Rift Evolution Research Articles

Comment on Strauss et al., 2020: Subsidence analysis of salt tectonics‐driven minibasins (Northern Calcareous Alps, Austria)

Comment on Strauss et al., 2020: Subsidence analysis of salt tectonics‐driven minibasins (Northern Calcareous Alps, Austria)

Stappen High–Bjørnøya Tectono-Sedimentary Element, Barents Sea

Abstract The Stappen High–Bjørnøya Tectono-Sedimentary Element (TSE) is located in the western Barents Shelf and is one of the prominent tectonic elements in the area. The Stappen High comprises a shallow platform, and Bjørnøya forms its highest point with exposed outcrops. Modern seismic reflection data of improved quality in the southern part of the TSE and vintage seismic data in the northern portion are utilised. Together with updated geological information at Bjørnøya, the study provides insights into the Paleozoic–early Cenozoic evolution of the Stappen High–Bjørnøya TSE. In this context, we discuss structural inheritance and the rift development, and account for confirmed and potential hydrocarbon systems and plays.

Cenozoic structure and tectonics of North subbasins in Beibu Gulf Basin, northern South China Sea

The spatial and temporal interaction of rifting at the northern South China Sea (SCS) margin and shearing along Red River Fault Zone (RRFZ) is crucial to understanding the regional geodynamics. Their obscure relationship has led to controversial models for the origin of the Beibu Gulf basin within the northeast edge of the SCS. Newly released seismic-reflection data and well logs permit a new comprehensive analysis of the tectono-sedimentary history, which provides important constraints on the basin-forming history. Based on the interpretation of 3D multi-channel seismic data, we analyzed the seismic stratigraphy, fault development and tectonic subsidence, with additional confirmation from analogue modelling. The results suggest that several key unconformities separate the rifting history of North subbasins into three distinct phases. The syn-depositional faults and depocenters experienced a tectonic transition from NE- to NNE-orientated during the first two stages. The extensional structures were overprinted by the conjugate strike-slip faulting during stage III (30–23 Ma), as evidenced by the NEE-trending predominant faults and reactivation of NW-trending blind faults. Together with other regional geologic records and plate reconstruction history, we propose an updated view of the region's tectonic history. The NE-trending rift system initiated at the beginning of the Cenozoic. Tectonic reconfiguration at ~40 Ma induced a regional change of stress field to become north-south stretching that caused a tectonic transition of the basin. Influenced by the regional shearing activity of the RRFZ as well as additional north-south stretching, the basin was reworked by widespread strike-slip deformation. We therefore emphasize that while the basin's overall architecture was defined by the development of NE-trending rifts, both reconfiguration of the stress field acting on pre-existing faults and strike-slip reactivation of blind faults led to the basin's current intricate structure.

Normal Fault Kinematics and the Role of Lateral Tip Retreat: An Example From Offshore NW Australia

AbstractUnderstanding how normal faults grow is key to determining the tectono‐stratigraphic evolution of rifts. According to recent studies, normal faults tend to grow in two temporally distinct stages: a lengthening stage, followed by a throw/displacement accumulation stage. However, this model is still debated and not widely supported by many additional studies. Relatively few studies have investigated what happens to a fault as it becomes inactive, that is, does it abruptly die, or does the at‐surface trace‐length progressively shorten by so‐called tip retreat? We, here, use a 3D seismic reflection data set from the Exmouth Plateau, offshore Australia, to develop a three‐stage fault growth model for seven normal faults of various sizes and to show how the throw‐length scaling relationship changes as a fault dies. We show that during the lengthening stage, which lasted <30% of the faults’ lives, faults reached their near‐maximum lengths, yet accumulated only 10%–20% of their total throw. During the throw/displacement accumulation stage, which accounts for c. 30%–75% of the faults’ lives, throw continued to accumulate along the entire length of the faults. All of the studied faults also underwent a stage of lateral tip‐retreat (last c. 25% of the faults’ lives), where the active at‐surface trace‐length decreased by up to 25%. The results of our study may have broader implications for fault growth models, slip rate variability during fault growth, and the way in which faults die, in particular the role of lateral tip‐retreat.

