Tectonic Subsidence Research Articles

Crustal and uppermost mantle structure of the NW Namibia continental margin and the Walvis Ridge derived from ambient seismic noise

SUMMARYThe Walvis Ridge (WR) is the most prominent hotspot track related to the opening in the South Atlantic Ocean. Several hypotheses have been developed to explain its origin and evolution. The presence of a massive magmatic structure at the landfall of the WR in Northwest Namibia raised speculation about the role of a hotspot during the opening of the South Atlantic ocean. To investigate its deeper velocity structure at the junction of the WR with the African continent was the focus of the amphibious seismological WALPASS experiment. In total 12 ocean-bottom seismometers and 28 broad-band land stations were installed between 2010 and 2012 to acquire seismological data. Here, we present the results of seismic ambient noise tomography to investigate to which extent the Tristan hotspot modified the crustal structure in the landward prolongation of the ridge and in the adjacent oceanic basins. For the tomography, vertical and hydrophone component cross correlations for >300 d for OBS stations and between 1 and 2 yr for land stations data were analysed. More than 49 000 velocity measurements (742 dispersion curves) were inverted for group velocity maps at 75 individual signal periods, which then had been inverted for a regional 3-D shear wave velocity model. The resulting 3-D model reveals structural features of the crust related to the continent–ocean transition and its disturbance caused by the initial formation of the WR ∼130 Ma. We found relatively thick continental crust below Northwest Namibia and below the near-shore part of the WR, a strong asymmetry offshore with typical, thin oceanic crust in the Namibe Basin (crossing over into the Angola Basin further offshore) to the North and a wide zone of transitional crust towards the Walvis Basin south of the WR.

The crustal structure and evolution of the Bunguran Trough, offshore Sarawak, Malaysia

The Bunguran Trough is a deep Tertiary basin depocentre in the West Luconia Delta Province, Sarawak Basin, offshore Malaysia. At the centre of the trough, the top of the acoustic basement is ∼15 km deep, buried beneath a thick pile of Late Eocene to Quaternary coastal-deltaic sediments of the proto-Rajang-Lupar River delta system. Sediment thickness, bathymetry and gravity data were utilized to re-construct the crustal structure of the West Luconia Delta and the Bunguran Trough. The results indicate an average crustal thickness of 5.3 km beneath the Bunguran Trough, which is extremely thin compared to the extended continental crust (∼15–22 km) beneath the adjacent West, Central and North Luconia provinces. Extreme crustal thinning is confined to the Bunguran Trough, which appears to have steeply faulted boundaries, but the nature of the faults is obscured by the thick sedimentary pile.Structural evidences from reflection seismic data suggest widespread extensional faulting throughout Sarawak Basin although some major tectonic lineaments may have had significant strike-slip movements due to later reactivation and basin inversion. Both extensional and strike-slip faulting may have contributed to the formation and subsidence of the Bunguran Trough. West Balingian Line is a major fault zone that seems to have had a significant strike-slip displacement history and probably had an important role during basin initiation. Seismic data suggests that this fault zone extends from the Balingian and Tatau provinces into West Luconia as a major strike-slip fault with a releasing bend along which the Bunguran Trough may have developed. Crustal extension in a pull-apart basin may have been initiated by sinistral motion along the fault during Late Eocene-Oligocene. Alternatively, the widespread normal faulting indicates that the Bunguran Trough may have developed as a rift basin which experienced at least two phases of extension before a major inversion event which is marked by the Middle Miocene Unconformity (MMU). The main phase of subsidence in the Bunguran Trough had occurred during the active extensional phase before the MMU.

Characteristics of the Earthquake Swarms in the Andaman Sea Region, India, from 1960 – 2020

The Andaman Sea region is the highly tectonically and seismically active region in India. This is the perfect site for studying earthquake swarms and how the swarms are reacting with the volcanic activities and tectonic processes. Using the International Seismological Centre (ISC) catalogue and USGS NEIC catalogue, we documented 17 earthquake swarms from 1960 to 2020. The hypocentre of the earthquake swarms was reported at 5 km. to 50 km and most of the incidents occurred below 20 km. depth. We examined the seismotectonic characteristics of these swarms. The moment tensor solutions of 6 swarms showing these events are controlled by pure normal faults with a significant strike-slip component, few are dominated by strike-slip faults but no thrust mechanisms are recorded. The b value distribution for the Andaman Sea region was calculated from the swarms’ data and the b value significantly varied between 1.4 –2.17. We conclude that the earthquake swarms in the Andaman and Nicobar Island region are probably the result of magma intrusion in the thin and hot oceanic crust. High b values, active volcanoes and cratered seamount support the volcanic background of these swarms.

