic low. Monthly measured and pressure transducer data established seasonal pressure responses and upward groundwater gradients. All basin groundwater is lost to evapotranspiration at the topographic low, where a thin salt pan has developed. Groundwater evolves from fresh to hypersaline near the alluvial fan/playa interface where there is an inverted salinity gradient and a groundwater pressure ridge. The pressure ridge and inverted salinity interface are due to: (1) osmotic pressure established between the oolitic sand of high hydraulic conductivity and the overlying low-hydraulic-conductivity lake mud at the fan/playa interface, and (2) the collision between fresh groundwater flow driven by a steep hydraulic head and hypersaline groundwater flow driven by a nearly flat hydraulic head.

Investigating the slope failures at the Lou rock glacier front, French Alps

AbstractOn August 14 2015 a large debris flow initiated by the occurrence of two slope failures at the front of the Lou rock glacier flooded part of the town of Lanslevillard, France. The present study aims to understand the meteorological and geomorphological context that led to these failures. Investigations were conducted by combining meteorological data, surface movements, and geophysical transects. The analysis indicates that the Lou rock glacier is directly connected to an active torrential channel and has a natural predisposition to frontal failure due to the steepness of its front. The slope failures were triggered after a heat wave followed by a three‐week period of almost continuous rainfall. Water flowing on top of the permafrost table was observed in the two head scarps, suggesting that regressive erosion consecutive to this concentrated subsurface water flow triggered the failures. For one of the slides, traces of previous failures were observable on historical aerial imagery dating back to the 1950's, while the second slide corresponded to a novel event and developed on the frontal slope of a fast‐moving and destabilized rock glacier lobe. We also discuss the increase in local predisposition to failure related to the remarkable morphological modifications such as frontal advance and development of surface cracks associated with the lobe destabilization.

Conjugation with the Central Asian Foldbelt: Interpretation of the 3DV and Tynda–Amurzet Transects

In this paper we consider the results of geological interpretation of 3DV (Tommot–Skovorodino segment) and Tynda–Amurzet geophysical transects crosscutting the Aldan Shield and the Stanovoi Granite–Greenstone Domain of the Siberian Platform; the Selengino–Stanovoi, Mongolia–Okhotsk, and Gobi–Hinggang foldbelts; and the Argun and Mamyn microcontinents with a total extent of sections of about 1000 km and depth of about 40 km. The data of previous studies and follow-up electric conductivity information have been used. The data of geological mapping, subject studies, and insights into the deep-seated structure of the considered territory obtained with a complex of geophysical methods are discussed. It is shown that Mesozoic strike-slip and thrust faults play the leading role in the present-day structure of the territory and in control rifting and mantle diapirism. It is suggested that Californian-type metamorphic nuclei, which are of structure-forming significance for adjacent territories in the west, have also developed in the studied region.

Spatial variations of magmatic crustal accretion during the opening of the Tyrrhenian back‐arc from wide‐angle seismic velocity models and seismic reflection images

AbstractThe structural complexity of back‐arc basins is related to the evolution of the associated subduction system. Here, we present an integrated geophysical and geological study that constrains the 3D spatial variability of magmatic activity along the Tyrrhenian back‐arc basin. We use wide‐angle seismic and gravity data, acquired in 2010 within the MEDOC experiment along a ~300 km‐long NW‐SE transect that extends from SE Sardinia Island to the NW Sicily continental margin, across the Cornaglia Terrace. The geophysical transect is coincident with a seismic reflection line from the Italian CROP experiment that we have re‐processed. The geophysical results, together with available basement dredges, support a basement along the profile fundamentally composed of continental‐type rocks, locally affected by subduction‐related magmatism. The continental nature of this region contrasts with the nature of the basement inferred along two geophysical cross‐sections located to the north of the Cornaglia Terrace in which seismic velocity of the lower crust supports significant magmatic crustal accretion. The comparison of these three cross‐sections supports that the highest magmatic activity occurred in the central and most extended region of the basin, whereas it was less important in the North and practically nonexistent in the South. These observations indicate abrupt variations of magmatism during the basin formation. As in other back‐arcs, the temperature, water content and composition of the mantle might have played an important role in such variation, but they fail to explain the abruptness of it. We propose that the interaction of the overriding continental lithospheres of Adria and Africa with the Apenninic‐Calabrian subduction system caused changes in slab rollback and trench retreat dynamics, which in turn resulted in variations of back‐arc stretching and magmatism. Based on our observations, we suggest that the Cornaglia Terrace formation process might share some similarities with the formation of oceanic crust in the Red Sea.

