Evolution Of Passive Margins Research Articles

The role of fault reactivation in the geomorphological evolution of coastal landforms on passive continental margins: Evidence from a tectonic estuary in southern Brazil

Coastal areas are unique, fast-changing environments shaped by the action of a wide range of climatic, anthropogenic and geological factors. Tectonic processes (e.g., subsidence, uplift, faulting and tilting) are major controls of landscape evolution and may be responsible for significant changes in river catchments, sea level and erosion rates on coastal zones. In continental passive margins, however, strong tectonic influence is typically attributed to early post-rift evolutionary stages, while changes in climate and/or sediment flux become increasingly dominant in the latest phases. However, there are key exceptions to this rule. We provide new results on an estuarine system formed due to reactivation of major structures crossing the southern edge of Serra do Mar escarpment, where Precambrian terrains, Cretaceous volcanic rocks and Neogene coastal deposits coexist. Morphostructural analysis revealed strong morphometric anomalies near Proterozoic fault zones marked by local subsidence and reactivation of oblique and normal faults, responsible for relevant changes in coastal landscape and tilting of the coastal plain towards the Atlantic Ocean. Despite major Late Holocene progradation along the southern Brazilian coast, local subsidence led to development and preservation of Babitonga Bay, an atypical tectonic estuary formed in the latest stages of passive margin evolution. Our findings imply that the common assumption of recent tectonic stability for intraplate passive margins is false and may lead to misinterpretation of similar coastal systems around the world.

Anatomy and kinematic evolution of an ancient passive margin involved into an orogenic wedge (Western Southern Alps, Varese area, Italy and Switzerland)

We make use of own geological mapping, interpretations of seismic reflection profiles and deep geophysical data to build a lithospheric-scale cross-section across the European Western Southern Alps (Varese area) and to model a progressive restoration from the end of Mesozoic rifting to present-day. Early phases of Alpine orogeny were characterized by Europe-directed thrusting, whereas post-Oligocene shortening led to basement-involving crustal accretion accompanied by backfolding, and consistent with the kinematics of the adjoining Ivrea Zone. Wedging was favored by a significant component of reactivation of the inherited Adriatic rifted margin. Our results also suggest that, during the collisional and post-collisional tectonics, lithosphere dynamics drove diachronically the onset of tectonic phases (i.e., wedging and slab retreat), from east to west, across the Western Southern Alps.

Late Palaeozoic-Neogene sedimentary and tectonic development of the Tauride continent and adjacent Tethyan ocean basins in eastern Turkey: New data and integrated interpretation

The eastern Taurus exemplifies continental rifting, passive margin development, Late Cretaceous melange genesis and ophiolite emplacement. Following Triassic rifting, a carbonate platform developed near sea level in the south (Munzur unit), whereas its northern extension (Neritic-pelagic unit) subsided into deep water during Late Jurassic-Late Cretaceous. Triassic-Cretaceous deep-water sediments and volcanics restore as distal deep-water slope/base of slope units. Jurassic-Cretaceous basic volcanics, interbedded with pelagic sediments, represent emplaced oceanic seamounts. Supra-subduction zone ophiolites formed to the north (c. 93 Ma), probably within an Inner Tauride ocean, and were emplaced southwards by trench-margin collision during latest Cretaceous (c. 75–66 Ma). The margin underwent flexural uplift/erosion and then subsidence/foredeep-infill. Part of the Tauride continent in the south (Malatya Metamorphics) deeply underthrust/subducted northwards, then exhumed rapidly by the late Maastrichtian (c. 65 Ma). To the south, oceanic lithosphere (e.g. Göksun ophiolite) was thrust northward beneath Tauride (Malatya) crust from a more southerly oceanic basin (Berit ocean), and intruded by Late Cretaceous subduction-related granitic rocks (88–82 Ma). Allochthonous units were assembled during the latest Cretaceous, followed by thick-skinned folding/thrusting, generally southwards, related to regional collision tectonics during Mid-Late Eocene. Part of the unmetamorphosed Tauride platform and its over-riding Late Cretaceous allochthon were apparently displaced >60 km northeastwards. Mid-Late Miocene regional collision drove variable folding and re-thrusting, in places northwards. Regional comparisons suggest that the Tauride carbonate platform (Geyik Dağ) narrowed eastwards, such that the palaeogeography of the E Taurides differed from farther west, influencing the late Mesozoic-Cenozoic structural development.

