Passive Margin Stage Research Articles

Neoarchean to Paleoproterozoic crustal evolution of the central Aravalli-Banded Gneissic Complex (NW India): Evidence from zircon U-Pb-Hf isotope systematics and whole-rock composition of Neoarchean sanukitoids, Paleoproterozoic granitoids and metasedimentary rocks

The central Aravalli-Banded Gneissic Complex in NW India is an Archean nucleus that was extensively reworked during the late Paleo- to Neoproterozoic. New results of combined zircon U-Pb-Hf isotopes and whole-rock geochemical data reveal that the basement was affected by felsic magmatism during the Neoarchean and Paleoproterozoic, followed by a passive margin stage. The results provide evidence for coeval emplacement of sanukitoids and TTGs at ca. 2540 Ma, and for granitoid magmatism at ca. 1875 Ma. Similar REE patterns of the synchronous sanukitoids and TTGs, coupled with high SiO2 contents (59.2–70.9 wt%), subchondritic εHf2.55 Ga values (−5.0 to −2.6) and Hf-model ages (3246–3123 Ma) of the Masuda sanukitoids indicate that the sanukitoid magmas were formed mainly by interaction of TTG magmas (generated by the melting of subducted Paleo- to Mesoarchean oceanic crust) with the overlying enriched peridotite. The calc-alkalic and magnesian compositions, enrichment in LREE and LILE along with pronounced depletion in HFSE (Nb, P and Ti) in ca. 1875 Ma granitoid gneisses imply subduction-related magmatism during Paleoproterozoic. The predominance of subchondritic εHf1.87 Ga values, ranging from −4.4 to +1.7, suggests that the parental magmas mainly resulted from the reworking of heterogeneous Meso- to Neoarchean felsic crust. Age-Hf isotope data of detrital zircon grains provide evidence that clastic sedimentary rocks covering the central Aravalli basement were deposited in a passive margin setting at < 1730 Ma, and the detritus was mainly sourced from adjoining Neoarchean and Paleoproterozoic granitoids. Comparison of our new data with other Indian and worldwide cratons indicate that the Aravalli orogen represents a unique terrane, where the Neoarchean TTGs and sanukitoids with subchondritic Hf isotopic signatures were emplaced nearly coeval in a close spatial relationship. The Aravalli Archean basement was subsequently reconfigured by voluminous felsic magmatism during Paleoproterozoic at 1875–1810 Ma. Furthermore, the compiled zircon U-Pb age and Hf-Nd isotope data for the late Paleoproterozoic subduction-related magmatic rocks provide evidence for substantial crustal reworking during assembly of the Columbia supercontinent.

The cryptic stratigraphic record of the syn‐ to post‐rift transition in the offshore Campos Basin, SE Brazil

AbstractRift basins typically comprise three main tectono‐stratigraphic stages; pre‐, syn‐ and post‐rift. The syn‐rift stage is often characterised by the deposition of asymmetric wedges of growth strata that record differential subsidence caused by active normal faulting. The subsequent post‐rift stage is defined by long‐wavelength subsidence driven by lithospheric cooling and is typified by the deposition of broadly tabular stratal packages that drape any rift‐related relief. The stratigraphic contact between syn‐ and post‐rift rocks is often thought to be represented by an erosional unconformity. However, the late syn‐rift to early post‐rift stratigraphic record is commonly far more complex since (i) the associated tectonic transition is not instantaneous; (ii) net subsidence may be punctuated by transient periods of uplift; and (iii) strain often migrates oceanward during rifting until continental breakup is achieved with crustal rupture. Previous publications on the Eastern Brazilian marginal basins have not historically used the tripartite scheme outlined above, with the post–pre‐rift interval instead being subdivided into rift, sag and passive margin tectono‐stratigraphic stages. In addition, the sag stage has been previously described as late syn‐rift, early post‐rift or as a transition between the two, with the passive margin stage being equivalent to the classically defined post‐rift, drift stage. Two (rather than one) erosional unconformities are also identified within the rift‐to‐sag succession. In this work, we use 2D and 3D seismic reflection and borehole data to discuss the expression of and controls on the syn‐ to post‐rift transition in the shallow and deep water domains of the south‐central Campos Basin, south‐east Brazil. We identified three seismic–stratigraphic sequences bounded by unconformities, named lower and upper pre‐salt and salt. The lower pre‐salt interval is characterised by wedge‐shaped packages of reflections that thicken towards graben and half‐graben‐bounding normal faults. This stage ends with the development of an angular unconformity, inferred to form as a result of the onset of the oceanward migration of deformation. The upper pre‐salt is typically defined by packages of subparallel and relatively continuous reflections that are broadly lenticular and thin towards fault‐bound basement highs, but that locally contain packages that thicken against faults. The pre‐salt to salt contact is defined by an erosional unconformity that is largely restricted to basement highs, and which is inferred to have formed due to base‐level fall and uplift associated with local fault reactivation, resulting in the formation of channels of possible fluvial origin. Based on its geometries and seismic facies, we conclude that the lower pre‐salt interval is syn‐rifting and syn‐tectonic, deposited during active continental extension and upper crustal faulting affecting the entire evolving margin, whereas the overlying upper pre‐salt is syn‐rifting and post‐tectonic in the Campos Basin, deposited when extension and faulting had migrated seaward to the future location of the spreading centre. The results of our study support the arising notion that the syn‐rift sequence does not only display syn‐tectonic sedimentary packages, and thus the tripartite tectono‐stratigraphic model for rift development is too simplistic and cannot be applied when assessing rifts in the context of the regional development of continental margins.

