Crustal Rifting Research Articles

Local Seismicity and Sediment Deformation in the West Svalbard Margin: Implications of Neotectonics for Seafloor Seepage

AbstractIn the Fram Strait, mid‐ocean ridge spreading is represented by the ultra‐slow system of the Molloy Ridge, the Molloy Transform Fault and the Knipovich Ridge. Sediments on oceanic and continental crust are gas charged and there are several locations with documented seafloor seepage. Sedimentary faulting shows recent stress release in the sub‐surface, but the drivers of stress change and its influence on fluid flow are not entirely understood. We present here the results of an 11‐month‐long ocean bottom seismometer survey conducted over the highly faulted sediment drift northwards from the Knipovich Ridge to monitor seismicity and infer the regional state of stress. We obtain a detailed earthquake catalog that improves the spatial resolution of mid‐ocean ridge seismicity compared with published data. Seismicity at the Molloy Transform Fault is occurring southwards from the bathymetric imprint of the fault, as supported by a seismic profile. Earthquakes in the northern termination of the Knipovich Ridge extend eastwards from the ridge valley, which together with syn‐rift faulting identified in seismic reflection data, suggests that a portion of the currently active spreading center is buried under sediments away from the bathymetric expression of the rift valley. This hints at the direct link between crustal rifting processes and faulting in shallow sediments. Two earthquakes occur close to the seepage system of the Vestnesa Ridge further north from the network. We suggest that deeper rift structures, reactivated by gravity and/or post‐glacial subsidence, may lead to accommodation of stress through shallow extensional faults, therefore impacting seepage dynamics.

Sedimentology and stratigraphic architecture of Barremian synrift barrier island–estuarine depositional systems from blended field and drone‐derived data

ABSTRACTThe Lower Cretaceous (Barremian) Camarillas Formation in the Galve Sub‐basin of eastern Spain is an exceptionally muddy, synrift, aggradational then retrogradational paralic succession. Deposition within these arid, equatorial paralic systems was strongly controlled by crustal rifting of the Iberia plate linked to the geodynamic evolution of the Atlantic Ocean, the Bay of Biscay and the Tethys Ocean. Although synsedimentary extensional tectonics controlled thickness and facies distributions, the parasequence stacking patterns point to a superimposed high‐order allogenic control on the paralic succession. Field data and drone imagery are combined to document changes in sedimentology and three‐dimensional stratigraphic architecture of these deposits to interpret changes in depositional environments as this basin filled. Three evolutionary stages are identified: (i) tide‐dominated estuary; (ii) mixed‐energy estuary, with a well‐developed wave‐dominated barrier island system; and (iii) barrier island–tidal inlet suite. An exceptional record of back‐barrier‐island depositional interactions is preserved in this high‐subsidence, extensional‐basin setting, including deposition of washover fans, flood‐tidal deltas and ebb‐tidal deltas. Drone‐derived imagery facilitates three‐dimensional architectural characterization of these complex paralic deposits, including multi‐episodic tidal inlets, and correlation of basin scale stratigraphic markers. Spatio‐temporal interactions between climate change, sea‐level variations and rift‐related subsidence generated complex estuarine and barrier island geobodies. The general transgressive trend recorded in the Camarillas Formation correlates well with global eustatic sea‐level rise during the Barremian.

Characteristics of crustal structures in the Yamato Basin, sea of Japan, deduced from seismic explorations

AbstractCrustal structures around the Yamato Basin in the southeastern Sea of Japan, inferred from recent ocean bottom seismography (OBS) and active‐source seismological studies, are reviewed to elucidate various stages of crustal modification involved from rifting in the crust of the surrounding continental arc to the production of oceanic crust in the Yamato Basin of the back‐arc basin. The northern, central, and southern areas of the Yamato Basin have crustal thicknesses of approximately 12–16 km, and lowermost crusts with P‐wave velocities greater than 7.2 km/s. Very few units have P‐wave velocities in the range 5.4–6.0 km/s, which corresponds to the continental upper crust. These findings, combined with previous geochemical analysis of basalt samples, are interpreted to indicate that a thick oceanic crust has been formed in these areas of the basin, and that this oceanic crust has been underplated by mantle‐derived magma. In the central Yamato Basin, the original continental crust has been fully breached and oceanic crust has been formed. Conversely, the presence of a unit corresponding to the continental upper crust and the absence of a high‐velocity part in the lower crust implies that the southwestern edge of the Yamato Basin has a rifted crust without significant intrusion. The Oki Trough has a crust that is 17–19 km thick with a high‐velocity lower crust and a unit corresponding to the continental upper crust. The formation of the Oki Trough resulted from rifting with magmatic intrusion and/or underplating. We interpret these variations in the crustal characteristics of the Yamato Basin area as reflecting various instances of crustal modification by thinning and magmatic intrusion due to back‐arc extension, resulting in the production of a thick oceanic crust in the basin.