Gondwana's interlinked peripheral orogens

After its Ediacaran-Early Cambrian assembly, Gondwana was flanked by a system of peripheral orogens, Terra Australis, Avalonian-Cadomian and newly defined North Indo-Australie, which display broad temporal correlations of their lithotectonic records. Prior to assembly, their initial histories were primarily controlled by the early Neoproterozoic breakup of Rodinia with second order variances reflecting the differing relationships of their basement continental blocks to that supercontinent. The Terra Australis Orogen developed on basement blocks that previously occupied interior locations within Rodinia and initial successions record development of a passive continental margin. The North Indo-Australie orogen records a similar history of passive margin development, but at least its Indian portion was likely separate from Rodinia. The basement blocks of the Avalonian-Cadomian Orogen previously occupied exterior locations around Rodinia with initial successions indicating the development of a convergent plate margin. As Gondwana assembled, Avalonian-Cadomian convergence terminated at about the same time as convergence commenced in the Terra Australis and North Indo-Australie orogens. The absence of a complete long-lived contemporaneous subduction girdle around Gondwana likely prevented its breakup, in contrast to Rodinia and Pangea, in which the presence of subduction girdles corresponds with lithospheric extension across the supercontinents as a precursor to their ultimate breakup.

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)

Geochemical, mineralogical and sedimentological analyses of reworked sediments (new) in the syn- to post-rift Middle Cretaceous-Quaternary detrital deposits from western Atlantic margin of Cameroon: evidence from sedimentation-erosion alternation in the context of passive margin evolution

Reworked detrital sediments were discovered in the syn-rift to post-rift Cretaceous to Quaternary (Pleistocene) detrital deposits of the Douala coastal sub-basin in the eastern margin of the South Atlantic Ocean at West Cameroon. This contribution presents the geochemical, mineralogical, and sedimentological data to constrain the origin of these reworked sediments. The investigated samples are characterized by the high values of Plagioclase Index of Alteration (PIA, 97.83–99.90), Chemical Index of Alteration (CIA, 71.9–99.08), and Chemical Index of Weathering (CIW, 98.67–99.90). Data gleaned from these indices suggest that the reworked sediments and their potential source rocks experienced severe weathering. The A-CN-K plot along with the mineralogical indicates a probably post-depositional K-enrichment. The high light rare earth elements (LREE)/heavy rare earth elements (HREE) ratios (15.82–79.13) suggest that the reworked were mainly derived from felsic igneous rocks. This interpretation is further confirmed by the Zr versus TiO2, TiO2/Al2O3, Th/Co versus La/Sc, and La/Th versus Th/Yb plots, which indicate felsic igneous and silicic source rocks. Meanwhile, the high variability of LREE/HREE ratios (15.82–79.13) and the positive Eu anomalies (1.10–1.32) of studied samples could indicate a little contribution of mafic source rocks. Analysis of the distribution of the heavy minerals (e.g. tourmaline, aluminum silicate, and rutile) revealed that these felsic and silicic source rocks are mainly granite and gneiss, probably belonging to the Nyong Group and the Western part of the Neoproterozoic Yaounde Group. The morphoscopic analyses of the host rocks (very angular to angular shapes) have revealed that the reworked sediments have proximal to subproximal onshore origins, probably from former sedimentary deposits, as is indicated by variation of their color. The tectonic discriminant diagram indicates the Oceanic Island Arc and Active Continental Margin which are inconsistent with the geological history of the Douala coastal sub-basin and could be explained by the recycling effect experienced by studied sediments which have been deposited in the coastal plain to the fluvial environment in the Low System Track context.