Chapter 13 Geology of Mesoproterozoic Chhattisgarh Basin, central India: current status and future goals

Abstract In the last two decades multiproxy studies involving process-based sedimentology, geochronology of interbedded tuff units from different stratigraphic levels, sediment geochemistry including stable isotope signatures and documentation of structural grains within selective stratigraphic intervals from the Chhattisgarh Basin, central India have resulted in a perception change on various aspects of the basin fill including its time frame, stratigraphic framework and depositional architecture in the space–time domain. In addition to establishing a Mesoproterozoic ( c. 1450–1000 Ma) time frame for the basin on a strong foothold, these studies also proposed revision of its stratigraphy by introducing new stratigraphic units at ‘formation’ and ‘group’ level. From collation of available data, their critical evaluation and presentation of new data, the present work proposes a four-tier lithostratigraphy for the Chhattisgarh Supergroup, namely, Singhora Group, Chadarpur Group, Raipur Group and Kharsiya Group. Further, application of sequence stratigraphic rationale allowed the basin succession to be subdivided into four and three nonconformity/unconformity-bound depositional sequences in its eastern and western parts, respectively. The present chapter also highlights the potential of the basin for carrying out studies related to Mesoproterozoic ocean oxygenation and outlines the necessity of well-planned geophysical transects to resolve issues related to tectonic setup of the basin.

New geophysical data on the depth structure of the Tien Shan intermontane depressions

New results of a detailed study of the Issyk Kul intermontane depression system by electromagnetic and geomagnetic methods are presented. A profile-based magnetometric survey across the water area of Lake Issyk Kul revealed a positive 30-km-wide magnetic anomaly at approximately +100 nT in intensity. The nature of this anomaly can be explained by the exposure of Late Paleozoic granitoid intrusions. The problems of the construction of a regional geoelectric model along the Barskaun magnetotelluric profile, which crosses the central Issyk Kul Depression in the meridional direction, are discussed. On the basis of generalization of previously obtained data from complex studies along the MANAS geophysical transect, a structural-geodynamic lithosphere section was constructed along this transect. In this section, the interrelationships between the surface and deep seismic and geoelectric structures are clearly reflected in the lithosphere of the Naryn-Atbashi intermontane depression system.

The role of inherited structures in the evolution of the Meknassy Basin, Central Tunisia, based on geological–geophysical transects

This paper uses seismic data, well data, and surface geologic data to present a detailed description of the Meknassy Basin in the Atlas fold and thrust belt of central Tunisia. These data reveal that the Meknassy Basin is bounded by major faults, along which Triassic evaporites have been intruded. The anticlines and synclines of the basin are delimited by two N–S main faults (the North–South Axis and the Sidi Ali Ben Oun fault) and are subdivided by associated N120° and N45° trending fault-related anticlines.The Meknassy Basin is characterized by brittle structures associated with a deep asymmetric geometry that is organized into depressions and uplifts. Halokinesis of Triassic evaporites began during the Jurassic and continued during the Cretaceous period. During extensional deformation, salt movement controlled the sediment accumulation and the location of pre-compressional structures. During compressional deformation, the remobilization of evaporites accentuated the folded uplifts. A zone of decollement is located within the Triassic evaporites.The coeval strike-slip motion along the bounding master faults suggests that the Meknassy Basin was initiated as a pull-apart basin with intrusion of Triassic evaporites. The lozenge structure of the basin was caused by synchronous movements of the Sidi Ali Ben Oun fault and the North–South Axis (sinistral wrench faults) with movement of NW–SE first-order dextral strike-slip faults. Sediment distribution and structural features indicate that a major tectonic inversion has occurred at least since Late Cretaceous and Cenozoic. The transpressional movements are marked by reverse faults and folds associated with unconformities and with remobilization of Triassic evaporites. The formation of different structural features and the evolution of the Meknassy Basin and its neighboring uplifts have been controlled by conjugate dextral and sinistral strike-slip movements and thrust displacement.