Shallow marine glauconitization during the Proterozoic in response to intrabasinal tectonics: A study from the Proterozoic Lower Bhander Sandstone, Central India

Glauconite forms authigenically by the replacement of quartz and feldspar substrates within the Precambrian Lower Bhander Sandstone (LBS) of the Vindhyan Basin. The glauconite exhibits consistently high K2O (7.08–8.62 wt%) and low to moderate TFe2O3 (6.20–18.5 wt%). The X-Ray Diffraction study reveals an evolved to highly evolved glauconitic mica structure. The deposition of mudstone-dominated LBS took place in a shallow marginal marine environment within a low gradient epeiric sea. The overall fining upward succession of the LBS comprises multi-storied, meter scale progradational cycles defined by red mudstone and sandstone–siltstone alternation in ascending order. The repetitive cycles indicate recurrent low-magnitude basin subsidence of the epeiric seafloor. Slump folds and slide planes, associated with the wedge-shaped mud pebble conglomerate, mark each episodes of basin subsidence. The glauconite occurs within sandstone and the sandy matrix of a wedge-shaped conglomerate at the base of a shallowing upward cycle which starts with a grey colored mudstone. The enrichment factors of redox-sensitive elements elucidate sub-oxic depositional condition during the glauconitization. The redox-sensitive element ratios (V/Cr, V/Sc) corroborates the sub-oxic depositional condition for the glauconitic interval and oxic environment for the encasing sediments. The formation of glauconite in the LBS corresponds to the deepening of the basin in response to the tectonic subsidence. The meter scale progradational cycles, recording the transition from suboxic to oxic conditions, corresponds to tectonically-induced deepening and subsequent filling. This study recognizes the significance of intrabasinal tectonics in facilitating suitable redox conditions for the glauconitization process within Proterozoic shallow epeiric sea deposit.

The continent-to-ocean transition in the Iberia Abyssal Plain

Abstract Conceptual models of magma-poor rifting are strongly based on studies of the nature of the basement in the continent-to-ocean transition of the Iberia Abyssal Plain, and suggest that exhumed mantle abuts extended continental crust. Yet, basement has only been sampled at a few sites, and its regional nature and the transition to seafloor spreading inferred from relatively low-resolution geophysical data are inadequately constrained. This uncertainty has led to a debate about the subcontinental or seafloor-spreading origin of exhumed mantle and the rift-related or oceanic nature of magmatic crust causing the magnetic J anomaly. Different interpretations change the locus of break-up by >100 km and lead to debate of the causative processes. We present the tomographic velocity structure along a 360-km-long seismic profile centered at the J anomaly in the Iberia Abyssal Plain. Rather than delineating an excessive outpouring of magma, the J anomaly occurs over subdued basement. Furthermore, its thin crust shows the characteristic layering of oceanic crust and is juxtaposed to exhumed mantle, marking the onset of magma-starved seafloor spreading, which yields the westward limit of an ~160-km-wide continent–ocean transition zone where continental mantle has been unroofed. This zone is profoundly asymmetric with respect to its conjugate margin, suggesting that the majority of mantle exhumation occurs off Iberia. Because the J anomaly is related to the final break-up and emplacement of oceanic crust, it neither represents synrift magmatism nor defines an isochron, and hence it poorly constrains plate tectonic reconstructions.

Joint inversion of receiver functions and surface wave dispersion in the Recôncavo–Tucano basin of NE Brazil: implications for basin formation