Controls on Asymmetric Rift Dynamics: Numerical Modeling of Strain Localization and Fault Evolution in the Kenya Rift

AbstractComplex, time‐dependent, and asymmetric rift geometries are observed throughout the East African Rift System (EARS) and are well documented, for instance, in the Kenya Rift. To unravel asymmetric rifting processes in this region, we conduct 2D geodynamic models. We use the finite element software ASPECT employing visco‐plastic rheologies, mesh‐refinement, distributed random noise seeding, and a free surface. In contrast to many previous numerical modeling studies that aimed at understanding final rifted margin symmetry, we explicitly focus on initial rifting stages to assess geodynamic controls on strain localization and fault evolution. We thereby link to geological and geophysical observations from the Southern and Central Kenya Rift. Our models suggest a three‐stage early rift evolution that dynamically bridges previously inferred fault‐configuration phases of the eastern EARS branch: (1) accommodation of initial strain localization by a single border fault and flexure of the hanging‐wall crust, (2) faulting in the hanging‐wall and increasing upper‐crustal faulting in the rift‐basin center, and (3) loss of pronounced early stage asymmetry prior to basinward localization of deformation. This evolution may provide a template for understanding early extensional faulting in other branches of the East African Rift and in asymmetric rifts worldwide. By modifying the initial random noise distribution that approximates small‐scale tectonic inheritance, we show that a spectrum of first‐order fault configurations with variable symmetry can be produced in models with an otherwise identical setup. This approach sheds new light on along‐strike rift variability controls in active asymmetric rifts and proximal rifted margins.

Long-term throw rates and landscape response to tectonic activity of the Tunka Fault (Baikal Rift) based on morphometry

Determining the fault displacement rates and the sequence of formation of intra-rift structures are essential aspects in the study of the evolution of intracontinental rifts. To better understand the development of the Tunka system of basins (Baikal Rift) and the influence of tectonics on landscape evolution, we conducted a morphometric analysis of the Tunka Fault and its transverse drainage network. We studied geomorphic parameters of 64 facets and 74 drainage basins within the Tunka mountain-front in the footwall of the Tunka Fault; these parameters include mountain front sinuosity, the ratio of facet height and width to base length, basin shape, hypsometric integral, asymmetry factor, and the valley width to height ratio. Our main objectives were to determine long-term throw rates for specific mountain front segments and estimate the timing of corresponding geomorphic structures, to characterize the geomorphological response of the transverse drainage systems to fault movements, and to understand the relationship between the morphometry and kinematics of different segments along the Tunka Fault. The analysis of the Tunka mountain front reveals evidence for strong tectonic control on its morphology. We found that the morphological features are strongly influenced by the Late Pleistocene – Holocene kinematic inversion along the eastern part of the Tunka Fault. The Late Pliocene–Quaternary throw rates estimated for specific geomorphic structures vary in the range of 0.8–1.0 mm year−1, which is compatible overall with the long-term throw rates of other basins of the Baikal Rift. Based on these rates, we estimate the age of fault-controlled subsidence of the Tunka and Khoytogol basins and Nilovsky Spur to be between 3.5 and 1.5 Ma. We also show that geomorphological response of the transverse drainage varies along the Tunka Fault, indicating a close relationship between fault kinematics and landscape response.

Tectono - Magmatic and stratigraphic evolution of final rifting and breakup: Evidence from the tip of the southwestern propagator in the south China sea

In this study, we explore the tectono-sedimentary and magma evolution of final rifting and continental breakup along the SW conjugate margins of the South China Sea (SCS). Based on new seismic observations, we define and map three rift domains, referred to as “box-shaped”, “wedging” and “proto-oceanic”. Each of these domains is characterized by: (1) the shape and thickness of the crust; (2) the specific architecture of the syn-rift mega-sequence that includes three system tracts each one including syn- and post-rift packages; and (3) different deformation styles.Results are synthetized in two Wheeler diagrams that provide a complete description of the tectono-magmatic and stratigraphic evolution of final rifting and breakup within two neighboring segments of the SW-SCS. We conclude that: 1) localization of deformation occurs simultaneous and perpendicular to the migration of magma-emplacement in front of a propagating spreading system, 2) the arrival of magma controls continental breakup, and 3) the final rifting stage and breakup are diachronous. The relationships between extension, sedimentation and magmatic processes observed in the SW-SCS differ significantly from the classical models derived from the N-Atlantic or the Alpine Tethys. Main differences include the way the crust is thinning/necking, the sharp transition to breakup and the style of magma emplacement linked to crustal breakup.