Impact of mechanical stratigraphy on deformation style and distribution of seismicity in the central External Dinarides: a 2D forward kinematic modelling study

The External Dinarides fold-thrust belt formed during Mid-Eocene–Oligocene times by SW-propagating thrusting from the Internal Dinarides towards the Adriatic foreland. Although previously considered as structurally quite uniform, recent work reported along-strike contrasting deformation styles in two structural domains within this fold-thrust belt. The two areas with very contrasting deformation styles are separated by the N–S-striking dextral Split-Karlovac Fault, a 250 km long, transpressive transfer fault. The southeastern domain is characterized by a thin-skinned SW-vergent nappe stack in contrast to the northwestern domain, where a set of blind, thick-skinned top-SW thrust duplexes prevail underneath the passive NE-vergent backthrusts. To better understand why the External Dinarides underwent such contrasting along-strike deformation, we reconsidered a temporal and spatial along- and across-strike distribution of Paleo-Mesozoic lithofacies to both sides of the Split-Karlovac Fault and estimated the role of mechanical stratigraphy on deformation styles in this part of the fold-thrust belt. Therefore, we constructed a new 2D kinematic forward model in the western backthrust-dominated domain. Our best-fit forward-modelled balanced cross section across the central Velebit Mtn. portrays a 75 km wide triangle zone. This zone took up at least 47 km of shortening during Eo-Oligocene times. It comprises a set of thin-skinned NE-vergent backthrusts detached in the upper Paleozoic atop a SW-vergent thick-skinned antiformal stack detached in the lower Paleozoic Adriatic basement. The NE-vergent backthrusts likely nucleated at lateral facies boundaries related to extensional half grabens that locally formed during Middle Triassic and Late Jurassic passive margin extension. During the Eo-Oligocene folding and thrusting, the selective inversion of inherited Mesozoic half grabens boundary faults into the NE-vergent backthrusts in the northwestern domain led to the observed along-strike changes in the deformation style of the External Dinarides. A seismotectonic analysis of instrumentally recorded earthquakes suggests contrasting seismic behaviour along the central and southern Velebit transects within the northwestern structural domain. The central Velebit Mtn. triangle structure appears to currently accommodate dominantly strike-slip motion, with reverse faulting being confined to east of the Split-Karlovac Fault. In contrast, seismicity along the southern Velebit cross section appears to be confined to the structurally lowermost parts of the triangle zone and the foreland, while it´s structurally higher parts are less seismically active. Also, a predominance of reverse faulting along this transect suggests ongoing accommodation of shortening in this part. Our results indicate that both the variations in the mechanical stratigraphy and the pre-orogenic structural inheritance obtained during rifting and passive margin stages exert control on contractional structures within the External Dinarides, including the distribution of present-day seismicity.

Post-salt magmatism in the Campos Basin, offshore SE Brazil: style, distribution, and relationship to salt tectonics

Substantial magmatism occurred during the development of the marginal basins of the central and southern South Atlantic. Situated on the Brazilian side of the central segment, the Campos and Santos basin represent a transitional margin, located between the magma-rich margin in the north and the magma-poor margin in the south. In addition to magmatism associated with the initiation of continental rifting, the southeast Brazilian basins experienced three main magmatic events; one during the early post-rift (i.e., sag stage) and two during the late post-rift (i.e., passive margin stage), both occurring after deposition of a thick, late Aptian salt layer. The products of post-salt, intrusive and extrusive magmatism in the southern Campos Basin are imaged in seismic reflection data and have been directly penetrated by boreholes, yet they remain poorly understood in terms of their style, detailed geometry, and distribution. Furthermore, the temporal, spatial and possibly genetic relationships between these post-salt igneous products and salt-tectonics are largely unknown. Documenting these possible salt-magma interactions is important to understanding the tectono-stratigraphic evolution of Campos Basin and other salt-bearing passive margin basins subject to magmatic activity. We use 2D and 3D seismic reflection and borehole data from the southeastern Campos Basin, offshore Brazil to map and characterize post-salt igneous sills and volcanoes, and to understand the interaction between salt tectonics and igneous products. We identified 99 sills emplaced within uppermost Aptian-to-Maastrichtian strata, 83 emplaced above the salt and 16 intra-salt, and 15 volcanoes within Upper Cretaceous-to-lower Paleogene strata, associated with a >310 km2 lava field. We also identify four, subsalt-sourced, vent-like structures, which vary in age from Paleogene to Late Cretaceous. Salt and overburden structures define a domain of extensional deformation on a base-salt basement high, and a domain of contractional deformation in the adjacent basement low. The relationship between igneous intrusions and salt tectonics is complex, i.e., in areas where they are common, sills appear to have locally restricted salt flow and the growth of large salt structures, whereas in other cases, salt structures and overlying faults appear to have partially acted as pathways to intruding magma, allowing it to ascend to relatively shallow depths. Extrusive igneous products exclusively developed after the main phase of salt movement. However, salt distribution influenced the location of volcanoes, given the latter developed where salt is welded or very thin. Salt flow and thickness also controlled the distribution of lava flow products, which accumulated in structural lows between large salt tectonic structures. Our work indicates a complex relationship between salt tectonics and magmatic emplacement that impact the post-rift evolution of Campos Basin.