SUMMARYThe crustal structure of the Recôncavo–Tucano basin, an aborted rift system that developed in NE Brazil during extension related to the opening of the South Atlantic Ocean, has been investigated through local constraints from receiver functions developed at 18 seismic stations in the region. Gravity modelling has proved unable to unequivocally localize crustal thinning under the basin depocentre and, together with a general lack of sediments from a putative thermal sag phase, this has led to a range of basin formation models invoking either pure or simple shear or a combination of both. In particular, the ‘flexural cantilever’ model has assumed simple shear extension in the upper crust and pure shear extension in the lower crust and mantle, enabling local erosion of the rift flanks after footwall uplift and regional erosion of the thermal sag phase after magmatic underplate of the basin's crust. Our results reveal that the crust is over 40 km thick beneath the Tucano and Recôncavo basins and that it contains a thick (5–8 km) layer of high velocity (Vs > 4.0 km s–1) material below ∼35 km depth. These observations contrast with structure immediately West (São Francisco Craton) and East (Borborema Province) of the basin, for which crustal thicknesses average 42 and 36 km, respectively, lower crustal velocities are below 4.0 km s–1, and local instances of crust as thin as 33.5 km are observed. We propose, in agreement with the ‘flexural cantilever’ model, that the fast velocity layer making the basin's lowermost crust resulted from mafic underplating after stretching and thinning during the syn-rift phase, restoring crustal thickness to pre-rift values (or larger) and providing the necessary buoyancy to trigger regional uplift. Moreover, although not pervasive, instances of thin crust along the footwall could be related to rift flank erosion. We thus conclude that, regardless of the mode of extension in the upper crust, our results favour models of basin formation invoking extension of the lower crust by pure shear.

The Moho reflectivity of the subduction beneath the Southwestern Alps from ambient seismic noise autocorrelations

SUMMARY The Western Alps shows a complex crustal organization due to the subduction of the European Plate beneath the Adriatic Plate and exhumation of the mantle wedge. The lithospheric structure of the Western Alps, that may hold significance for understanding orogenic processes and evolution, has been the subject of many geophysical studies, but the Moho profile remains unclear and this has led to controversies about the depth and extent of the European Plate beneath the Adriatic Plate. With the goal of retrieving detailed information on crustal constitution, we use autocorrelation of seismic ambient noise as a tool to map the body wave reflectivity structure at the subduction zone under the southwestern Alps. We use data recorded by the China–Italy–France Alps (CIFALPS) seismic transect, that includes 45 stations located approximately 5–10 km apart along a profile crossing the Alpine continental subduction in the Western Alps. We analyse the data set in four different frequency bands between 0.09 and 2 Hz. We automatically pick the arrival time of the Moho reflection in the second derivative of the envelope of the autocorrelation stack using prior Moho information. The 0.5–1 Hz frequency band mostly gives the best result due to the clear changes in reflectivity along the waveforms of the autocorrelation stacks after the picked arrival times of the Moho reflections. We find spatial coherence between 18 and 23 km depth in the western portion of the profile, indicating relatively homogeneous crustal rocks, and highly reflective structure under the central mountain range, due to the existence of a highly faulted zone. The very thin crust and the underlying mantle wedge known as the Ivrea body show instead high transparency to seismic waves and absence of reflections. The subduction profile of the European Plate shows a steep trend as compared to previous studies. We discuss autocorrelation stacks and Moho depths obtained from the arrival times of the picked reflectivity change in comparison with previous studies to validate the different reflection structures. Stacked ambient noise autocorrelations reliably image varied crustal properties and reflectivity structures in the highly heterogeneous region of the southwestern Alps.

Crustal structure across the Teisseyre-Tornquist Zone offshore Poland based on a new refraction/wide-angle reflection profile and potential field modelling

This paper presents a 230-km long refraction/wide-angle reflection profile that was acquired in the southern Baltic Sea across the Teisseyre-Tornquist Zone (TTZ), the boundary between the East European Craton (EEC) and the West European Platform (WEP). This profile is nearly parallel to the western Polish coast, halfway between Poland and the Danish island of Bornholm. The data acquisition was conducted with 15 ocean bottom seismometers (OBS) and 2 land stations. We applied the trial-and-error iterative ray-tracing technique using all identified seismic phases to derive crustal models by minimizing misfit between calculated and observed P-wave travel-times for individual layers. Final velocity model was further verified by forward potential field modelling, testing various P-wave velocity (Vp) to density relationships. The Moho boundary was inferred at 33–38km depth, deepening towards the EEC, with the local ~3km rise in a 40-km-wide zone north of the Caledonian Deformation Front, corresponding to the elevated middle-crust velocities Vp > 6.5 km/s. The lower and middle crust are mostly continuous along the BalTec profile with only minor perturbations between the EEC and WEP. Nevertheless, the thickness of these crustal layers is poorly constrained by seismic data along the SW section of the profile. In contrast, the upper crust and sedimentary cover can be subdivided into three domains, corresponding, from the NE to SW, to the EEC, TTZ and WEP, respectively. The EEC is characterised by the flat top of the basement, uniformly inclined towards the SW. The TTZ shows rapid thinning of cratonic upper crust and thickening of sedimentary layer. The WEP reveals, despite limited seismic coverage, a 13-km thick sedimentary overburden. The lateral changes of seismic structure within the upper crust and sedimentary cover along the BalTec profile can be attributed to protracted phases of late Palaeozoic-Mesozoic extension punctuated by the Carboniferous and Late Cretaceous inversion phases.