Transformation of sediment delivery and dispersal patterns controlled by relay-ramp evolution along the boundary fault of a lacustrine rift: The Eocene Shahejie formation, Dongying Sag, Bohai Bay Basin, NE China

The boundary fault of the Dongying Sag is divided into isolated and scattered segments and small-scale relay zones, which are important geomorphic features controlling sedimentation in the early stage of rift evolution. Previous studies have shown changes in facies in deposits of thousands of meters in thickness on fault-bounded margins, where deep-water fan systems are vertically stacked with delta deposits in the same way as in many basins around the world. However, changes in sedimentation processes and sedimentary systems and their relationship with the evolution of the main boundary fault remain a matter of debate. We focus on the transition from shallow-water to deep-water systems and their responses to relay-ramp evolution in the Yanjia Subsag of the Eocene Dongying Sag in the Bohai Basin. A relatively simple assemblage of local structures provides an ideal opportunity to investigate the impact of relay-ramp evolution on the early synrift deposits. An integrated dataset of 3D seismic volumes, cores and wireline logs is used to analyze the evolution of the Chennan Fault, to delineate thirteen lithofacies, and to recognize two depositional styles (including fan deltas and nearshore subaqueous fans) and their distribution during different stages of the relay-zone evolution. In the early stage of rifting, the depositional style changes, from the development of axial and transverse systems in the early to middle period to the development of transverse systems in the late period; this is mainly controlled by the evolution of the relay ramp. The change is accompanied by a rapid decrease in the rate of sediment supply, architectural changes in transport conduit from the relay ramp to a canyon, and local geomorphic changes that control transport routes, sediment delivery, and depositional locations. Floods ultimately become dominant transport mechanism with a new system, such that hyperpycnal flows form, leading to another transformation of the sedimentary systems from a fan-delta system to a nearshore subaqueous fan system. On the basis of these transformations in sedimentary systems we are able to propose new recommendations for reservoir exploration in continental rift basins, which include exploration potential, source reservoir cap assemblage, and spatiotemporal evolution of plays.

Overturned Eocene – Lower Pliocene alluvial stratum on the southern coast of Lake Baikal and its neotectonic significance

The study is focused on a section of sediments exposed on the right bank of Mishikha River, Russia. These sediments have a wide range of ages, from the Eocene to the Lower Pliocene. The stratigraphic subdivision of the section is based on the lithogeochemical data and X-ray phase analysis of the mineral compositions. The particle-size analysis shows the alluvial origin of the deposits. Their ages are constrained by spore-pollen spectra in three palynozones: I – Eocene – Oligocene, II – Early – Middle Miocene (subzone a – Tsuga, Picea in the lower part, and Quercus, Taxodiaceae, Momipites, Carya in the upper part; subzone b – Fagus, Quercus, Tsuga), and III – the Late Miocene – beginning of the Pliocene (subzone ν – Ulmus, Juglans, Carya; subzone g – Carya, Alnus). The section shows a combination of normal and overturned sedimentary layers. The tectonic displacement of the block with its flip was accompanied by the entry into contact of the unlithified Pliocene sediments with a rigid bed and the development of a landslide. The lower age limit of deformations is constrained from the youngest (beginning of the Pliocene) spore and pollen spectrum extracted from deformed layers. It is suggested that the overturned layers result from strike-slip deformations of the sediments at the beginning of the late orogenic stage of the Baikal rift development. The regional correlations of the sedimentary strata give grounds to conclude that the Mishikha section is characteristic of alluvial sedimentation that dominated at the eastern end of the Tankhoi tectonic step (Mishikha-Klyuevka paleovalley), in contrast to the Tankhoi block in the central part of the step, wherein a thick Lower Miocene stratum of swampy-oxbow sediments accumulated. The stratons of the Mishikha section correlate with sedimentary units detected by drilling in the Selenga delta at the central part of the South Baikal basin.