Episodic exhumation of the Appalachian orogen in the Catskill Mountains (New York State, USA)

Abstract Increasing evidence indicates the eastern North American passive margin has not remained tectonically quiescent since Jurassic continental breakup. The identification, timing, resolution, and significance of post-orogenic exhumation, notably an enigmatic Miocene event, are debated. We add insight by constraining the episodic cooling and exhumation history of the Catskill Mountains (New York, USA) utilizing apatite fission-track thermochronology and apatite (U-Th)/He data from a ∼1 km vertical profile. Multi-kinetic inverse thermal modeling constrains three phases of cooling: Early Jurassic to Early Cretaceous (1–3 °C/m.y.), Early Cretaceous to early Miocene (∼0.5 °C/m.y.), and since Miocene times (1–2 °C/m.y.). Previous thermochronologic studies were unable to verify late-stage cooling and/or exhumation (typically post-Miocene and younger) because late-stage cooling was commonly a spurious artifact of earlier mono-kinetic annealing algorithms. Episodic cooling phases are correlative with rifting, passive-margin development, and drainage reorganization causing landscape rejuvenation. Geomorphologic documentation of increased offshore mid-Atlantic sedimentation rates and onshore erosion support the documented accelerated Miocene cooling and exhumation.

Evolution and demise of passive margins through grain mixing and damage

How subduction-the sinking of cold lithospheric plates into the mantle-is initiated is one of the key mysteries in understanding why Earth has plate tectonics. One of the favored locations for subduction triggering is at passive margins, where sea floor abuts continental margins. Such passive margin collapse is problematic because the strength of the old, cold ocean lithosphere should prohibit it from bending under its own weight and sinking into the mantle. Some means of mechanical weakening of the passive margin are therefore necessary. Spontaneous and accumulated grain damage can allow for considerable lithospheric weakening and facilitate passive margin collapse. Grain damage is enhanced where mixing between mineral phases in lithospheric rocks occurs. Such mixing is driven both by compositional gradients associated with petrological heterogeneity and by the state of stress in the lithosphere. With lateral compressive stress imposed by ridge push in an opening ocean basin, bands of mixing and weakening can develop, become vertically oriented, and occupy a large portion of lithosphere after about 100 million y. These bands lead to anisotropic viscosity in the lithosphere that is strong to lateral forcing but weak to bending and sinking, thereby greatly facilitating passive margin collapse.

Trace fossils, depositional context, and paleogeography of the upper Tal Group (upper lower Cambrian), Lesser Himalaya, India: a Gondwanan succession with no affinities to the Avalonia microcontinent – discussion of paper by Singh et al. (2019)

Terminal Ediacaran–late early Cambrian deposition, faunas and passive margin evolution of the north Indian margin are recorded in the Nigali Dhar syncline succession. Restudy of the upper Tal Group (upper lower Cambrian Koti Dhaman Formation, KDF) ichnofauna from the Khud-Drabil section reduces it to 18 confidently named forms. The lower KDF (Lower Quartzite Member) Cruziana-Rusophycus assemblage is in subtidal (not intertidal) sandsheet facies. The overlying black Shale Member (SM) records trans-East Gondwanan deepening, not intertidal facies, in the Palaeolenus Zone. The SM, with low diversity Planolites-Palaeophycus assemblage, is overlain by subtidal (not intertidal) sandsheet facies of the middle KDF (Arkosic Sandstone Member, ASM) with shallow burrowers and furrowers (Gordia marina assemblage, new; Cruziana ichnofacies). KDF faunas with Cruziana and Rusophycus are similar to coeval, shallow marine associations elsewhere in Gondwana and NW Laurentia. Interpretation of a second KDF section 20 km from Khud-Drabil has confused an understanding of Lesser Himalaya geologic evolution as it claims Ordovician Cruziana species in the ASM and an angular SM–ASM unconformity caused by the Cambrian–Ordovician boundary Kurgiakh orogeny. However, upper lower Cambrian microfaunas occur in and above the ASM, while the angular SM–ASM unconformity is consistent with submarine sliding. KDF-type ichnofaunas do not show a tropical location of Avalonia, which has the distinctive lithofacies and biotas of a high-latitude continent unrelated to Gondwana.