Oblique rifting along transfer zones: The structural evolution model revealed by physical modeling

Oblique rifting is a common process that acts during the rupture of continental masses and can evolve to the passive margin stage. The inter-rift (or intra-rift) interaction can occur by transfer zones (TZs). These zones can be formed at the most diverse scales and, on a large scale, can result in the development of oblique rifts. To understand how TZs nucleate and evolve at the plate scale, we simulated, through physical modeling, the development of oblique transfer zones that connect two parallel rifts (with no lateral overlap) developed orthogonally to the extensional direction. Our results showed that obliquity and rheology could play an important role in the structural architecture of rifts that originated in such a structural environment. When the obliquity ranges from low to intermediate (i.e., 20°≤ α ≤ 45°), these rifts can be compartmentalized into two main structural domains - extremity, and central regions - with diachronic evolution and distinct kinematics. Extremity regions are characterized by oblique-slip deformation with intense rotation/tilting of the substrate. The central region is dominated by transcurrent (and oblique-slip) deformation with the presence of marginal plateaus, pull-apart, and oblique basins. Small variations in the obliquity (around 5°) are enough to affect parameters like faults' dip and geometry, and depocenters’ width and depth, thereby promoting changes in the morphology, geometry, architecture, and configuration of TZs. In a more mature stage, and on a continental scale, the TZs evolve into oblique rifts whose margins are composed of oblique and transcurrent fault segments (transfer/transcurrent) that together define a typical zig-zag geometry. The sizes and numbers of these segments depend on the obliquity and the rheology. Transcurrent fault segments are more common when the obliquity is 20° and 25°, in the model with quartz sand only. The modeling data presented here can be compared with the equatorial Atlantic margin of South America, assuming that it may represent an important plate-scale oblique transfer zone (20°) developed between the South and Central Atlantic.

Provenance evolution of the Paleoproterozoic metasedimentary cover sequences in northern Fennoscandia: Age distribution, geochemistry, and zircon morphology

The metasedimentary basin-fill successions of the Central Lapland belt, Peräpohja belt, and Kuusamo belt form an important part of the northern Fennoscandia Shield. They each record ca. 600 Ma (2.5–1.9 Ga) of sedimentation history and tectonic evolution, providing important information about past depositional conditions and tectonic settings. Detrital zircon geochronology shows that Archean age populations between 3.00 and 2.70 Ga have provenances from the surrounding basement complexes of granitoid/TTG rocks, whereas the Paleoproterozoic age populations (≤2.25–1.86 Ga) can be linked to the surrounding granitoids in northern Finland and Sweden. Zircon morphology studies indicate that basin sedimentation and/or provenance area(s) were likely influenced by eolian processes ca. 2.3–2.1 (or possible earlier). Major and trace element geochemistry indicates minor to moderate weathering trends, closely resembling Archean-Proterozoic sandstones and shales. Provenance signatures of tectonic settings vary from rift to arc and collisional. We propose five basin evolution stages for the Central Lapland belt, Peräpohja belt, and Kuusamo belt: a rifting stage from 2.5 to 2.1 Ga including (i) initial rifting/early syn-rift, (ii) syn-rift, and (iii) syn-rift to early post-rift; (iv) a passive margin (post-rift) stage from 2.1 to 1.94–1.92 Ga; and (v) a foreland basin system 1.94–1.92 to 1.88 Ga. The rifting stage includes bimodal subaerial to subaqueous volcanism with continental to marine sedimentation, and the initial accumulation of carbonate and evaporate successions, indicating later local tectonic stability and restricted hydrological circulation. The early development of an epeiric sea setting was followed by a passive margin stage and the development of aulacogens, controlled by cyclic sea-level fluctuations superimposed on long-term thermal subsidence. This stage was characterized by the accumulation of hemipelagic deposits together with gravity-driven sandstone deposits and associated subaqueous volcanism. The foreland basin system comprises an initial compressional stage with the development of forebulge and foredeep depozones and magmatic arc collision. This was followed by the main compressional stage, where basin inversion resulted in a foreland fold-thrust belt. The foreland basin system includes syn- and post-orogenic deposystems and a transition from underfilled marine sedimentation to overfilled non-marine deposits.