Episodic mafic dike swarms (2.0 and 1.7 Ga) in the Quadrilátero Ferrífero Province, Brazil: Implications for the refinement of the São Francisco Craton magmatic barcode and the North China link

Late Archean to Paleoproterozoic dike swarms crosscut the basement and supracrustal sequences of the São Francisco Craton. Their spatial distribution and orientation patterns, as paleomagnetic signature, provide key evidence to the positioning of the cratonic area in ancient paleocontinental reconstructions through the refinement of the “magmatic barcode”. Zircon U–Pb data from dikes in the cratonic domain of the Quadrilátero Ferrífero Mineral Province, in the southern of São Francisco Craton, allowed the recognition of two magmatic stages: a Rhyacian-Orosirian pulse, belonging to the Paraopeba swarm, and a Statherian pulse that belongs to the Pará de Minas II swarm. The Ediacaran Brasiliano Orogeny later affected dikes from both swarms.The Paraopeba swarm consists of pulses related to the Rhyacian-Orosirian orogenic cycle, with dikes providing U–Pb zircon ages of 2019 + 23–11, 1944 ± 29 and 1918 ± 12 Ma. The zircon εHf values from these dikes are predominantly negative, suggesting reworking of preexisting crust, whereas the TDM ages vary from 4000 to 2500 Ma, which can be interpreted as a long period of crustal residence. The Statherian Pará de Minas II swarm, with a U–Pb zircon age of 1717 ± 9 Ma, is chronocorrelated to the lower stratigraphic sequence of the Espinhaço rift basin. Zircon grains from this dike have predominantly positive εHf values and TDM model ages between 3050 and 1950 Ma. Unlike the Paraopeba swarm, the Pará de Minas magmatism is marked by a striking input of juvenile mantle material with minor crustal contamination. The Ediacaran Brasiliano tectono-thermal event caused isotopic disturbance in both Paraopeba and Pará de Minas II swarms, resulting in lower intercepts at 595 ± 65 and 584 ± 27 Ma, respectively.Our data pinpoint dike emplacement related to tectonic events described in the southern of São Francisco. In the final stages of Rhyacian orogenic cycle, at ca. 2019–1918 Ma, data suggests an important crustal contamination. Thereafter, a crustal rifting event at ca. 1717 Ma involved dike swarm emplacement with magmatic source from a mantle plume. The Rhyacian-Orosirian ages obtained here are the first zircon U–Pb geochronological data presented for the Paraopeba dike swarm and allow for a refinement of the São Francisco Craton barcode, reinforcing previous suggestions of proximity with the North China Craton crosscut by the similarly aged Hengshan and Xiwangshan/Xuwujia dike swarms, whether or not those two cratonic pieces were part of the Columbia supercontinent.

The Statherian Natividade Basin evolution constrained by U–Pb geochronology, sedimentology, and paleogeography, central Brazil

The tectonostratigraphic evolution of the intracontinental Natividade Basin is evaluated by zircon U/Pb analysis of one sample of acidic metavolcanic rock observed at the bottom of the depositional succession and three samples of metasedimentary rocks collected at different stratigraphic levels. The metavolcanic sample yielded a 1824 Ma zircon U–Pb age, and the youngest detrital zircon grains from the metasedimentary sequence resulted in a maximum depositional age of 1776 Ma for the paleobasin. The isotopic data associated with the analysis of stratigraphy, paleogeography, and sedimentary environments converge to the interpretation that the Natividade Group evolved from the late Paleoproterozoic (Statherian), possibly entering the early Mesoproterozoic. The sedimentation is interpreted to have occurred between the deposition of the Araí and the Traíras/Serra da Mesa groups. This understanding is due to the absence of continental sediments in the Natividade Basin (which could corroborate the correlation with the Araí Basin) and the absence of detrital zircon grains younger than 1.5 Ga (which would support the correlation with the Traíras/Serra da Mesa groups). The metavolcanic rocks are observed only locally and are interpreted as the result of melting by crustal rifting that was greatest in the south, related to the Araí Group, and decreased northwards of the study region. The mechanical subsidence responsible for the deposition of the Araí Group continental sediments was not observed in the Natividade Basin. The thermo-flexural subsidence responsible for the Natividade Basin evolution allowed the deposition of complex sedimentary environments related to mixed siliciclastic platforms and shallow water turbidite facies. Based on bibliographic studies and fieldwork data, our geological model suggests that the Almas Block was possibly a high paleorelief area, separating the Araí Basin to the south and the Natividade Basin to the north. This assertion is based on the following facts for the study area: i) there is no occurrence of the Veadeiros Supergroup sediments; ii) Nowadays in the region, there are many high relief areas related to quartzite layers and several granitic units that are resistant to denudation processes; iii) deposition of the shallow water turbidite present at the Natividade Group needed elevated source areas to control the sedimentation; iv) The predominance of carbonates and mass flow deposits in the southern portion of Natividade Group, near to Almas Block, suggests a lower depth of the sea to the south of the basin. Further north, the predominance of fine-grained terrigenous deposits suggest a deeper basin; v) it is common in rift-sag environments the preservation of elevated blocks as an isostatic compensation mechanism. The integrated analysis conducted allowed the following conclusions: the Natividade Basin evolution is not coeval with the Araí or the Traíras/Serra da Mesa basins and is classified as a sag-type basin in which the sedimentary environments resulted in massive carbonate accumulation in a complex platform controlled by the basement paleorelief.