Tectonostratigraphic evolution of the Orphan Basin and Flemish Pass region – Part 1: Results from coupled kinematic restoration and crustal area balancing

The Orphan Basin and Flemish Pass region on the Newfoundland continental margin is a frontier area in terms of oil and gas exploration and remains poorly understood in terms of structural evolution and rift development. The area has few exploration wells and, until recently, sparse seismic data coverage. Existing gravity inversion and seismic refraction data from the area suggest that stretched continental crust in the Orphan Basin is highly attenuated (locally to < 5 km) but previously published structural restorations have been unable to fully restore the continental crust below the Orphan Basin to pre-deformed thicknesses (30–32 km). Here we perform a structural restoration of a regional, WNW-ESE oriented cross-section to investigate crustal structure, rifting style, and structural evolution of the Orphan Basin and Flemish Pass region. Interpretation is constrained by modern regional depth converted broadband 2D and 3D seismic reflection data, well data, and published gravity inversion results. Present-day crustal thicknesses are used as constraints for crustal area balancing and estimation of crustal thinning across the basin. We find that low-angle extensional detachments are widespread in the area, with offsets on individual structures of up to 42 km. Zones of attenuated continental crust are coincident with the presence of low-angle detachment surfaces displaying isostatic uplift of detachment footwalls. Comparison of total extension derived from crustal area balancing vs. structural restoration suggests approximately 39% of extension across the cross-section is unresolvable in seismic data. We attribute this to (i) ductile deformation prior to brittle faulting, (ii) subseismic-scale faulting, (iii) uncertainties in detachment breakaway positions, and (iv) unresolvable later offset of detachment surfaces. Widespread and relatively evenly distributed extension across the Orphan Basin and Flemish Pass region from Middle-Late Jurassic to Early Cretaceous influences sediment distribution across the region. Calibrated regional seismic mapping of the Orphan Basin suggests that significant thicknesses of Jurassic strata exist throughout the area, including in the northwestern part of the study area, where Kimmeridgian and Tithonian strata reach combined thicknesses of up to 4.9 km.

Multiphase deformation history of the Porcupine Basin, offshore west Ireland

AbstractThe Porcupine Basin is a large underexplored sedimentary basin located offshore west of Ireland within the structurally complex European North Atlantic Margin. The basin has evolved through multiple Jurassic–Recent phases of deformation and although the overall plate tectonic context of the margin is well‐documented, there are still uncertainties regarding the phases of tectonic activity, and their associated strain distribution and fault kinematics. Based on the analysis of large volumes of 2D and 3D seismic data from the Porcupine Basin, we provide an overview of the nature and origin of multiple fault systems, both tectonic and nontectonic, with links to regional tectonic events where possible. Three distinct basin‐wide phases of tectonically induced extensional faulting are recognized: (a) Late Jurassic N‐S to NE‐SW trending rift faults, (b) Late Cretaceous E‐W to ENE‐WSW trending normal faults and (c) Mid Eocene N‐S trending faults. The Jurassic faults were active over a period of 11.5 Myr between the Oxfordian and Kimmeridgian but with an intervening 4.5 Myr period of quiescence providing a two‐stage rift evolution. The Late Cretaceous faulting in the Porcupine Basin broadly correlates with extension in the Celtic Sea basins and is tentatively attributed to the rotational spreading of the Bay of Biscay in the south. The Mid Eocene phase of extension, which coincides with the onset of spreading between Europe and Greenland (Atlantic spreading), resulted in partial reactivation of the Jurassic faults. A series of non‐tectonic fault systems occur within specific stratigraphic intervals suggesting a compactional or gravitationally driven origin, including basinward dipping normal faults within a Palaeocene–early Eocene deltaic sequence, and multiple tiers of polygonal faults within Cretaceous, late Eocene and Neogene strata.

Tectonic inheritances in rifts: The meaning of NNE lineaments in the continental rift of SE-Brazil

The effect of previous structures inheritance is known to be important in the development of tectonic rifts. A series of overlapping structures generally can be represented by lineaments marking the successive tectonic events. We studied the NNE structural lineaments corridor in the central region of the Ribeira Belt. We used a digital elevation model (DEM) and new and previous fieldwork data to investigate the structural control of such lineaments and their relevance for the Brazilian continental margin. Our results suggest that the NNE direction is a crustal weakness zone characterising corridors of intense ductile and brittle deformation which was recurrently reactivated. Aligned NNE Neoproterozoic-Ordovician ductile and brittle structures as foliations, shear zones, lithological boundaries, and fractures filled by pegmatitic veins coincide with the lineaments. During the Cretaceous rift, a transtensional sinistral regime generated NNE T-fractures filled by mafic dykes. In the Cenozoic, the NNE direction is represented by transfer and domino faults developed within a mega accommodation zone in an intracontinental rift system. Our results suggest that the NNE direction was active in this region throughout the Phanerozoic and has high relevance for the structural development of the continental margin of southeastern Brazil.