Gravimetric and magnetic fabric study of the Sintra Igneous complex: laccolith-plug emplacement in the Western Iberian passive margin

The geometry and emplacement of the ~ 96 km2, Late Cretaceous Sintra Igneous complex (SIC, ca. 80 Ma) into the West Iberian passive margin is presented, based on structural data, gravimetric modeling, and magnetic fabrics. A granite laccolith (~ 76 km2, < 1 km thick, according to gravimetric modeling) surrounds a suite of gabbro–diorite–syenite plugs (~ 20 km2, ~ 4 km deep) and is encircled by cone sheets and radial dykes. Anisotropy of Magnetic Susceptibility was interpreted from 54 sites showing fabrics of para- and ferro-magnetic origin. Most fabrics can be interpreted to have a magmatic origin, according to the scarcity of solid-state deformation in most part of the massif. Magnetic foliations are shallowly dipping in the granite laccolith and contain a sub-horizontal ENE–WSW lineation. The gabbro–syenite body displays concentric magnetic foliations having variable dips and steeply-plunging lineations. The SIC can be interpreted to be intruded along an NNW–SSE, 200 km-long fault, perpendicular to the magnetic lineation within the laccolith, and was preceded by the intrusion of basic sills and plugs. The SIC intruded the Mesozoic series of the Lusitanian Basin during the post-rift, passive margin stage, and its geometry was only slightly modified during the Paleogene inversion that resulted in thrusting of the northern border of the intrusion over the country rocks.

Speculative petroleum systems of the southern Pelotas Basin, offshore Uruguay

The Pelotas Basin of Brazil and Uruguay represents a frontier basin with under-explored hydrocarbon potential. Although oil and gas accumulations have yet to be identified, only 21 exploratory wells have been drilled in an area of more than 330,000 km2, 20 of which are located in the Brazilian portion of the basin. A detailed study of the petroleum system of offshore Uruguay has strong potential to contribute to a better characterization of the capacity of the basin to generate and accumulate hydrocarbons. Three stages have previously been recognized during the evolution of Pelotas basin: (1) a prerift phase which preserved Paleozoic and Mesozoic units of the Paraná Basin; (2) an Early Cretaceous volcano-sedimentary synrift phase; and (3) a Cretaceous to Cenozoic postrift phase deposited during the passive margin stage. In this study, we use sequence stratigraphy methodology to interpret 2D multichannel seismic sections of the southern segment of the Pelotas Basin in the Uruguayan Atlantic margin. This analysis allows us to identify depositional sequences, systems tracts and the distribution of the main elements of the potential petroleum systems. Following our analysis, we propose six speculative petroleum systems (SPS) in the Pelotas Basin. The first SPS is related to the prerift phase and is represented by a Lower Permian restricted marine source rock and reservoirs related to Permian to Upper Jurassic aeolian and fluvial sandstones. The second SPS corresponds to the synrift phase and is constituted by a Barremian lacustrine source rock with reservoirs of alluvial/fluvial sandstones of the same age. The other four proposed SPS are associated with the postrift phase, represented by marine source rocks related to Aptian-Albian, Cenomanian-Turonian and Paleocene transgressions, all of which are identified in the region and interpreted in seismic lines from Uruguay. These postrift SPS have predominantly siliciclastic reservoirs represented by Early Cretaceous aeolian sandstones and Cretaceous to Cenozoic deltaic sandstones and turbidites.

Ediacaran–Cambrian basin evolution in the Koonenberry Belt (eastern Australia): Implications for the geodynamics of the Delamerian Orogen

Contention surrounds the Ediacaran–Cambrian geodynamic evolution of the palaeo-Pacific margin of Gondwana as it underwent a transition from passive to active margin tectonics. In Australia, disagreement stems from conflicting geodynamic models for the Delamerian Orogen, which differ in the polarity of subduction and the state of the subduction hinge (i.e., stationary or retreating). This study tests competing models of the Delamerian Orogen through reconstructing Ediacaran–Cambrian basin evolution in the Koonenberry Belt, Australia. This was done through characterising the mineral and U–Pb detrital zircon age provenance of sediments deposited during postulated passive and active margin stages. Based on these data, we present a new basin evolution model for the Koonenberry Belt, which also impacts palaeogeographic models of Australia and East Gondwana. Our basin evolution and palaeogeographic model is composed of four main stages, namely: (i) Ediacaran passive margin stage with sediments derived from the Musgrave Province; (ii) Middle Cambrian (517–500Ma) convergent margin stage with sediments derived from collisional orogens in central Gondwana (i.e., the Maud Belt of East Antarctica) and deposited in a backarc setting; (iii) crustal shortening during the c. 500Ma Delamerian Orogeny, and; (iv) Middle to Late Cambrian–Ordovician stage with sediments sourced from the local basement and 520–490Ma igneous rocks and deposited into post-orogenic pull-apart basins. Based on this new basin evolution model we propose a new geodynamic model for the Cambrian evolution of the Koonenberry Belt where: (i) the initiation of a west-dipping subduction zone at c. 517Ma was associated with incipient calc-alkaline magmatism (Mount Wright Volcanics) and deposition of the Teltawongee and Ponto groups; (ii) immediate east-directed retreat of the subduction zone positioned the Koonenberry Belt in a backarc basin setting (517 to 500Ma), which became a depocentre for continued deposition of the Teltawongee and Ponto groups; (iii) inversion of the backarc basin during the c. 500 Delamerian Orogeny was driven by increased upper and low plate coupling caused by the arrival of a lower plate asperity to the subduction hinge, and; (iv) subduction of the asperity resulted in renewed rollback and upper plate extension, leading to the development of small, post-orogenic pull-apart basins that received locally derived detritus.