Contrasting Modes of Carbonate Precipitation in a Hypersaline Microbial Mat and Their Influence on Biomarker Preservation (Kiritimati, Central Pacific)

Microbial mats represented the earliest complex ecosystems on Earth, since fossil mineralized examples (i.e., microbialites) date back to the Archean Eon. Some microbialites contain putative remains of organic matter (OM), however the processes and pathways that lead to the preservation of OM within microbialite minerals are still poorly understood. Here, a multidisciplinary study is presented (including petrographic, mineralogical and organic geochemical analyses), focusing on a modern calcifying mat from a hypersaline lake in the Kiritimati atoll (Central Pacific). The results show that this mat has a complex history, with two main growth phases under hypersaline conditions, separated by an interruption caused by desiccation and/or freshening of the lake. The mineral precipitates of the mat are predominantly aragonitic and two contrasting precipitation modes are observed: the main growth phases of the mat were characterized by the slow formation of irregular micritic particles with micropeloidal textures and subspherical particles, linked to the degradation of the exopolymer (EPS) matrix of the mat; whereas the interruption period was characterized by the rapid development of a thin but laterally continuous crust composed of superposed fibrous aragonite botryoids that entombed their contemporaneous benthic microbial community. These two precipitation modes triggered different preservation pathways for the OM of the mat as the thin crust shows a particular lipid biomarker signature, different from that of other layers and the relatively rapid precipitation of the crust protecting the underlying lipids from degradation, causing them to show a preservation equivalent to that of a modern active microbial community, despite them being >1100 years old. Equivalent thin mineral crusts occur in other microbialite examples and, thus, this study highlights them as excellent targets for the search of well-preserved biomarker signatures in fossil microbialites. Nevertheless, the results of this work warn for extreme caution when interpreting complex microbialite biomarker signatures, advising combined petrographic, mineralogical and geochemical investigations for the different microbialite layers and mineral microfabrics.

Hyperextension and polyphase rifting: Impact on inversion tectonics and stratigraphic architecture of the North West Shelf, Australia

Restoration of interpreted 2D deep seismic lines across Australia's North West Shelf indicates that rifting style changed significantly during multiple phases of extension and continental breakup. Early phase rifting during the Neo-Proterozoic and Early Paleozoic, characterised by low-angle detachment faults, crustal-scale necking and hyperextension of the crust (Metamorphic Core Complex Mode), was localised over pre-existing Proterozoic orogenic belts formed adjacent to major cratonic blocks. Subsequent extensional events in the Late Paleozoic and Mesozoic reactivated these low-angle rift fault systems. However, a series of narrow rift basins (Narrow Rift Mode), bound by high-angle normal faults that cut into the upper mantle, also began forming due to changes in the rheological architecture of the lithosphere following previous extension, crustal thinning and post-rift cooling. Wide rift mode extension, wherein extensional deformation was distributed over broad areas, may have occurred prior to, or in conjunction with, the development of more localised narrow rifts (dual mode rifting) in areas such as the Northern Carnarvon and Browse basins. Extensional deformation progressively localised into the narrow rift basins, which in some cases, matured into seafloor spreading centres during breakup, typically outboard of the major low-angle detachment fault systems. The resulting general structural architecture evolution is consistent with several other hyperextended margins from around the world.The rift events were punctuated by periods of thermal sag, often in conjunction with pulses of compressional deformation associated with plate-scale tectonic events. Structural restoration of 2D regional seismic lines shows a marked difference in the way the different polyphase rift basin elements responded to shortening. Low-angle detachment faults were relatively well oriented for reactivation under compression, leading to the development of inversion anticlines and associated convergent unconformities towards the inboard basin margin. In contrast, high-angle normal fault systems associated with the pre-existing narrow rift basins were poorly oriented for reactivation during compression and tend to be characterised by minor wrench/flower structures, generally with relatively limited uplift and erosion on the basin margins. Regional in-plane compressional stresses also worked in combination with vertical loading due to thermal decay to downwarp the lithosphere, forming broad rapidly subsiding synclinal basins above axes of the pre-existing rift basins. The evolving structural architecture developed during the compressional events had significant impacts on the associated sequence stratigraphic architecture.