New Insights into Pre-to-Post Ediacaran Zircon Fingerprinting of the Mamfe PanAfrican Basement, SW Cameroon: A Possible Link with Rocks in SE Nigeria and the Borborema Province of NE Brazil

The pre- to post-Late Neoproterozoic geological histories in the south to southwestern part of Mamfe Basin (SW Cameroon) were reported following analysis of the zircon crystals from their host rocks. A genetic model was developed for the zircon host rocks’ formation conditions, and the registered post-emplacement events were presented. The obtained ages were correlated with the data available for rocks in the Cameroon Mobile Belt, SE Nigeria, and the Borborema Province of NE Brazil. Separated zircons from Araru black to whitish gneiss, Araru whitish-grey gneiss, and Mboifong migmatite were analyzed for their morphology and texture U-Th-Pb composition, and U-Pb ages. Published U-Pb zircon ages for Otu granitic pegmatite, Babi mica schist, and Nkogho I-type anatectic granite were updated. Zircon ages in Araru black to whitish gneiss; Araru whitish-grey, Mboifong migmatite, Babi mica schist, Nkogho I-type anatectic granite, and Otu granitic pegmatite date the Eburnean tectono-magmatic/metamorphic event in Cameroon and SE Nigeria. The Late Paleoproterozoic to Early Mesoproterozoic ages record extensional (continental rift) settings and anorogenic magmatism in the Borborema Province in the NE of Brazil. These ages date collisional phases between the São Francisco–Congo and West African cratons and the Saharan metacraton with metamorphism and magmatism in Cameroon. They also date the Kibarian tectono-magmatic/metamorphism and PanAfrican tectono-magmatic/metamorphism in SE Nigeria. The Late Paleoproterozoic to Early Mesoproterozoic ages date the Cariris Velhos orogeny in the Borborema Province in NE Brazil, with Early Tonian crustal rifting, magmatism, and metamorphism and the collisional phase of the Brasiliano orogeny with syn-collisional plutons and extensive shear zoning and post-collisional granite intrusions.

Hydrothermal silicification in the intraplate Samambaia seismogenic fault, Brazil: Implications for fault loss of cohesion and healing in rifted crust

This study investigates the Samambaia seismogenic fault at the border of the Potiguar rift Basin, Brazil, to evaluate the relationships among hydrothermal processes, seismicity, and preexisting tectonic fabric. The fault is a 27 km-long structure composed of three left-bend, en echelon segments spaced 1.0–1.5 km apart. The segments strike N31°E – N35°E, dip 70° NW and have focal depths between 1 and 9 km. The seismogenic fault coincides with a swarm of silica-rich veins that range in size from a few centimeters wide and 1–2 m long to 20–50 m wide and hundreds of m long. These structures encompass a V1 vein set composed of quartz veins parallel to the Proterozoic-Archean basement metamorphic foliation and a V2 vein set composed of chalcedony, opal, cryptocrystalline silica, and quartz geodes that cut across Cretaceous siliciclastic and carbonate sedimentary units in the basin and the basement. Fluid inclusions from the V1 vein set yield trapping temperatures of 91 °C–336 °C, whereas the V2 vein set yields temperatures of 147 °C–388 °C. The fluid inclusions and oxygen isotope analyses indicate a magmatic fluid source, although contamination by lateritic soil–derived fluids can be recognized in the V2 vein set. We conclude that in the Samambaia fault, fluid pressure reduces the effective stress until the fault experiences failure, contributing to fault weakening and seismicity. Subsequent cooling of hydrothermal fluids leads to fault healing after coseismic slip and precipitation of Si-rich cements. This process was associated with variation from hydrostatic to lithostatic pressure during cementation, reducing permeability along the fault. Then, the fault locks and fluid pressures begin to rise again. We interpret the superposed V1 and V2 vein sets as the expression of fault-valve behavior caused by repeated fluid-driven reactivation through time along preexisting structures in an extended, rifted crust.