Mesozoic rift to post-rift tectonostratigraphy of the Sverdrup Basin, Canadian Arctic

Jurassic-Cretaceous rift successions and basin geometries of the Sverdrup Basin are reconstructed from a review and integration of stratigraphy, igneous records, outcrop maps, and subsurface data. The rift onset unconformity is in the Lower Jurassic portion of the Heiberg Group (approximately 200–190 Ma). Facies transgress from early syn-rift sandstones of the King Christian Formation to marine mudstones of the Jameson Bay Formation. The syn-rift succession of marine mudstones in the basin centre, Jameson Bay to Deer Bay formations, ranges from Early Jurassic (Pleinsbachian) to Early Cretaceous (Valanginian). Early post-rift deposits of the lower Isachsen Formation are truncated by the sub-Hauterivian unconformity, which is interpreted as a break up unconformity at approximately 135–130 Ma. Cessation of rift subsidence allowed for late post-rift sandstone deposits of the Isachsen Formation to be distributed across the entire basin. Marine deposition to form mudstone of the Christopher Formation throughout the Canadian Arctic Islands and outside of the rift basin records establishment of a broad marine shelf during post-rift thermal subsidence at the start of a passive margin stage. The onset of the High Arctic Large Igneous Province at approximately 130 Ma appears to coincide with the breakup unconformity, and it is quite typical that magma-poor rifted margins have mainly post-rift igneous rocks. We extend the magma-poor characterization where rifting is driven by lithospheric extension, to speculatively consider that the records from Sverdrup Basin are consistent with tectonic models of retro-arc extension and intra-continental rifting that have previously been proposed for the Amerasia Basin under the Arctic Ocean.

Mesozoic lithofacies palaeogeography and petroleum prospectivity in North Carnarvon Basin, Australia

The North Carnarvon Basin, which lies in the North West Shelf of Australia, is highly rich in gas resources. As a typical passive marginal basin, it experienced the pre-rifting, early rifting, main rifting, late rifting, post-rifting sagging and passive margin stages. The basin was mainly filled with thick Mesozoic-Cenozoic sediments, of which the Mesozoic hosts the principal source, reservoir and seal intervals. Mesozoic palaeogeography has an important control on the oil and gas distribution. Triassic gas-prone source rocks of deltaic origin determine the high endowment of natural gases in the North Carnarvon Basin. The more restricted distribution of oil accumulations is controlled by oil source rocks in the Upper Jurassic Dingo Claystone. The Muderong Shale deposited in the Early Cretaceous marine transgression provides the effective regional seal for the underlying oil and gas reservoirs.

Neoproterozoic metamorphic events along the eastern margin of the East Sahara Ghost Craton at Sabaloka and Bayuda, Sudan: Petrology and texturally controlled in-situ monazite dating