South Atlantic passive margin evolution: A thermochronology case study from the Rio de Janeiro-Três Rios section, SE Brazil

The southeastern Brazilian passive margin records a complex post-rift evolution, with two parallel high-elevation features formed after the opening of the South Atlantic. We applied apatite fission track (AFT) and U–Th/He (AHe) low temperature thermochronology to constrain the thermo-tectonic history of the Serra do Mar escarpment in the area of Rio de Janeiro state. New AFT central ages for basement areas collected from a N–S transect orthogonal to the shoreline between the cities of Rio de Janeiro and Três Rios, range between 98.5 ± 5.3 and 54.1 ± 4.2 Ma, with mean track lengths between 12.34 ± 0.40 and 14.63 ± 0.17 μm. Uncorrected AHe ages lie between 68.1 ± 5.9 and 60.2 ± 7.3 Ma and are consistent with AFT results. Inverse thermal history models constrained by AFT and AHe data imply the earliest cooling onset from the Barremian (Early Cretaceous), with steady rates more common for samples closer to coastal areas. Maximum depths of denudation are between 2.5 and 4.5 km. Published thermochronological data from adjacent areas combined with the new results show a seemingly simpler post-rift evolution for the area, although suggesting structural control of age distribution and exhumation.

The unsteady post-rift stage of the South American passive margin based on the tectono-sedimentary evolution of the onshore Paraíba Basin, NE Brazil

The post-rift tectono-sedimentary history of passive margins has been an issue of increased interest after the perception of their unsteady nature during the Neogene and Quaternary. The evolution of the eastern South America margin might be benefited from investigations of the sedimentary record in a number of marginal basins. The Paraíba Basin is of particular interest to discuss the post-rift evolution of the South American passive margin, because it represents the last bridge with the African plate. This work aims to analyze the post-rift sedimentary record of the onshore Paraíba Basin as a contribution to discuss the neotectonic history of the South American passive margin. Surface and subsurface data were integrated from outcrops, deep stratigraphic profiles, airborne geophysical maps and remote sensing products. The depositional and denudational processes in this basin were a result of tectonic subsidence and uplifting. Tectonics after the main rifting phase are indicated by abundant structures, including normal, reverse, and strike–slip faults, as well as folds that affected the Neogene and Late Pleistocene sedimentary deposits. The ductile and brittle deformation resulted mainly from multiple reactivations of E–W, NE–SW and NW–SE trending basement shear zones. The deformation mechanism included distension and compression under a strike–slip regime. Tectonic reactivations even during the late Pleistocene support an unsteady South Atlantic passive continental margin throughout the post–rifting stage.

Seismic stratigraphic framework of the continental shelf offshore Delmarva, U.S.A.: Implications for Mid-Atlantic Bight Evolution since the Pliocene

Understanding how past coastal systems have evolved is critical to predicting future coastal change. Using over 12,000 trackline kilometers of recently collected, co-located multi-channel boomer, sparker and chirp seismic reflection profile data integrated with previously collected borehole and vibracore data, we define the upper (< 115 m below mean lower low water) seismic stratigraphic framework offshore of the Delmarva Peninsula, USA. Twelve seismic units and 11 regionally extensive unconformities (U1-U11) were mapped over 5900 km2 of North America's Mid-Atlantic continental shelf. We interpret U3, U7, U9, U11 as transgressive ravinement surfaces, while U1,2,4,5,6,8,10 are subaerial unconformities illustrating distinct periods of lower sea-level. Based on areal distribution, stratigraphic relationships and dating results (Carbon 14 and amino acid racemization estimates) from earlier vibracore and borehole studies, we interpret the infilled channels as late Neogene and Quaternary courses of the Susquehanna, Potomac, Rappahannock, York, James rivers and tributaries, and a broad flood plain. These findings indicate that the region's geologic framework is more complex than previously thought and that Pleistocene paleochannels are abundant in the Mid-Atlantic. This study synthesizes and correlates the findings of other Atlantic Margin studies and establishes a large-scale Quaternary framework that enables more detailed stratigraphic analysis in the future. Such work has implications for inner continental shelf systems tract evolution, the relationship between antecedent geology and modern coastal systems, assessments of eustacy, glacial isostatic adjustment, and other processes and forcings that play a role in passive margin evolution.