Tectonic Setting of the Kenya Rift in the Nakuru Area, Based on Geophysical Prospecting

In this paper, we present results of tectonic and geophysical investigations in the Kenya Rift valley, in the Nakuru area. We compiled a detailed geological map of the area based on published earlier works, well data and satellite imagery. The map was then integrated with original fieldwork and cross sections were constructed. In key areas, we then performed geophysical survey using Electrical Resistivity Tomography (ERT), Hybrid Source Audio MagnetoTelluric (HSAMT), and single station passive seismic measurements (HVSR). In the study area, a volcano-sedimentary succession of the Neogene-Quaternary age characterized by basalts, trachytes, pyroclastic rocks, and tephra with intercalated lacustrine and fluvial deposits crops out. Faulting linked with rift development is evident and occurs throughout the area crosscutting all rock units. We show a rotation of the extension in this portion of the Kenya rift with the NE–SW extension direction of a Neogene-Middle Pleistocene age, followed by the E–W extension direction of an Upper Pleistocene-Present age. Geophysical investigations allowed to outline main lithostratigraphic units and tectonic features at depth and were also useful to infer main cataclasites and fractured rock bodies, the primary paths for water flow in rocks. These investigations are integrated in a larger EU H2020 Programme aimed to produce a geological and hydrogeological model of the area to develop a sustainable water management system.

Structural architecture and the episodic evolution of the ediacaran Campo Alegre Basin (southern Brazil): Implications for the development of a synorogenic foreland rift and a post-collisional caldera-volcano

During the last decades, tectonic models provided new insight into the evolution of the Luis Alves, Curitiba, and Paranaguá terranes, which are all limited by thrust and transpressive shear zones, nowadays outcropping only as deep crustal horizons and presenting poorly known lateral displacements. An essential puzzle piece to understanding the juxtaposition processes and evolution of these blocks in the Neoproterozoic lies in the Campo Alegre Basin in Southern Brazil, a volcano-sedimentary sequence deposited during the middle to late Ediacaran. Based on new U–Pb geochronological, structural, and aero-geophysical data, at least two main stages of filling and subsidence have been identified in this region, namely the basin and the caldera stages. In the Basin Stage, the regional collisional tectonics triggered the far-field stress resulting in a local extension at ~605 ± 5 Ma through the reactivation of NNW-SSE inherited basement structures. The deposition of the sedimentary basin finishes with the Initial Volcanic Activity, corresponding to a bimodal mildly alkaline, predominantly mafic and effusive volcanism. After the transition to a post-collisional setting, probably at ca. 595 Ma, regional extension led to the Caldera Stage of the basin, which had its volcanic peak at ca. 583-580 Ma, contemporaneous with the intrusive A-type magmatism of the nearby Graciosa Province. The Main Volcanic Activity corresponds to a predominantly alkaline silica-saturated, effusive to explosive magmatic manifestation culminating with the formation of a caldera-volcano. The volcanic products from both the initial and the main volcanic activities were raised to the surface mainly through NNW-SSE and ENE-WSW oriented conduits, respectively reactivated and neo-formed during the collisional process. The crustal-scaled discontinuities associated with the development of the sedimentary basin have further controlled the subsidence of the caldera structure, which might be the main mechanism of preservation for these ancient volcano-sedimentary sequences in the evolution of the Campo Alegre Basin.

Geochemistry and Petrogenesis of Basaltic Rocks and Enclosed Xenoliths from the Ghab Pliocene Volcanic Field in Northwestern Syria

Basaltic rocks and their content of ultramafic xenoliths are common in the Ghab Pliocene Volcanic Field in northwestern Syria in the form of lava flows, cinder cones and pyroclastic deposits. The rocks occur within the Ghab pull- apart graben that formed by sinister strike slip faults within the zone that defines the boundary between the African and the Arabian plates. Three petrographic types occur: basanite, olivine basalt and more commonly alkali olivine basalt. The peridotite xenoliths are spinel lherzolite and harzburgite. Geochemical analysis indicates that the basalts are mostly alkaline to subalkaline. A distinctive feature of these rocks is the narrow compositional variations in the content of most major oxides and minor elements, SiO2 (44.33 - 46.43 wt%) and MgO (4.01 - 8.28 wt%). Some of the refractory and high field strength elements and incompatible minor elements in the basalts are relatively high (Cr average = 303 ppm and Ni average = 185 ppm) compared with their content in average basalts. These geochemical characteristics reflect crystallization of the Ghab basalts from pristine and primary magmas that have experienced minimal fractional crystallization and crustal contamination. Similarly, chemical compositions of the ultramafic xenoliths vary within a restricted geochemical range. They are compatible with the generation of these rocks from partial melting of a primitive mantle pyrolite to yield the xenoliths. These magmatic processes operated during the Pliocene in a regional transtensional stress environment attending the development of the Dead Sea Rift.