Recent geodynamic models for the evolution of the East Sahara Ghost Craton (ESGC) propose delamination of the subcontinental lithospheric mantle due to the formation of Pan-African orogenic belts along its margins. To identify the behavior of the craton's basement during the Neoproterozoic we studied metapelitic rocks from the eastern margin of the craton at Sabaloka and Bayuda, both of which lie about 230km apart along the boundary with the Arabian-Nubian Shield and are considered to be parts of the ESGC. The results are compared to those published for the central part of the craton at Uweinat for a better understanding of the Neoproterozoic tectonic evolution of the craton as a whole.Petrology and texturally controlled in-situ monazite dating reveals that the Sabaloka granulites and the Bayuda metasediments experienced two different metamorphic events. The events at Sabaloka and Bayuda are neither synchronous nor similar in intensity and/or nature, with the events in Bayuda being older than those in Sabaloka and also of lower intensity. At Sabaloka, monazite inclusions in garnet record the timing of an early Neoproterozoic high temperature crustal thickening event at 780–720Ma, while monazite grains in coarse-grained cordierite–spinel symplectites indicate the timing of a Neoproterozoic high to ultra-high temperature overprint at ∼602Ma. Applying the same approach of monazite dating to the pelitic schists from the Dam El Tor fold-and-thrust-belt in the Kurmut Terrane at Bayuda, a crustal thickening event leading to amphibolite-facies metamorphism is dated at ∼890Ma. It was followed by low-grade metamorphism at ∼670Ma leading to recrystallization of matrix minerals. This study thus indicates that the nature and timing of Neoproterozoic tectonometamorphic events along the eastern margin of the ESGC were heterogeneous, both temporally and in their intensity. The ∼890Ma age obtained from the Kurmut Terrane metasediment provides, for the first time, a direct age of the amphibolite-facies metamorphism, slightly later than the magmatism during the Bayudian event dated at 920–900Ma in the Rahaba-Absol Terrane. We infer that tectonism in the juvenile oceanic arc and ophiolitic terranes of the Arabian Nubian Shield (ANS) in the east triggered the end of the passive margin stage of the ESGC and led to the subduction related magmatic–metamorphic Bayudian event. The passive margin stage of the craton further south at Sabaloka seems to have ended later at around 780–720Ma, which may have caused the first high temperature crustal thickening event there. Published magmatic formation ages of 810–806Ma from basalts and ophiolites indicate a rifting stage that formed oceanic re-entrants in the Bayuda and Nubian desert areas along the margin of the craton, subsequent to the Bayudian event. After collision with some of the eastern juvenile ANS terranes the oceanic re-entrants were closed, causing magmatism and a low-grade metamorphic overprint on the amphibolite-facies rocks dated here at ∼670Ma. At Sabaloka, the high-T to UHT granulite-facies decompression event at ∼602Ma and the widespread granitic magmatism all along the eastern margin between 650 and 570Ma was most likely due to the final collision of the ESGC and the ANS. The heterogeneity of Neoproterozoic tectonometamorphic events along the eastern margin of the ESGC thus indicates a local tectonic control on these metamorphic events.In contrast to the eastern margin, no evidence for a metamorphic event during the Neoproterozoic has been found in the center of the craton at Uweinat. The granulites at Uweinat record a Neorchean UHT metamorphism and a Paleoproterozoic UHT reworking. Therefore, it is more likely that the proposed delamination of the subcontinental lithospheric mantle of the entire ESGC was not synchronous and was caused by different orogenic events in different parts of the craton.

Detrital zircon age patterns and provenance assessment for pre-glacial to post-glacial successions of the Neoproterozoic Macaúbas Group, Araçuaí orogen, Brazil

The Neoproterozoic Macaúbas Group represents the precursor basin system of the Araçuaí–West Congo orogen (SE Brazil – SW Africa), located between the São Francisco and Congo cratons. The Macaúbas basin evolved from continental rift settings to an inland-sea basin (a gulf) partially floored by oceanic crust and connected to aulacogens. Its evolution was coeval with at least one of the major Neoproterozoic glaciations. In this paper, we present more than 450 new U–Pb ages on detrital zircon grains from four units of the Macaúbas Group, covering glacial-related to post-glacial sedimentation stages. The results suggest that the main sedimentary supplies of the Macaúbas basin should have come from secondary sources, located in the São Francisco and Congo cratons. The pre-glacial units of the Macaúbas Group are the only ones that show important contribution from 1100 to 1200Ma sources, and lack Archaean zircons, which are common in younger rocks of the group. These features indicate an important change in the source areas, coherent with the climate change to glacial conditions and with a regional unconformity previously mapped. Sources between 1500–1600Ma are more significant in the continental rift stages, decreasing in the passive margin stage. This suggests changes driven by the end of a glaciation and/or the breakup of the continental lithosphere. Tonian zircons occur throughout all the basin stages, and significantly increase in the late rift and passive margin successions, recording the most abundant rift-related magmatism yet found in the Araçuaí–West Congo orogen. Detrital zircon grains as young as 750Ma reveal contributions from Cryogenian sources to passive margin successions, suggesting sedimentary supplies from the South Bahia Alkaline Province and/or Rio Negro magmatic arc, in Brazil, and/or from rocks found in Africa (e.g., La Louila volcanic rocks). The long lasting evolution of the Macaúbas basin (c. 930Ma to c. 600Ma) would have involved more than one continental rifting event before the onset of an ocean-floor spreading phase around 660Ma.