87Sr/86Sr of coral reef carbonate strata as an indicator of global sea level fall: Evidence from a 928.75-m-long core in the South China Sea

Reconstructing the precise timing of sea level fall is important for understanding earth system during the geologic time. However, previous studies are limited by the absence of effective indicators for sea level fall. To overcome this limitation, we investigated the 87Sr/86Sr and petrologic features of carbonate strata in a 928.75-m-long coral reef core (CK2) from Xisha Islands, northern South China Sea and their relationship to sea level fluctuations. Ten exposed surfaces were identified by their petrologic features. The Mn and Sr content, Mn/Sr, Sr/Ca and δ18O suggest that most of the coral reef carbonate in the CK2 core preserved the original seawater 87Sr/86Sr except at intervals of 672–616, 596–575, 521–491, 423–414, and 192.5–161 m, which suffered alteration of meteoric water and showed higher 87Sr/86Sr during low sea level period. The 160 carbonate 87Sr/86Sr showed a continuously increasing trend, which is consistent with that of seawater over the same period. The continuously increasing 87Sr/86Sr curve of CK2 was divided into seven units by six 87Sr/86Sr hiatuses. Inconsistency between the sedimentation rate of CK2 and tectonic subsidence rate in the Xisha area suggest that the continuously increasing 87Sr/86Sr was controlled by the coupled effect of tectonic subsidence and sea level change, and the hiatus and higher 87Sr/86Sr interval in the 87Sr/86Sr curve were controlled by sea level fall. The higher 87Sr/86Sr usually produced during low sea level (<5 m), 87Sr/86Sr hiatus usually produced when sea level changed from deeper to shallower. The depth of hiatuses and higher intervals of 87Sr/86Sr correspond well with the exposed surface in CK2 core, verifying that the hiatuses and higher intervals of 87Sr/86Sr in the coral reef carbonate succession provide an effective indicator of sea level fall. Using this indicator and the CK2 87Sr/86Sr profile, we identified eleven large-scale sea level falls and three dramatic regressions during 19.6–0.2 Ma and constrained the age based on strontium isotope stratigraphy. These interpreted sea level falls are mostly consistent with other records of global sea level changes, confirming the reliability of our records. The 87Sr/86Sr profile of a coral reef carbonate is an efficient indicator of past sea level falls, including their timing.

Low-accommodation foreland basin response to long-term transgression: A record of change from continental-fluvial and marginal-marine to open-marine sequences over 60,000 km2 in the western Canada foreland basin

Low-accommodation foreland basins have been of recent interest because their stratigraphic architectures provide insight into the interaction of tectonic subsidence and eustasy. Aptian-aged strata in Alberta, Canada (McMurray Formation and Wabiskaw Member) were deposited under the influence of a continental-scale river that longitudinally transported and debouched sediment into the southward-transgressing Boreal Sea in the distal, low-accommodation Western Canada Foreland Basin (WCFB). In this study, we present a high-resolution stratigraphic framework for the McMurray-Wabiskaw interval, which was deposited during a long-term transgression. Using wireline logs from >20,000 wells and more than 500 cores, regional mapping focused on resolving stratigraphic relationships and the distribution of depositional environments across approximately 60,000 km2 in the Athabasca Oil Sands Region of northeastern Alberta. The studied stratigraphic interval is part of a third-order sequence: the McMurray Formation developed during a period of relative sea-level fall, lowstand, and early transgression, while the overlying Wabiskaw Member was deposited during late transgression. Within the McMurray Formation, superimposed fourth-order sea-level fluctuations in this low accommodation setting created a complex amalgam of deltaic strata vertically and laterally juxtaposed with valley-fill deposits. Relative sea-level rises resulted in rapid transgressions in which the shorelines migrated >400 km landward. Thin (5–15 m thick), widespread deltaic parasequence sets were deposited during subsequent sea-level highstands. During relative sea-level falls, fluvial meandering channel belts developed in downcutting valleys, with associated shorelines dramatically prograding basinward. In the Wabiskaw Member, continued sea-level rise coupled with potentially decreasing sediment supply resulted in the development of tide-influenced/dominated channels and bars in estuaries. The final phase of the third-order transgression was marked by a regional transgressive ravinement event and several southwestward backstepping wave-dominated shorefaces/strandplains. This low-accommodation foreland basin succession demonstrates a transition from deposition in continental fluvial and marginal-marine settings subject to high-frequency eustatic changes, to open-marine conditions over a long-term transgression.