The role of inheritance in forming rifts and rifted margins and building collisional orogens: a Biscay-Pyrenean perspective

A long-standing challenge in tectonics is to evaluate the role of inheritance and define the initial conditions of a geodynamic system, which are prerequisites to understand and model its evolution with some accuracy. Here we revisit the concept of “inheritance” by distinguishing “interface shape inheritance”, which includes the transient thermal state and gravitational potential energy, and “persisting inheritance”, which encompasses long-lasting structural and compositional inheritance. This new approach allows us to investigate, at each stage of a Wilson Cycle, the interplay between inheritance (innate/“genetic code”) and the physical processes at play (extension/compression, magmatism etc.). The aim of this paper is to provide a conceptual framework that integrates the role of inheritance in the study of rifts, rifted margins and collisional orogens based on the work done in the OROGEN project, which focuses on the Biscay-Pyrenean system. The Biscay-Pyrenean rift system resulted from a multistage rift evolution that developed over a complex lithosphere pre-structured by the Variscan orogenic cycle. There is a general agreement that the Pyrenean-Cantabrian orogen resulted from the reactivation of an increasingly mature rift system along-strike, ranging from mature rifted margins in the west to an immature and segmented hyperextended rift in the east. However, different models have been proposed to explain the preceding rifting and its influence on the subsequent reactivation. Results from the OROGEN project highlight the sequential reactivation of rift-inherited decoupling horizons and identify the specific role of exhumed mantle, hyperextended and necking domains during compressional reactivation. They also highlight the contrasting fate of rift segment centresversussegment boundaries during convergence, explaining the non-cylindricity of internal parts of collisional orogens. Results from the OROGEN project also suggest that the role of inheritance is more important during the initial stages of collision, which may explain the higher complexity of internal parts of orogenic systems with respect to their external parts. In contrast, when the system involved in the orogeny is more mature, the orogenic evolution is mostly controlled by first-order physical processes as described in the Coulomb Wedge theory, for instance. This may account for the simpler and more continuous architecture of external parts of collisional orogens and may also explain why most numerical models can reproduce mature orogenic architectures with a better accuracy compared to those of initial collisional stages. The new concepts developed from the OROGEN research are now ready to be tested at other orogenic systems that result from the reactivation of rifted margins, such as the Alps, the Colombian cordilleras and the Caribbean, Taiwan, Oman, Zagros or Timor.

Petrological and geochemical characteristics of mafic rocks from the Neoproterozoic Sugetbrak Formation in the northwestern Tarim Block, China

The Neoproterozoic Sugetbrak Formation in the Aksu area, which is located at the northwest margin of Tarim Block, comprises mafic rocks and provides key records of the evolution of the Rodinia supercontinent. However, the genetic relationship among these mafic rocks exposed in different geographical sections are still unclear. In this study, the petrology, geochemistry, and Sr-Nd-Pb isotope geochemistry of the mafic rocks exposed in the Aksu-Wushi and Yuermeinark areas have been studied in some detail along three sections. The authors found that the mafic rocks in these three typical sections were mainly composed of pyroxene and plagioclase, containing a small amount of Fe-Ti oxides and with typical diabasic textures. All the mafic rocks in this region also showed similar geochemical compositions. They were characterised by high TiO2 contents (1.47% –3.59%) and low MgO (3.52% –7.88%), K2O (0.12% –1.21%). Large ionic lithophile elements (LILEs) (Rb, Sr, and Cs) were significantly depleted. Meanwhile, high field strength elements (HFSEs) were relatively enriched. In the samples, light rare earth elements (LREEs) were enriched, while heavy rare earth elements (HREEs) were depleted. Based on the Zr/Nb, Nb/Y, and Zr/TiO2 ratios, the Aksu mafic rocks belong to a series of sub-alkaline and alkaline transitional rocks. The mafic rocks along the three typical sections showed similar initial values of 87Sr/86Sr (ISr) (0.7052–0.7097) and εNd(t) (–0.70 to –5.35), while the Pb isotopic compositions with 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of 16.908–17.982, 15.487–15.721, 37.276–38.603, respectively. Most of the samples plot into the area near EM-Ⅰ, indicating that the magma of the mafic rocks might have derived from a relatively enriched mantle with some crustal materials involved. The geochemical element characteristics of most samples showed typical OIB-type geochemical characteristics indicating that the source region had received metasomatism of recycled materials. Combining with the regional geological background and geochemical data, we inferred that the mafic rocks of the Sugetbrak Formation in the Aksu area were formed in an intraplate rift environment. Summarily, based on our study, the mafic rocks of the Sugetbrak Formation in the Aksu area were derived from a common enriched mantle source, and they were product of a magmatic event during the rift development period caused by the breakup of the Rodinia supercontinent.©2023 China Geology Editorial Office.