Surface processes forcing on extensional rock melting

Surface processes and magmatism condition the structural evolution of continental rifts and passive margins through mechanical and thermal effects on the lithosphere rheology. However, their inter-relationships in extensional settings are largely unknown. Here, I use coupled thermo-mechanical geodynamic and landscape evolution numerical modeling to assess the links between erosion of rift shoulders, sedimentation within the rift basin and extensional rock melting. Results suggest that, when the crust is thinner than ~40 km, the extension rate is slower than ~2 cm/yr and the mantle potential temperature is below ~1230 °C, efficient surface processes may double crustal melting by Moho lowering and inhibit mantle decompression melting by ~50% through sediment loading within the rift basin. It is thus likely that surface processes significantly influenced the magmatic activity of a number of extensional settings worldwide – e.g. the Mediterranean, the Gulf of California, the Iberia-Newfoundland margin, and the South China Sea. Because magmatism and surface processes affect jointly the geological carbon cycle, the surface processes forcing on extensional rock melting investigated here involves an additional means of linkage between plate tectonics and climate changes.

Geophysical evidence for breakup volcanism in the Angola and Gabon passive margins

Abstract Serial two-dimensional reflection seismic profiles spanning the continent-ocean transition in the offshore Kwanza and southern Gabon basins have been interpreted and integrated with 2.5D gravity and magnetic anomaly modelling. These continental margins are characterized by the presence of a distal domain in which continental crust has been highly stretched and faulted, overlain by thick sequences of Early Cretaceous syn-rift sediments, that are in turn overlain by a thick but highly deformed Aptian evaporite unit. The evaporites are overlain by post-breakup sediments (Cretaceous to Recent in age). Oceanwards, syn-rift sediments are overlain by a volcanic unit that thickens basinward. This volcanic unit forms the base and the lateral-equivalent of the distal evaporite basin, developing into positive-relief volcanic edifices that acted as the oceanward margin of the evaporite basin. These volcanic units extend from the distal domain of the passive margin to the oceanic crust and are interpreted to represent volcanism that immediately predates lithospheric breakup and formation of oceanic crust, as indicated by the progressive transition into a constant-thickness oceanic crust. Volcanism synchronous with the late stages of passive margin development, and related hydrothermalism, can account for the high alkalinity that dominated the pre-salt lacustrine environments and could also have contributed to the modification of marine waters that led to the deposition of the thick Aptian evaporites in the South Atlantic Ocean. Breakup volcanism and its associated thermal impact can also help explain the apparently anomalous shallow water depth of these margins preserved during the latest rift and earliest post-rift stages.

Deep sea rock record exhumed on oceanic volcanic islands: the Cretaceous sediments of Maio, Cape Verde

Revising the deep water stratigraphy exposed on Maio offers a key section recording the sedimentary evolution in the distal domain of the Central Atlantic and the related North West African Atlantic margin (NWAAM). The oceanic volcanic island is one of nine islands in the Cape Verde archipelago, and is unique to the Central Atlantic due to the exposures of ophiolites and the overlying Mesozoic deep water sediments uplifted during the Cenozoic. This provides the opportunity to assess at outcrop, the exhumed sediments, stratigraphy and palaeo-environments of the early Central Atlantic and contribute towards the knowledge of passive margin evolution along the NWAAM, part of the largest continental remnant of Gondwana.Combined with the previous recording of calpionellids, the first collection of lower Valanginian ammonites from the lowest sedimentary succession provides conclusive evidence that Jurassic sediments are not present on Maio. Bed-by-bed sampling and ensuing micro-palaeontological analysis of these 71 samples, together with a re-interpretation of Stahlecker's remaining historical palaeontological collection providing a comprehensive dataset to build a high-resolution stratigraphic framework for the Mesozoic sediments. This reveals pelagic deep water limestones of the Morro Fm. were deposited until the upper Barremian. A major lithological change from carbonate- to siliciclastic-dominated facies, corresponding to the Morro-Carquiejo Fm. boundary is recognised as a regional hiatus spanning part of the Aptian identified across the Central Atlantic. The overlying Albian and younger Carquiejo Fm. is seen as an equivalent to the Albian-Cenomanian black shales of DSDP Leg 41, yet organic content is absent due to degradation. Future studies can build on this multi-disciplinary investigation revising the stratigraphy of Maio.