Mantle and Crustal Contributions to the Mount Girnar Alkaline Plutonic Complex and the Circum-Girnar Mafic-Silicic Intrusions of Saurashtra, Northwestern Deccan Traps

ABSTRACT Continental flood basalt (CFB) provinces, while dominated by tholeiitic basalts and basaltic andesites, often also contain alkaline mafic to felsic lavas and intrusions. The tholeiitic and alkaline magmas may reflect different degrees of partial melting of the same mantle source, or the alkaline magmas may be derived from metasomatised, incompatible element-enriched mantle sources. The tholeiitic and alkaline suites, even if closely associated spatially or temporally, require independent magmatic plumbing systems. In the Saurashtra region of the northwestern Deccan Traps CFB province, India, tholeiitic lavas have been intruded by the ~66 Ma Mount Girnar plutonic complex, which comprises olivine gabbros (often with cumulate textures), diorites, and monzonites, profusely intruded by dykes and veins of foid-bearing syenites and lamprophyres. In the region surrounding the complex the tholeiitic lavas have been intruded by a large (12 km-diameter) silicic ring dyke, as well as tholeiitic dykes and sills. The region thus provides an excellent opportunity to study potential petrogenetic relationships between tholeiitic, alkaline, and silicic magmatism in a CFB province, evaluated here using field, petrographic, mineral chemical, and whole-rock geochemical (including Sr–Nd–Pb isotopic) data. Initial (at 65 Ma) Sr–Nd–Pb isotopic ratios of an olivine gabbro and diorites of the Girnar plutonic suite are in the ranges (87Sr/86Sr)t = 0.70499 to 0.70584, (143Nd/144Nd)t = 0.512675 to 0.512484 (εNdt = +2.4 to −1.4) and (206Pb/204Pb)t = 18.270–18.679. Foid-bearing syenites and lamprophyres have broadly similar isotopic ratios and marked enrichments in the most incompatible elements. Thermobarometric calculations indicate crystallisation of mineral phases in the Girnar plutonic suite at varied crustal pressures (0.02–0.9 GPa). Small but significant Sr–Nd–Pb isotopic variations within the plutonic suite rule out closed-system fractional crystallisation as a viable process, whereas a lack of correlation between isotopic ratio and degree of magmatic evolution (rock type) also negates any simple scheme of combined assimilation–fractional crystallisation. The circum-Girnar tholeiitic intrusions, hitherto practically unstudied, are low-Ti and moderately to fairly evolved (MgO = 8.0–3.9 wt %); olivine gabbro and picrite dykes with cumulus olivine show higher MgO (10.1–15.7 wt %), Ni (360–700 ppm), and Cr (410–1710 ppm) contents. The circum-Girnar tholeiitic intrusions have a large range of Sr–Nd–Pb isotopic ratios (e.g. εNdt = +4.2 to −18.7) indicating open-system processes. We infer that magmas of the alkaline Girnar plutonic suite were derived from enriched mantle, with only minor crustal residence or material input, possibly reflecting a very thin basement crust under the complex. In contrast, magmas forming the circum-Girnar tholeiitic intrusions were derived from depleted mantle (εNdt &amp;gt; +4.2) by high degrees of melting, and they experienced olivine fractionation or accumulation in crustal chambers and significant contamination by ancient granitic basement crust. These features probably reflect a much thicker crust surrounding the plutonic complex than directly under it. The circum-Girnar silicic ring dyke has Sr–Nd–Pb isotopic ratios suggesting an origin by anatexis of the basement crust. Based on a range of evidence, the tholeiitic and silicic circum-Girnar dykes and sills are petrogenetically and structurally unrelated to the alkaline Girnar plutonic suite.