Sedimentology and sequence stratigraphy from a mixed (carbonate–siliciclastic) rift to passive margin transition: The Early to Middle Cambrian of the Argentine Precordillera

Through the analysis of facies stacking patterns, geometries and bounding surfaces, sequence stratigraphy provides a framework to understand sedimentary facies distribution and evolution. Many published works contain noteworthy examples where a sequence stratigraphy approach was applied to Middle–Late Paleozoic, Mesozoic and Cenozoic sedimentary units. Early Paleozoic examples, however, remain relatively scarce. This article addresses the exceptionally well-exposed Early to Middle Cambrian La Laja Formation and equivalent units of the Argentine Precordillera carbonate bank. Given the stratigraphic and faunal similarities, the Precordillera is considered as a Laurentia-derived allochthonous terrane, which detached from the Laurentia margin during Early to Middle Cambrian and docked against western Gondwana margin by Mid-Ordovician time. This work focuses on the lowermost mixed carbonate–siliciclastic interval that records the early evolution of the Precordillera terrane, providing a link to better understand the paired Laurentia margin and Lower Paleozoic paleogeography. The study was built upon a detailed facies and microfacies analysis where thirteen facies associations were recognized, representing a spectrum of environments ranging from deep subtidal, shallow subtidal to inter-supratidal sabkha facies. Our analysis allowed the recognition of three sequences (S1, S2 and S3) separated by unconformities (D1, D2, D3 and D4). A reciprocal sedimentation model is suggested to explain coarse-grained siliciclastic intervals overlying and bypassing subtidal limestones in S1 and S2. However the fine-grained siliciclastic-rich carbonate intervals within S2 seem to represent transgressive deposits, probably influenced by higher accommodation and climatically driven siliciclastic influx. Lateral facies and thickness variations suggest that the La Laja Formation developed during the progressive margin stabilization with differential subsidence. The sequence boundary separating the La Laja Formation from the overlying Zonda Formation represents an important architectural change from differential subsidence during the late rift stage in the Early–Middle Cambrian La Laja Formation to the passive margin stage with flat-lying remarkably continuous rhythmic dolomites in the overlying Upper Cambrian Zonda Formation. Further strengthening the stratigraphic similarities with the Lower Paleozoic of Laurentia, a similar transition has also been suggested for coeval units of the Laurentia paired margin. This work highlights the importance that Precordillera stratigraphy is linked to the early evolution of the paired margins during the Iapetus Ocean opening and Lower Paleozoic paleogeography reconstructions.

A review of the tectonic evolution of the Sunsás belt, SW Amazonian Craton

Abstract The Sunsas–Aguapei province (1.20–0.95 Ga), SW Amazonian Craton, is a key area to study the heterogeneous effects of collisional events with Laurentia, which shows evidence of the Grenvillian and Sunsas orogens. The Sunsas orogen, characterized by an allochthonous collisional-type belt (1.11–1.00 Ga), is the youngest and southwesternmost of the events recorded along the cratonic fringe. Its evolution occurred after a period of long quiescence and erosion of the already cratonized provinces (>1.30 Ga), that led to sedimentation of the Sunsas and Vibosi groups in a passive margin setting. The passive margin stage was roughly contemporary with intraplate tectonics that produced the Nova Brasilândia proto-oceanic basin (

DEFORMATION STAGES IN CUU LONG BASIN

Seperate of deformation stages, deformation phases and reconstracted their strees fields at Cuu Long basin is very important for explore oil/gas. Based on analysis and synthesis of geological data and geophysical data, it showing that the Cuu Long basin exists four major deformation stages: - Early-Middle Jurassic Post Collisional Orogenic Stage (D1) - Late Jurassic-Cretaceous Active Continental Triệu nămrgin stage (D2) - Eocene-Early Miocene Rifting stage (D3) includes six deformation phases: D3.1, D3.2, D3.3, D3.4, D3.5 and D3.6. Phases of D3.1, D3.3 và D3.5 are extention and thermal subsidence formed basin. Phases of D3.2, D3.4 and D3.6 are compression formed folds, faults and fractures - Late Early Miocene - Quaternary shelf of passive continental margin stage (D4)

Upper Vendian-lowest Ordovician sequences of the western Gondwana margin, NE Spain

Abstract The intra-Vendian (Ediacaran)–intra-Tremadocian succession of the Cadenas Ibéricas in NE Spain is divided into nine sequences. Overall, these are interpreted as second-order sequences. Those that encompass Lower and lower Middle Cambrian carbonates, with identified transgressive systems tract (TST) and highstand systems tract (HST) phases, may constitute elements of composite sequences. The lowermost sequence is of Late Vendian age. In Lower and lower Middle Cambrian units, sequence tops indicate drowning, reflecting extensional tectonics. Rifting effects are traceable up to mid-Mid Cambrian times. The remaining sequences probably represent a sag phase, either accentuating the preceding extensional local basin regime or heralding the Gondwana passive margin stage. Of the southerly Gondwana deposits those of other areas of the Iberian Peninsula and of the Moroccan Anti-Atlas fold belt show similar conditions during the Early Cambrian, that is, an Early Cambrian extensional regime, and, as for the Moroccan fold belt, four sequences imaging TST and HST phases.