Geophysical transect across the North China Craton: A perspective on the interaction between Tibetan eastward escape and Pacific westward flow

The North China Craton (NCC) is composed of the Eastern and Western Blocks amalgamated along the Central Orogenic Belt (Trans-North China Orogen, TNCO). Geological, geochemical, geochronological and geophysical studies have confirmed that the lithosphere beneath the Eastern Block has undergone significant destruction and modification during the Mesozoic, whereas the Western Block preserves the Archean lithosphere. The mechanisms and crustal response to the lithospheric disruption remain unclear. Here we construct a lithosphere-scale transect across the Western and Eastern Blocks and the intervening TNCO at the southern margin of the NCC. We interpret the lithosphere thickness, crustal P-wave velocity, density and thermal structure, and also seismic activity. Our results show that: (1) crustal thinning is consistent with lithospheric thinning from west to east of this transect; (2) the seismogenic structure is distinctly different among the Western Block, the TNCO and the Eastern Block; (3) a marked increase in crustal density and average P-wave velocity exists at the contacts between the Western Block and TNCO, and the TNCO and Eastern Block, as well as beneath the Tanlu fault at the eastern margin of the Eastern Block; and (4) a positive correlation is displayed between increasing P-wave velocity (density, temperature) and the occurrence of Mesozoic basalts and high-Mg andesites. Under the tectonic background of the amalgamation pattern between the Eastern and Western Blocks within the NCC, we attribute the lateral variation of crustal geophysical features to the lateral variation of crustal composition or the interaction of mantle flow from the eastward escape from Tibetan collision, and the westward mantle flow beneath the eastern NCC, without excluding the west–east variation of crustal response to lithospheric disruption.

SAHKE geophysical transect reveals crustal and subduction zone structure at the southern Hikurangi margin, New Zealand

The Seismic Array Hikurangi Experiment (SAHKE) investigated the structure of the forearc and subduction plate boundary beneath the southern North Island along a 350 km transect. Tomographic inversion of first‐arrival travel times was used to derive a well‐resolved 15–20 km deep P wave image of the crust. The refracted phases and migrated reflection events image subducting slab geometry and crustal structure. In the west, Australian Plate Moho depth decreases westward across the Taranaki Fault system from 35 to ∼28–30 km. In the east, subducted Pacific Plate oceanic crust is recognized to have a positive velocity gradient, but becomes less distinct beneath the Tararua Ranges, where the interface increases in dip at about 15 km depth from &lt;5° to &gt;15°. This bend in the subducted plate is associated with vertical clusters in seismicity, splay fault branching, and low‐velocity high‐attenuation material that we interpret to be an underplated subduction sedimentary channel. We infer that a step down in the decollément transfers slip on the plate interface at the top of a subduction channel to the oceanic crust and drives local uplift of the Tararua Ranges. Reflections from the Wairarapa Fault show that it is listric and soles into the top of underplated sediments, which in turn abut the Moho of the overriding plate at ∼32 km depth, near the downdip end of the strongly locked zone. The change in dip of the Hikurangi subduction interface is spatially correlated with the transition from geodetically determined locked to unlocked areas of the plate interface.