Post-orogenic exhumation in the western Pyrenees: evidence for extension driven by pre-orogenic inheritance

We discuss the drivers of the Pyrenean post-orogenic exhumation, including drainage migration, flexural rebound and tectonic reactivation.We provide new low-temperature thermochronological data and inverse thermal modeling from both the hinterland and foreland of the western Pyrenees. Our new thermochronological ages range from 6.6 to 61.4 Ma and reveal a Late Miocene exhumation phase in several massifs. The contrasting thermal histories define a domain of focused exhumation in the western Pyrenees that coincides with the present-day extensional tectonics in a region to the north of the Axial Zone. Based on the inferred cooling rates and paleogradient estimates, we highlight an exhumation phase of c. 1 mm yr –1 between 11 and 9 Ma in the Axial Zone, well above rates expected for a post-orogenic evolution. The thermal evolution inferred from three boreholes of the Aquitaine foreland basin reveals that sediments eroded from the hinterland did not accumulate in the Piedmont region but were transported offshore in the Bay of Biscay. We infer that the significant c. 10 Ma post-orogenic exhumation event must be related to the modern normal faulting regime of the western Pyrenees, associated with contrasting crustal thickness and densities, inherited from the Mesozoic rift evolution of the northern Pyrenees. Supplementary material: Laboratory analytical procedures, radial plot visualization of AFT detrital data and Age/eU relationship of AHe and ZHe samples are available at https://doi.org/10.6084/m9.figshare.c.5212581

Carajás Mineral Province - Example of metallogeny of a rift above a cratonic lithospheric keel

The Carajás Mineral Province (CMP) is located on the southeastern margin of the Amazonian Craton, in northern Brazil. It is the largest producer of high-grade iron ore and a major world supplier of copper, nickel, manganese and gold. This article presents the CMP as a product of oblique rift evolution (2.76–2.06 Ga) controlled by the Canaã and Cinzento strike slip system. The Carajás rift is hosted in the tonalite-trondhjemite-granodiorite (TTG)-greenstone Rio Maria terrains (3.0–2.85 Ga). It overlies a subcratonic lithospheric mantle (SCLM) keel as indicated by seismic tomography and is filled by the Carajás plutonic-volcanic sedimentary sequences (CPVSS) that host the CMP. The rifting stage (Grão Pará Group) began with intense tholeiitic fissure volcanism with basalt and rhyolite flows, coeval intrusions of alkali granites and mafic-ultramafic bodies, BIFs, shales and restricted tuff beds. The Azul and Águas Claras Formations consist mainly of shales and sandstones deposited during the sag stage. The calc-alkaline signature of the Grão Pará metavolcanic rocks was demonstrated to reflect the composition of the melted TTG basement. The high magnesium and platinum-group elements (PGE) contents of Puma and Onça mafic-ultramafic complexes indicate extensive partial fusion, conditions that are indicative of a plume origin. We are proposing that CMP's IOCG deposits were formed from saline carbonate fluids exsolved from deep-seated metasomatic mantellic magmas (MMM), channelled and concentrated along jogs and step-overs in strike-slip fault systems in the Carajás rift.The CMP comprises three main ore-forming stages: (1) a plume stage, ca. 2.76–2.73 Ga, creating supergiant exhalative sedimentary biogenic Fe deposits (Serra Norte, Serra sul and Serra Leste), syngenetic exhalative Cu–Au–Zn (Pojuca deposit), exhalative Mn (Antonio Vicente), Ni-PGE-reef deposits in a magma mixing zone of layered intrusions (e.g. Luanga deposit), and Siqueirinho IOCG deposit; (2) a metasome stage including crustal extension and mantle flash melting (60–100 km depth), ca. 2.68–2.50 Ga, forming the most important IOCG deposits like Salobo, Alemão and Igarapé Bahia. and (3) a sag stage, ca. 2.68–2.06 Ga, with sedimentary Mn deposited in a redox-controlled stratified oceanic environment. The deposits related to granitic intrusion, ca. 1.88 Ga, with Cu-(Au–Mo–W–Sn) greisen (Breves deposit) and Cu-(Au–Mo–F) veins (Gameleira and Alvo 118 deposits) are associated with the A-type magmatism that affected the entire central Amazonian Craton. These deposits are, in contrast, related to an intracratonic silicic large igneous province (SLIP) and have no relation to the Archean Carajás cratonic lithosphere keel rift evolution. The CMP is directly related to the thinning of the lithosphere above a mantle plume, beneath a sub-continental lithospheric mantle (SCLM) keel.