Hafnium isotopic constraints on crustal assimilation in response to the tectono–magmatic evolution of the Okinawa Trough

Crustal assimilation and the input of subducted sediment in a subduction zone play vital roles in generating arc and back-arc lavas along a continental margin. However, distinguishing the impacts of these two processes on lava geochemistry is not straightforward or well understood. In this paper, we present new Hf–Nd–Sr–Pb isotopic data for 20 igneous rocks ranging in composition from basalt to rhyolite, which when combined with data from the literature allow us to fingerprint the contributions of crustal and slab-derived materials along the Okinawa Trough (OT). Consistent with previous results, the basaltic lavas were generated by melting of an Indian-type mantle source metasomatised by slab-derived fluids and subducted pelagic/detrital sediments. We confirm that Hf behaves as a non-conservative element during subduction, constrained by Hf-Nd isotopic systematics. Our new data suggest that crustal assimilation processes worked independently of the extent of magma differentiation but were determined by the stage of tectonic development in individual regions. In the southern OT (seafloor spreading stage), crustal assimilation is insignificant, and the addition of subducted sediment is 0.4%–1.5%. In the middle OT (crustal rifting stage), the basaltic and rhyolitic suites do not show distinctive Hf–Nd–Sr–Pb isotopic compositions, and both record moderate crustal assimilation (<10%) and < 0.6% addition of subducted sediment. In contrast, in the southwestern OT (crustal extension stage), the basaltic and dacitic lavas record the highest involvement of 10%–40% crustal material. We conclude that the various stages of back-arc development exerted first-order controls on the extent of crustal assimilation in the OT and had marked effects on the geochemical and isotopic compositions of OT lavas.

Zircon record of an Archaean crustal fragment and supercontinent amalgamation in quaternary back-arc volcanic rocks

Magmatism has profoundly influenced the evolution of the geosphere, hydrosphere, atmosphere, and biosphere in back-arc basins. However, the timing of the magmatism in the Okinawa Trough (OT) is not well constrained by the age spectra of zircons. Here, for the first time, we carry out an integrated study combining in situ analysis of zircon U–Th–Pb and Hf–O isotopes, and trace element compositions of zircons from the volcanic rocks from the southernmost part of the OT. We found that the young (< 100 ka) zircons in these volcanic rocks have old (108 Ma to 2.7 Ga) inherited cores, which were captured as the magma ascended through the rifting continental crust. In particular, the inherited Archean zircons strongly suggest that remnants of the old East Asian continental blocks underlie the embryonic crustal rifting zone. Moreover, the ages of most of the inherited zircons correspond to five supercontinent amalgamation events. Specifically, the Archaean inherited zircons, which have positive εHf(t) and low δ18O values, correspond to the formation of juvenile continental crust. In contrast, the negative εHf(t) and high δ18O values of the post-Archaean inherited zircons indicate that their parental magma contained recycled older crust due to the enhanced crust-mantle interactions during the evolution of the early continental crust. Therefore, the inherited zircons in the back-arc volcanic rocks not only reflect the evolution of the local magmatism, but they also contain a record of the Archaean crustal fragment and of several global continental amalgamation events.