Development of the Côte d´Ivoire Basin: reading provenance, sediment dispersal, and geodynamic implications from heavy minerals

The combined approach of conventional and varietal heavy mineral analysis was applied to constrain changes in source rock area, dispersal routes of detritus, and geodynamic setting of the Upper Cretaceous to Neogene Cote d'Ivoire basin. The heavy mineral assemblage of Albian to Turonian sediments reflect a narrow source rock area and short transport distances. A prominent change in the heavy mineral association of Late Cenomanian to Neogene sediments indicates enlargement of source area and longer fluvial transport distances, or longer storage of sediment within the littoral energy fence. The sudden change within the heavy mineral associations marks the transition from the late syn-rift stage to the early passive margin stage of the Southern Atlantic. During Albian to Late Cenomanian time, the basin geometry was mainly controlled by steep slopes and sediments were transported directly to the depocenter. The situation changed in Late Cenomanian time. A small shelf area developed where sediments from different sources were mixed and reworked before deposition. We believe that the W–E direction of the long-shore drift, which transports today huge quantities of near-shore sand, was established at the end of the Paleogene. This resulted from the drifting of southern West Africa across the paleoequator and subsequent change in surface wind directions.

Stratigraphy and sedimentary history of the Nepal Tethys Himalaya passive margin

The sedimentary history of the Nepal Tethys Himalaya began with deposition of thick carbonates in the Cambro?–Ordovician, followed by a mixed siliciclastic–carbonate epicontinental succession recording two major deepening events in the Early Silurian and Late Devonian. Fossiliferous carbonate ramp deposits in the Tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins (“rift stage”). Break-up in the mid-Early Permian, locally associated with effusion of tholeiitic lava flows, was followed by a transgressive sandy to shaly, locally coal-bearing or bioclastic unit capped by condensed pelagic carbonates in the Middle to Late Permian (“juvenile ocean stage”). Subsidence of the cooling stretched crust led close to bathyal water depths in the Olenekian, but then slowed down in the Middle Triassic to increase again sharply in the Late Triassic owing to renewed extensional tectonic activity and sediment loading during up- and out-building of the Indian continental terrace. Deposition of tropical platform carbonates finally became widespread in the middle Liassic (“mature passive margin stage”). The initial fragmentation of Gondwana in the Middle Jurassic led to rejuvenation of the Indian craton and deposition of quartzo-feldspathic hybrid arenites, capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late Bathonian/middle Callovian. Next, a discontinuous pelagic grey marly limestone unit was followed by the ammonoid-rich offshore Spiti Shale in the Late Jurassic. The final disintegration of Gondwana began in the Berriasian, when quartzose siliciclastics derived again from the rejuvenated Indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the Valanginian? followed in the Hauterivian to Albian by more felsic differentiates such as the trachyandesites exposed in the Lesser Himalaya 120 km to the south. A widespread drowning episode, fostered by waning volcaniclastic supply during a global eustatic rise, is documented by a major glauconitic horizon deposited at middle southern latitudes in the late Albian, overlain by “Scaglia-like” pelagic limestones in the latest Albian. The final part of sedimentary history, during the rapid northward flight of India and its collision with Eurasia, is not documented anywhere in Nepal due to later erosion of Upper Cretaceous to Lower Tertiary strata.

Foredeep basins: the main features and model of formation

The comparative study of the foredeep basins of the northern Caucasus, Alpine and Apennine foredeeps revealed that there are no significant differences between outer and inner foredeeps situated at the front and the back of the crustal-scale thrust. So the driving mechanism should be essentially the same for internal and external foredeeps and elastic flexure should not be supposed as the only (sometimes as the main) mechanism of foredeep formation. The common feature of the foredeep evolution is a phase of slow subsidence lasting for 50 million years or more (passive margin stage sensu stricto) and then relative uplift followed by rapid subsidence. This twofold event of relative uplift (the duration is several million years) giving way to rapid subsidence may be repeated several times. The relative uplift or nearly equal to zero subsidence of the foredeep basins appeared to be synchronous with phases of external compression within adjacent mountain belts, as well as rapid subsidence took place between phases of external compression. Thus, the first stage of relative uplift could be recognised as the first stage of the foredeeps formation. To describe the development of the foredeeps at both sides of the compressional belts a model of evolution of the Earth's outer shell has been employed. The model predicts that when the Earth's outer shell has been once disturbed by external intraplate- or mantle-induced forces, small-scale convection within the asthenosphere arises. This convection causes deformation in the lithosphere of orogenic belts over a long period of time after the external forces stop. This manifests in uplift of belts and subsidence at their periphery. The suggested model shows a good agreement with data on the foredeeps structure and evolution. In particular, application of the model enabled us to explain the thickness of sediments in the foredeep basins and their shape, the formation of the foredeeps not only at the front but also at the back of asymmetric compressional thrust belts, uplift of foredeeps during compression in the belts and rapid subsidence after this compression stops. Numerical results for the Great Caucasus region showed that the first compressional event at the orogenic stage took place there before the formation of the Maykopian sediments, i.e. 39.5 Ma, in the end of the closure of the Arabian Ocean. The three further compressional events can also be recognised: one of them being between 16.6 and 15.8, the other between 14.3 and 12.3 Ma, and the last one between 7.0 and 5.2 Ma.