Diamondiferous lamproites of the Luangwa Rift in central Africa and links to remobilized cratonic lithosphere

Mesozoic diamondiferous lamproite pipes occur along the Kapamba River within the Luangwa Valley of eastern Zambia, which is a ca. 300–200 Ma old Karoo-age precursor branch to the East African Rift System. The Luangwa Rift developed above a reactivated mega-shear zone that cuts through the Proterozoic Irumide Belt between the Congo-Tanzania-Kalahari cratons and thus it provides a rare snapshot of early-stage cratonic rift evolution.The primary mineralogy of the fresh volcanic rocks suggests that they represent a continuum between primitive olivine lamproites and slightly more evolved olivine-leucite lamproites. Mineral compositions and evolutionary trends, such as the strong Al-depletion at Ti-F enrichment in groundmass phlogopite and potassic richterite, resemble those of classic lamproite provinces in circum-cratonic settings (e.g., the Leucite Hills of Wyoming and the West Kimberley field in Australia). However, there are some similarities to orangeites from the Kaapvaal craton (formerly Group-2 kimberlites), type kamafugites from the East African Rift, and ultramafic lamprophyres from a key region of the rifted North Atlantic craton, which implies a complex interplay between source-forming and tectonic processes during Karoo-age lamproite magma formation beneath south-central Africa.The bulk compositions of the Kapamba volcanic rocks fall within the range of ‘cratonic’ low-silica lamproites, but there is overlap with orangeites, in particular with the more evolved leucite- and sanidine-bearing orangeite varieties. Modelling of the process by which most of the original leucite was transformed into analcime suggests that the primitive alkaline magmas at Kapamba contained ~6–9 wt% K2O and had high K2O/Na2O ratios between ~1.6–6.2 at >10 wt% MgO – confirming the ultrapotassic nature of the mantle-derived magmatism beneath the Luangwa Rift. The virtually CO2-free, H2O-F-rich Kapamba lamproites present an extension of the geochemical continuum displayed by the members of the CO2-H2O-rich kamafugite/ultramafic lamprophyre group. Hence, we suggest that the Kapamba lamproites and the type kamafugites, located within separate branches of the East African Rift System, represent melting products of similar K-metasomatized cratonic mantle domains, but their formation occurred under contrasting volatile conditions at different stages during rift development (i.e., incipient versus slightly more advanced rifting).Temperature estimates for peridotite-derived olivine xenocrysts from the Kapamba lamproites suggest that the Luangwa Valley is an aborted cratonic rift that retained a relatively cold (≤42 mW/m2) lithospheric mantle root down to ~180–200 km depth during the Mesozoic. Olivine major and trace element compositions support the presence of an Archean mantle root (up to 92.4 mol% forsterite contents) that is progressively metasomatized toward its base (e.g., increasing Ti-Cu contents with depth). For south-central Africa, it appears that significant volumes of Archean cratonic mantle domains ‘survived’ beneath strongly deformed and granite-intruded Proterozoic terranes, which suggests that the continental crust is more strongly impacted during collisional or rift tectonics than the ‘stabilizing’ mantle lithosphere.