Morphotectonic Analysis of the East Manus Basin, Papua New Guinea

Backarc basin systems are important sites of extension leading to crustal rupture where basin development typically occurs in rifting phases (or stages) with the final successful stages identified by the formation of spreading ridges and new oceanic crust. The East Manus Basin is a young (&amp;lt;1 Ma), active, rapidly rifting backarc basin in a complex tectonic setting at the confluence of the oblique convergence of the Australian and Pacific plates. Here we undertake the first comprehensive spatial-temporal morphotectonic description and interpretation of the East Manus Basin including a link to the timing of, and tectonic controls on, the formation of seafloor massive sulfide mineralization. Key seafloor datasets used in the morphotectonic analysis include multi-resolution multibeam echosounder seafloor data and derivatives. Morphotectonic analysis of these data defines three evolutionary phases for the East Manus Basin. Each phase is distinguished by a variation in seafloor characteristics, volcano morphology and structural features: Phase 1 is a period of incipient extension of existing arc crust with intermediate to silicic volcanism; Phase 2 evolves to crustal rifting with effusive, flat top volcanoes with fissures; and Phase 3 is a nascent organized half-graben system with axial volcanism and seafloor spreading. The morphotectonic analysis, combined with available age constraints, shows that crustal rupture can occur rapidly (within ∼1 Myr) in backarc basins but that the different rift phases can become abandoned and preserved on the seafloor as the locus of extension and magmatism migrates to focus on the ultimate zone(s) of crustal rupture. Consequently, the spatial-temporal occurrence of significant Cu-rich seafloor massive sulfide mineralization can be constrained to the transition from Phase 1 to Phase 2 within the East Manus Basin. Mineralizing hydrothermal systems have utilized interconnected structural zones developed during these phases. This research improves our understanding of the early evolution of modern backarc systems, including the association between basin evolution and spatial-temporal formation of seafloor massive sulfide deposits, and provides key morphotectonic relationships that can be used to help interpret the evolution of paleo/fossilized backarc basins found in fold belts and accreted terrains around the world.

The Western Limit of Iapetan Rifting in the Eastern United States: A New Assessment

AbstractSpecifying the extent and location of rifted, crystalline Precambrian crust in the eastern United States is important for seismic hazard evaluation and for models that relate upper-mantle structure to ancient tectonic features and ongoing tectonism. As currently depicted in the National Seismic Hazard Maps (NSHM), the western limit of Iapetan rifted crust is beneath the Appalachian plateau physiographic province, west of the Valley and Ridge province. New estimates of crustal thickness using EarthScope Transportable Array and other data do not support the presence of rifted crust beneath the Blue Ridge, Valley and Ridge, and Appalachian plateau physiographic provinces. Crustal thicknesses exceed 45 km throughout most of this region. The crust thins to the southeast beneath the southeastern part of the Piedmont physiographic province and is only 36 km thick near the edge of the Atlantic coastal plain. We suggest that the western limit of Iapetan rift-extended crust is east of the Blue Ridge province and is associated with the prominent Appalachian gravity gradient. This location coincides with palinspastic reconstructions based on geologic data for the Iapetan rifted margin. Recognition of thick crust beneath the Blue Ridge and Valley and Ridge provinces, unextended by Iapetan rifting, will support more robust modeling of the effects of mantle structure (such as delamination and abrupt changes in lithospheric thickness) on ongoing tectonism and earthquake activity in the eastern United States and will provide more accurate seismotectonic zonation in the NSHM.

A magmatic barcode for the São Francisco Craton: Contextual in-situ SHRIMP U[sbnd]Pb baddeleyite and zircon dating of the Lavras, Pará de Minas and Formiga dyke swarms and implications for Columbia and Rodinia reconstructions

Eoarchean to Rhyacian crust is preserved in the São Francisco Craton of eastern Brazil. To position this crustal segment in paleocontinental reconstructions, precise, accurate and robust geochronological data are necessary, especially for the diverse regional-scale mafic dyke swarms that crosscut the cratonic basement. This geochronological database can then be used to construct a magmatic barcode and compare it to the barcode of other cratons around the world, in search of similarities that might help to position these pieces in the paleocontinental puzzles. New UPb SHRIMP contextual in-situ (thin section) dating of baddeleyite and zircon from six samples of three different dyke swarms in the southern São Francisco Craton, in addition to novel lithogeochemical and NdSr isotopic data, allow to pinpoint dyke emplacement at ca. 2.55 Ga (Lavras I swarm; εNd(t) = −6 to +2; TDM not calculable), ca. 1.8–1.7 Ga (Pará de Minas I and II dyke swarms; εNd(t) = −10 to −5; TDM = 2.5–3.0 Ga) and at ca. 900 Ma (Formiga dyke swarm; εNd(t) = −7 to 0; TDM = 1.4–2.3 Ga). The new geochronological data suggest a link between the regional dyke swarms and extensional stresses during the onset of crustal rifting related to the evolution of the Minas, Espinhaço and Macaúbas basins, respectively. A barcode comparison shows strong similarity between the São Francisco and North China cratons (Lavras-Taipingzhai/Naoyumen swarms, Pará de Minas-Taihang/Miyun swarms, Formiga/Pedro Lessa-Sariwon/Dashigou swarms; and possible correlations of the poorly dated 2.2–2.0 Ga Paraopeba swarm with similar aged swarms in North China), suggesting proximity of those two cratonic blocks, whether they were part or not of Proterozoic paleocontinents such as Columbia and Rodinia. The novel geochronological data support previous interpretations based on paleomagnetic data and provide further refinements of the geochronological record of the southern hemisphere cratonic blocks, allowing for better-tied global correlations.