Preservation of the Iberian Tethys paleomargin beneath the eastern Betic mountain range

We obtain P-wave receiver functions from teleseismic earthquake recordings at a dense seismic broadband transect, deployed along 170 km across the Betic mountain range in southeastern Spain. Migrated images show the crustal structure of the orogen in detail. In particular, they reveal the situation of the subducted Iberian paleomargin, with full preservation of the proximal domain and the ∼50 km wide necking domain. Crustal thinning across the necking domain affects mainly the lower continental crust. The Variscan crust of the Tethys margin is bending downward beneath the Betics, reaching ∼45 km depth, and terminates abruptly at a major slab tear. The distal domain of the paleomargin cannot be reconstructed, but the migrated section suggests that material has been exhumed through the subduction channel and integrated into the Betic orogen. This supports an origin of the HP-LT Nevado-Filabride units from subducted, hyperextended Variscan crust. According to our profile, the present-day eastern Betics appear to have a much more significant contribution from metamorphic Iberian crust than previously thought.

A new perspective on the continent–ocean boundary of Palawan continental block identified with geophysical data

The location of the Continent–ocean boundary (COB) between the Palawan continent block (PCB) and the central oceanic basin remains controversial. There are more than 100 km difference among the scholars' view about the specific positions of the COB. To obtain a more reasonable location of the COB in the PCB, a comprehensive interpretation was conducted based on new shipborne geophysical data, including bathymetry, gravity, magnetic and reflection seismic data. In particular, upward continuation was applied to the new stern-towing magnetic data, and analytical signal analysis (ASA) processing was adopted for the total field magnetic anomalies, including both the public and stern-towing magnetic data. The most reasonable COB of PCB is just identified with the geophysical data based on the comprehensive structures' anomalies, such as the foot of massif slope and the magmatic volcanoes in topographic data, and the gentle gradient of the free-air and Bouguer gravity anomalies, and the clear dividing boundary in magnetic ASA sharp anomaly, and the seismic profiles structure's display in the Continent–ocean transition (COT). Meanwhile, structural modeling along the seismic profiles are used to constrained the identification. The refined convex–concave's COB is distributed at the seaward limit of the continental crust along the foot of northwest Palawan slope with a hyper-extended thinner crust, seaward-dipping listric faults, and is reasonably located between the extensive thinned continental crust and expansive subducting oceanic crust.

The structure of the tectonosphere the Mozambique and Madagascar ridges by geophysical data

The study area occupies the southwestern part of the Indian Ocean, where the Mozambique and Madagascar ridges are located. Ideas about the tectonic structure of these ridges are still debatable. The anomalous structure of the crust of these ridges can be explained either by underplating — thickening of the oceanic crust from below due to magmatism, or by stretching and thinning of the continental crust. Based on the data on anomalous gravitational and magnetic fields, seismotomography, and other geological and geophysical information, density modeling was performed for four profiles, which revealed differences in the crustal structure and evolution of the Mozambique and Madagascar ridges.

Timing of Eocene compressional plate failure during subduction initiation, northern Zealandia, southwestern Pacific

SUMMARY Rapid onset of subduction tectonics across the western Pacific convergent margins in the early Eocene was followed by a slower phase of margin growth of the proto Tonga-Kermadec subduction system north of Zealandia during a middle Eocene phase to tectonic adjustment. We present new age constraints from International Ocean Discovery Program Expedition 371 borehole data on deformation events in northern Zealandian sediments that document the formation of the convergent margin boundary northwest of New Zealand. The deformation shows a shortening event that lasted up to 20 myr and acted over distances of ∼1000 km inboard of the evolving plate margin, just northwest of New Zealand. Multichannel seismic profiles tied to our new borehole sites show shortening occurred predominantly between 45 Ma and 35 Ma with some deformation related to slope failure continuing into the Oligocene. The termination of shortening is linked to opening of the backarc basins of the southwest Pacific and the migration of the Tonga-Kermadec Trench to the east which may have removed the structural evidence of the Eocene plate margin. Paleogene deformation observed inboard of the evolving proto Tonga-Kermadec subduction system indicates that the lithosphere of northern Zealandia, a region of thin continental crust, was strong enough to act as a stress guide. Compressive stresses that caused intra plate folding and faulting developed behind the initiating subduction system with the finite period of deformation indicating the time frame over which an active convergent margin lay along the northern margin of Zealandia.