DEEP CRUSTAL PROFILE ACROSS THE SOUTHERN KAROO BASIN AND BEATTIE MAGNETIC ANOMALY, SOUTH AFRICA: AN INTEGRATED INTERPRETATION WITH TECTONIC IMPLICATIONS

Two geophysical onshore-offshore lines on the southern margin of Africa form the Agulhas-Karoo Geophysical Transect (AKGT) and cross prominent geological features such as the Karoo Basin, Cape Fold Belt (CFB) and the Beattie Magnetic Anomaly (BMA). Geophysical data acquired along this AKGTransect between 2004 and 2007 within the Inkaba yeAfrica (lyA) framework, provide the platform for constructing a deep crustal section (IyA-200501) for the centre 100 km of the western AKGT-transect in order to resolve these features at depth. We present a detailed deep crustal model constructed from the joint interpretation of: i. archive data comprising surface geology, aeromagnetic data, nearby deep boreholes, teleseismic receiver functions and regional seismic reflection profiles, and ii. line coincident newly acquired high-resolution geophysical data consisting of near vertical seismic reflection data, shallow P- and S-wave velocity data, wide-angle refraction data, high resolution magnetotelluric data and impedance spectroscopy measurements on borehole samples. Our model differentiates four components in the up to 45 km thick crust: 1. a ~2 to 5 km thick folded Karoo Supergroup, disrupted by low-angle thrust faults rooted in a zone of local decollements in the lower Ecca Group and resting paraconformably on 2. a continuous undeformed sub-horizontal ~1.5 to 10 km thick wedge of the Cape Supergroup (CSG). This CSG wedge stretches from the Escarpment in the north to the tectonic front of the CFB in the south, and rests on an unconformity that dips about three degrees to the south. The angular unconformity is interpreted as an erosional peneplain that separates the CSG wedge from component 3. the ~13 to 21 km mid-crust basement below. The mid-crust contains a distinct north-dipping seismic fabric, here interpreted as ~1.4 to 1.0 Ga Mesoproterozoic Namaqua-Natal Metamorphic Belt (NNMB) crust. A south-dipping mid-crustal detachment, interpreted as a ductile thrust zone, separates the mid-crust from component 4. a highly reflective ~10 to 24 km thick lower crust. The latter is interpreted as an older Palaeoproterozoic section of the NNMB (or even Archean cratonic basement), and bounded by a ~2 to 5 km thick, highly reflective bottom layer below that lies sub- parallel to a clear Moho. This bottom layer is interpreted as a mafic underplate, metasomatic reaction zone, or lower-crust to mantle transition zone. Collectively the seismic reflection and wide-angle refraction data support an interpretation that the NNMB mid-crustal layer contains the BMA source, possibly connected to two zones of strong reflectivity: a ~10 to 12 km wide northern zone and a ~5 to 7 km wide southern zone, both about 5 km thick and 7 to 8 km below surface. We interpret the BMA source to be at least in part, a Namaqua-like massive to disseminated, deformed/metamorphosed stratiform sulphide-magnetite ore body with metasomatic overprint. The seismic reflection and -refraction data support an interpretation that a Pan-African suture zone at the BMA is absent and that instead, the NNMB continues below the CFB tectonic front, probably up to the continental margin and the Agulhas Fracture Zone. The seismic reflection data also supports a thin-skinned tectonic thrust model for the evolution of the CFB without significant fore-deep stratigraphic thickening of the Karoo Basin strata. A compatible tectonic model implies a Palaeozoic collision orogen setting, coupled to a south verging subduction zone much farther south of the CFB. Similarly, the geophysical data support a south dipping subduction zone during the amalgamation of the NNMB in the Mesoproterozoic. Current reconstructions of the Rodinia supercontinent link the NNMB and the Grenville Province of North America across the Grenville-Kibaran orogen. Our seismic section tests this reconstruction through a direct comparison with seismic profiles on the opposite flank of the orogen. Although the once adjacent continental blocks are now 1000s of kilometres apart, the seismic images show a good correlation and support the reconstruction.