High-temperature–low-pressure metamorphism and the production of S-type granites of the Hillgrove Supersuite, southern New England Orogen, NSW, Australia

ABSTRACTThe high-temperature–low-pressure Wongwibinda Metamorphic Complex of the southern New England Orogen is bound by S-type granite plutons of the Hillgrove Supersuite to the north, east and south. New U–Pb geochronology of five samples of the Hillgrove Supersuite demonstrates that plutonism in the complex involved two pulses: ca 300 Ma and ca 292 Ma. This indicates that plutonism partially overlaps the age of high-T–low-P metamorphism (296.8 ± 1.5 Ma), but also postdates it. Zircon grains identified as xenocrysts based on age (≥310 Ma) have U–Pb–Hf isotopic character that largely overlaps detrital grains in the host Girrakool Beds, indicating that accretionary complex crust is the likely source of these xenocrysts. The 176Hf/177Hf initial character for zircon for the ca 300 Ma plutons (three samples) is less radiogenic than those in the ca 292 Ma plutons (two samples). The progression in 176Hf/177Hf initial character for zircon infers an increasing mantle component in the Hillgrove Supersuite with time. These data are evidence of a rift tectonic setting, where mantle-derived magmas are predicted to more readily migrate to shallower crustal levels as the crust thins and becomes hotter. Additionally, early episodes of partial melting in the system melt-depleted the metasedimentary sources, thus reducing the S-type component as anatexis progressed. The evolution of the Hillgrove Supersuite coincides with a period of early Permian slab roll back and extension accompanied by crustal rifting and thinning, leading to high-T–low-P metamorphism, anatexis and S-type granite production and the development of rift basins such as the Sydney–Gunnedah–Bowen system.

Fracture patterns of the drainage basin of Wadi Dahab in relation to tectonic-landscape evolution of the Gulf of Aqaba - Dead Sea transform fault

Crustal rifting of the Arabian-Nubian Shield and formation of the Afro-Arabian rifts since the Miocene resulted in uplifting and subsequent terrain evolution of Sinai landscapes; including drainage systems and fault scarps. Geomorphic evolution of these landscapes in relation to tectonic evolution of the Afro-Arabian rifts is the prime target of this study. The fracture patterns and landscape evolution of the Wadi Dahab drainage basin (WDDB), in which its landscape is modeled by the tectonic evolution of the Gulf of Aqaba-Dead Sea transform fault, are investigated asa case study of landscape modifications of tectonically-controlled drainage systems. The early developed drainage system of the WDDB was achieved when the Sinai terrain subaerially emerged in post Eocene and initiation of the Afro-Arabian rifts in the Oligo-Miocene. Conjugate shear fractures, parallel to trends of the Afro-Arabian rifts, are synthesized with tensional fracture arrays to adapt some of inland basins, which represent the early destination of the Sinai drainage systems as paleolakes trapping alluvial sediments. Once the Gulf of Aqaba rift basin attains its deeps through sinistral movements on the Gulf of Aqaba-Dead Sea transform fault in the Pleistocene and the consequent rise of the Southern Sinai mountainous peaks, relief potential energy is significantly maintained through time so that it forced the Pleistocene runoffs to flow via drainage systems externally into the Gulf of Aqaba. Hence the older alluvial sediments are (1) carved within the paleolakes by a new generation of drainage systems; followed up through an erosional surface by sandy- to silty-based younger alluvium; and (2) brought on footslopes of fault scarps reviving the early developed scarps and inselbergs. These features argue for crustal uplifting of Sinai landscapes syn-rifting of the Gulf of Aqaba rift basin. Oblique orientation of the Red Sea-Gulf of Suez rift relative to the WNW-trending Precambrian Najd faults; and extrusion of volcanic rocks in directions parallel to the rift boundaries geometrically suggest rifting on tensional fractures that mutually bridge the Najd fault-related shear fractures. These aspects might envisage reactivation of the preexisting Precambrian fracture patterns in the Arabian-Nubian shield by the Oligo-Miocene to Pleistocene rift-controlled stress field.