Lithospheric Structure of the East European Craton at the Transition from Sarmatia to Fennoscandia Interpreted from the TTZ-South Seismic Profile (SE Poland to Ukraine)

The TTZ-South seismic profile follows the Teisseyre-Tornquist zone (TTZ) at the SW margin of the East European craton (EEC). Investigation results reveal the upper lithospheric structure as representing the NW-vergent, NE-SW striking overthrust-type, Paleoproterozoic (~1.84–1.8 Ga) Fennoscandia-Sarmatia suture. The Sarmatian segment of the EEC comprises two crustal-scale tectonic thrust slices: the Moldavo-Podolian and Lublino-Volhynian basement units, overriding the northerly located Lysogoro-Radomian unit of Fennoscandian affinity. The combined results of the TTZ-South and other nearby deep seismic profiles are consistent with a continuation of the EEC cratonic basement across the TTZ to the SW and its plunging into the deep substratum of the adjacent Paleozoic platform. Extensional deformation responsible for the formation of the mid to late Proterozoic (~1.4–0.6 Ga), SW-NE trending Orsha-Volhynia rift basin is probably also recorded. The thick Ediacaran succession deposited in the rift was later tectonically thickened due to Variscan deformation. The Moho depth varies between 37 and 49 km, resulting in the thinnest crust in the SE, sharp depth changes across the TTZ, and slow shallowing from 49 to 43 km to the NW. The abrupt Moho depth increase from 43 to 49 km is considered to reflect the overlying lower crust tectonic duplication within the suture zone.

Early Pliocene marine transgression into the lower Colorado River valley, southwestern USA, by re-flooding of a former tidal strait

Abstract Marine straits and seaways are known to host a wide range of sedimentary processes and products, but the role of marine connections in the development of large river systems remains little studied. This study explores a hypothesis that shallow-marine waters flooded the lower Colorado River valley at c. 5 Ma along a fault-controlled former tidal strait, soon after the river was first integrated into the northern Gulf of California. The upper bioclastic member of the southern Bouse Formation provides a critical test of this hypothesis. The upper bioclastic member contains wave ripple-laminated bioclastic grainstone with minor red mudstone, pebbly grainstone with hummocky cross-stratification (HCS)-like stratification and symmetrical gravelly ripples, and calcareous-matrix conglomerate. Fossils include upward-branching segmented coralline-like red algae with no known modern relatives but confirmed as marine calcareous algae, echinoid spines, barnacles, shallow-marine foraminifers, clams, and serpulid worm tubes. These results provide evidence for deposition in a shallow-marine bay or estuary seaward of the transgressive backstepping Colorado River delta. Tsunamis generated by seismic and meteorological sources likely produced the HCS-like and wave-ripple cross-bedding in poorly-sorted gravelly grainstone. Marine waters inundated a former tidal strait within a fault-bounded tectonic lowland that connected the lower Colorado River to the Gulf of California. Delta backstepping and transgression resulted from a decrease in sediment output due to sediment trapping in upstream basins and relative sea-level rise produced by regional tectonic subsidence.

Detachment-controlled subsidence pattern at hyper-extended passive margin: Insights from backstripping modelling of the Baiyun Rift, northern South China Sea

How detachment fault controls the basin subsidence at hyper-extended passive margin remains mysterious. Based on a detailed geological interpretation and backstripping on 29 high-resolution multi-channel seismic profiles, we calculate the basin-scale tectonic subsidence across the Baiyun Rift to study the relationship between tectonic subsidence and detachment faulting. Our results confirm that detachment faulting is the primary factor controlling the syn-rift subsidence. A decaying subsidence rate, or deficit subsidence is observed in the eastern Baiyun Rift from the early to late Eocene, which is in marked contrast to the intensive brittle stretching. We attribute this to the doming of the detachment fault footwall involving the syn-rift magmatism during its hyper-extension process. While in the western Baiyun Rift the average subsidence rate is over 45 m Myr−1 higher than that in the eastern as the response of flexural loading in the late Eocene. During the post-rift stage, the subsidence rate shows a similar oceanward increasing trend, and an anomalous rapid subsidence event during the middle Miocene in the eastern Baiyun Rift, which can be ascribed to the magma-assisted weakened rheological condition of the lithosphere deriving from the syn-rift detachment faulting. We propose that this relationship between subsidence pattern and detachment faulting has global applicability for other hyper-extended rift systems developing detachments and magmatic-related dome structures.