Hotspot origin for asymmetrical conjugate volcanic margins of the austral South Atlantic Ocean as imaged on deeply penetrating seismic reflection lines

We have interpreted 27,550 km of deep-penetrating, 2D-seismic reflection profiles across the South Atlantic conjugate margins of Uruguay/Southern Brazil and Namibia. These reflection profiles reveal in unprecedented detail the lateral and cross-sectional, asymmetrical distribution of voluminous, postrift volcanic material erupted during the Barremian-Aptian (129–125 Ma) period of early seafloor spreading in the southernmost South Atlantic. Using this seismic grid, we mapped the 10–200 km wide, continental margin-parallel limits of seaward-dipping reflector (SDR) complexes — that are coincident with interpretations from previous workers using seismic refraction data from the South American and West African conjugate margins. Subaerially emplaced and tabular SDRs have rotated downward 20° in the direction of the mid-Atlantic spreading ridge and are up to 22 km thick near the limit of continental crust. The SDR package is wedge shaped and thins abruptly basinward toward the limit of oceanic crust where it transitions to normal, 6–8 km thick oceanic crust. We have developed a model for the conjugate rifted margins that combine diverging tectonic plates and northwesterly plate motion relative to a fixed mantle position of the mantle plume. Our model explains an approximately 30% higher volume of SDRs/igneous crust on the trailing Namibian margin than on the leading Brazilian margin during the syn- and postrift phases. Our model for volcanic margin asymmetry in the South Atlantic does not require a simple shear mechanism to produce the asymmetrical volcanic material distribution observed from our data and from previously published seismic refraction studies. Determining the basinward extent of the extended continental basement is crucial for understanding basin evolution and for hydrocarbon exploration. Although these conjugate margins have evolved asymmetrically, their proximity during the early postrift stage suggests a near-equivalent, early basin evolution and similar hydrocarbon potential. Understanding the tectonic and magnetic processes that produce these observed asymmetries is critical for understanding volcanic passive margin evolution.

Comparing contiguous high- and low-elevation continental margins: New (U-Th)/He constraints from South Brazil and an integration of the thermochronological record of the southeastern passive margin of South America

The southeastern coast of South America is an example of the complexity of passive continental margins, as it displays both high- and low-elevation segments despite sharing a similar pre-rift geological history and structural configuration. As such, it is a prime candidate for investigating debated questions concerning the evolution of passive margins, such as the tectonic mechanisms driving uplift, their relationship to rifting and continental break-up, and why some margins are elevated when others are not. In this contribution, we present new (U-Th)/He data from a low-altitude portion of the South American passive margin in South Brazil, the Sul-rio-grandense Shield, and interpret it in the context of the regional thermochronological record. New results produce widespread apparent ages and reveal a complex exhumation history from the early Paleozoic onwards, including reheating during Paleo-Mesozoic sedimentation. For most of the study area, however, final exhumation was achieved at the latest during the rifting and early opening of the South Atlantic Ocean (135 Ma to 100 Ma). In spite of the presence of major Neoproterozoic shear zones, the inherited NE-SW structural framework seems not to have strongly influenced the thermochronological record. The new data were integrated into a large compilation of apatite fission track and (U-Th)/He results from southeast South America, in order to compare regional trends and investigate possible tectonic controls in the exhumation history. Low-elevation areas of the passive margin consistently record complex pre-rift cooling histories, while high-elevation areas experienced significant Upper Cretaceous/Paleogene uplift associated with the reactivation of Neoproterozoic shear zones. Because the inherited structural features of both segments are similar, plate dynamics alone cannot be responsible for the variating response. Hence, mantellic processes associated with post-rift alkaline magmatism may have affected the contrasting exhumation histories. This process was probably controlled by important South Atlantic fracture zones.