Plume-induced continental rifting and break-up in ultra-slow extension context: Insights from 3D numerical modeling

Breaking the lithosphere in extension without exceeding the driving far-field forces available on Earth is a tough quantitative modeling problem. One can tear it apart by propagation of an existing oceanic basin or weakness zone or one must assist rifting with magmatic processes, which drop the effective stress and weakens locally the lithosphere. While previous 3D models have demonstrated that non-cylindrical plumes produce almost cylindrical rift structures in a lithosphere under slight far-field loading, our contribution goes one step further by producing models of complete continental break-up. We investigate in details how the rheological stratification of the continental lithosphere interacting with active mantle plume influences the geometry and dynamics of rifting to continental break-up in 3D. We find that, irrespective of the rheological stratification, a plume-induced rifting process always occurs in two stages: an early crustal rifting stage and a late lithospheric necking (breakup) stage. In case of a rheologically decoupled lithosphere, initial brittle deformation is concentrated in the upper crust and strongly localized due to compensating ductile flow of lower-crustal material (core complex extension mode). On the contrary, rheological coupling between upper crust and lithospheric mantle results in highly distributed brittle deformation in the crust above mantle plume head (wide rift mode). Both core complex-like and wide rifting are followed by an abrupt transition to narrow rift stage when a localized ascent of mantle plume material focuses high strain along faults zones breaking through the entire lithosphere. The Main Ethiopian Rift, the Basin and Range province, and the East Shetland Basin may be natural examples of regions that have passed through these two stages of extension. Across-strike and along-strike asymmetry of break-up patterns arising spontaneously within initially symmetrical and laterally homogenous environment seems to be an intrinsic characteristic of plume-induced rifting.

Spreading vs. Rifting as modes of extensional tectonics on the globally expanded Ganymede

The formation of Ganymede's sulci is likely related to extensional tectonics that affected this largest icy satellite of Jupiter. Through geometric and structural analyses we reconstructed the pre-deformed terrains and we recognized two different modes of extension associated with sulci. In the first mode, smooth sulci constitute spreading centers between two dark terrain plates, similar to the fast oceanic spreading centers on Earth. Here extension is primarily accommodated by crustal accretion of newly formed icy crust. In the second mode, dark terrain extension is mainly accommodated by swaths of normal fault systems analogous to Earth's continental crustal rifts. A comparison with terrestrial extensional analogues, based on the fault displacement/length (Dmax/L) ratio, spacing and morphology, showed that magmato-tectonic spreading centers and continental crustal rifts on Earth follow the same relative patterns observed on Ganymede. Our results suggest that the amount of extensional strain may have previously been underestimated since the occurrence of spreading centers may have played a major role in the tectonic evolution of the globally expanded Ganymede. We also discuss a possible model for the origin of the different modes of extension in the context of the global expansion of the satellite.

Geochemical Characteristics and Tectonic Setting of Amphibolites in Ifewara Area, Ife-Ilesha Schist Belt, Southwestern Nigeria

Trace and Rare Earth Elements (REEs) data are used to constrain the geochemical evolution of the amphibolites from Ifewara in the Ife-Ilesha schist belt of southwestern Nigeria. The amphibolites can be grouped into banded and sheared amphibolites. Major element data show SiO2 (48.34%), Fe2O3 (11.03-17.88%), MgO (5.76-9.90%), CaO (7.76-18.6%) and TiO2 (0.44-1.77%) contents which are similar to amphibolites in other schist belts in Nigeria. The Al2O3 (2.85-15.55%) content is varied, with the higher values suggesting alkali basalt protolith. Trace and rare earth elements composition reveal Sr (160-1077ppm), Rb (0.5-22.9ppm), Ni (4.7-10.2ppm), Co (12.2-50.9 ppm) and Cr (2-7ppm). Chondrite-normalized REE patterns show that the banded amphibolites have HREE depletion and both negative and positive Eu anomalies while the sheared variety showed slight LREE enrichment with no apparent Eu anomaly. The study amphibolites plot in the Mid Oceanic Ridge Basalts (MORB) and within plate basalt fields on the Zr/Y vs Zr discriminatory diagrams. They are further classified as volcanic arc basalt and E-type MORB on the Th- Hf/3- Ta and the Zr-Nb-Y diagrams. The amphibolites precursor is considered a tholeiitic suite that suffered crustal contamination, during emplacement in a rifted crust.

Magmatism in Western Cuddapahs: The Mafic Sills and Lava Flows of Vempalle and Tadpatri Formations

Abstract: The western part of Cuddapah basin has been considered as a foreland to the fold-thrust belt represented by eastern Nallamalais and Nellore schist belt. The igneous activities, in the form of lava flows and sills, in western Cuddapahs have been a subject of controversy since tectonogenetic models ranging from a mantle plume to lithospheric extension have been suggested. The present work covers a detailed study of the mafic sills and lava flows of Vempalle and Tadpatri formations in western Cuddapahs, which earlier workers have also noted. A detailed field and laboratory work in this study brings out, for the first time, five distinct petrographic and geochemical varieties of sills. Geochemical studies also suggest that the different sill types have originated from varying degrees of partial melting of a metasomatized sub-crustal lithospheric mantle, the metasomatism of source being attributed to older Archaean convergent settings/greenstone belts that the basin unconformably overlies. The possible contribution of any small plume is only heating and doming of overlying mantle that induced incipient crustal rifting. Our geochemical studies on the lava flows of Vempalle Formation also suggest that the magma was derived from small degree of partial melting of spinel-garnet peridotite mantle in a rift setting.

Ambient noise tomography of the East African Rift in Mozambique

Seismic ambient noise tomography is applied to central and southern Mozambique, located in the tip of the East African Rift (EAR). The deployment of MOZART seismic network, with a total of 30 broad-band stations continuously recording for 26 months, allowed us to carry out the first tomographic study of the crust under this region, which until now remained largely unexplored at this scale. From cross-correlations extracted from coherent noise we obtained Rayleigh wave group velocity dispersion curves for the period range 5–40 s. These dispersion relations were inverted to produce group velocity maps, and 1-D shear wave velocity profiles at selected points. High group velocities are observed at all periods on the eastern edge of the Kaapvaal and Zimbabwe cratons, in agreement with the findings of previous studies. Further east, a pronounced slow anomaly is observed in central and southern Mozambique, where the rifting between southern Africa and Antarctica created a passive margin in the Mesozoic, and further rifting is currently happening as a result of the southward propagation of the EAR. In this study, we also addressed the question concerning the nature of the crust (continental versus oceanic) in the Mozambique Coastal Plains (MCP), still in debate. Our data do not support previous suggestions that the MCP are floored by oceanic crust since a shallow Moho could not be detected, and we discuss an alternative explanation for its ocean-like magnetic signature. Our velocity maps suggest that the crystalline basement of the Zimbabwe craton may extend further east well into Mozambique underneath the sediment cover, contrary to what is usually assumed, while further south the Kaapval craton passes into slow rifted crust at the Lebombo monocline as expected. The sharp passage from fast crust to slow crust on the northern part of the study area coincides with the seismically active NNE-SSW Urema rift, while further south the Mazenga graben adopts an N-S direction parallel to the eastern limit of the Kaapvaal craton. We conclude that these two extensional structures herald the southward continuation of the EAR, and infer a structural control of the transition between the two types of crust on the ongoing deformation.

Regional stratigraphy and subsidence of Orphan Basin near the time of breakup and implications for rifting processes

AbstractThe stratigraphic, subsidence and structural history of Orphan Basin, offshore the island of Newfoundland, Canada, is described from well data and tied to a regional seismic grid. This large (400 by 400 km) rifted basin is part of the non‐volcanic rifted margin in the northwest Atlantic Ocean, which had a long and complex rift history spanning Middle Jurassic to Aptian time. The basin is underlain by variably thinned continental crust, locally &lt;10‐km thick. Our work highlights the complex structure, with major upper crustal faults terminating in the mid‐crust, while lower crustal reflectivity suggests ductile flow, perhaps accommodating depth‐dependent extension. We describe three major stratigraphic horizons connected to breakup and the early post‐rift. An Aptian–Albian unconformity appears to mark the end of crustal rifting in the basin, and a second, more subdued Santonian unconformity was also noted atop basement highs and along the proximal margins of the basin. Only minor thermal subsidence occurred between development of these two horizons. The main phase of post‐rift subsidence was delayed until post‐Santonian time, with rapid subsidence culminating in the development of a major flooding surface in base Tertiary time. Conventional models of rifting events predict significant basin thermal subsidence immediately following continental lithospheric breakup. In the Orphan Basin, however, this subsidence was delayed for about 25–30 Myr and requires more thinning of the mantle lithosphere than the crust. Models of the subsidence history suggest that extreme thinning of the lithospheric mantle continued well into the post‐rift period. This is consistent with edge‐driven, small‐scale convective flow in the mantle, which may thin the lithosphere from below. A hot spot may also have been present below the region in Aptian–Albian time.