Late Neoproterozoic–early Paleozoic seismic structure–stratigraphy of the SW Tarim Block (China), its passive margin evolution and the Tarim–Rodinia breakup

A complete structural and stratigraphic record of extensional tectonics and passive margin evolution is displayed by the late Neoproterozoic–early Paleozoic syn- and post-rift sequences exposed in the SW Tarim Block. The syn-rift strata, represented by the upper Neoproterozoic Nanhua and Sinian Systems, occur in NE–SW– and WNW–ESE–trending graben and half–graben structures and comprise clastic terrestrial and bimodal volcanic rocks that are interspersed with diamictites. Nearly 100- to 150-km-long, sub-elliptical basins with ~400-m-thick upper Neoproterozoic Nanhua System deposits are nested within these grabens. Stratigraphically higher, upper Sinian units include littoral to shallow marine clastic rocks, grading upwards into massive dolomites and marking an important transition from terrestrial to a shallow marine depositional environment in the passive margin evolution of SW Tarim. Unconformably overlying these Neoproterozoic syn–rift rock units are the lower Cambrian sandstone, mudstone and dolomite, and the middle Cambrian dolomitic limestone and stromatolites collectively making up the post–rift sequence. These rocks represent a shallow marine transgressive sequence, deposited on a submerged carbonate platform of the rifted margin of SW Tarim. The regional unconformity between the syn– and post–rift sequences marks a sharp breakup unconformity, pointing to a major tectonic transition from continental rifting to drifting in the history of the Tarim–Rodinia breakup. This extensional tectonics and passive margin evolution during the late Neoproterozoic–early Cambrian indicate that the collisional Pan–African orogenic events, which affected several other East Gondwana continents, did not involve the Tarim Block.

Gondwana fragments in the Eastern Alps: A travel story from U/Pb zircon data

We compare detrital U/Pb zircon age spectra of Carboniferous and Permian / Lower Triassic sedimentary rocks from different structural positions within the Austroalpine nappe pile with published ages of magmatic and metamorphic events in the Eastern Alps and the West Carpathians. Similarities between sink and possible sources are used to derive provenance of sediments and distinct frequency peaks in sink and source age pattern are used for paleogeographic plate tectonic reconstructions. From this, travel paths of Austroalpine and West Carpathian basement units are traced from the Late Neoproterozoic to the Jurassic. We place the ancestry of basement units on the northeastern Gondwana margin, next to Anatolia and the Iranian Luth-Tabas blocks. Late Cambrian rifting by retreat of the Cadomian Arc failed and continental slivers re-attached to Gondwana during a late Cambrian / early Ordovician orogenic event. In the Upper Ordovician crustal fragments of the Galatian superterrane rifted off Gondwana through retreat of the Rheic subduction. An Eo-Variscan orogenic event at ~390 Ma in the Austroalpine developed on the northern rim of Galatia, simultaneously with a passive margin evolution to the south of it. The climax of Variscan orogeny occurred already during a Meso-Variscan phase at ~350 Ma by double-sided subduction beneath Galatia fragments. The Neo-Variscan event at ~330 Ma was mild in eastern Austroalpine units. This orogenic phase was hot enough to deliver detrital white mica into adjacent basins but too cold to create significant volumes of magmatic or metamorphic zircon. Finally, the different zircon age spectra in today's adjacent Carboniferous to Lower Triassic sediments disprove original neighbourhood of basins. We propose lateral displacement of major Austroalpine and West-Carpathian units along transform faults transecting Apulia. The intracontinental transform system was released by opening of the Penninic Ocean and simultaneous closure of the Meliata Hallstatt Ocean as part of the Tethys.

From passive margin to continental collision: Geochemical and isotopic constraints for E-MORB and OIB-like magmatism during the neoproterozoic evolution of the southeast Brasília Belt

Metamorphic rocks derived from mafic rocks can be found in metamorphic belts, and their geochemical and isotopic characteristics may be valuable sources of information for interpretation of ancient geologic magmatic scenarios. The southern Brasília Belt, between the Amazonian and São Francisco cratons in Central Brazil exhibits different amphibolite rocks that were emplaced into Veríssimo Sequence and Araxá Group metasedimentary sequences, during the Neoproterozoic. The obtained results indicate that the studied amphibolite rocks from Pires do Rio-Catalão area represent three magmatic events associated with three magmatic environments: 1) a passive margin development at 979 Ma, a forearc/back-arc basin at ∼819 Ma, and a continental collision at ∼651 Ma.The first magmatic event, related to continental extension and passive margin development is represented by the emplacement at 979 ± 17 Ma of E-MORB low-K tholeiitic basalts with positive εNd(t) values between +4.4 and −2 and TDM between 1.26 and 1.82 Ga, related with the Veríssimo metavolcanosedimentary Sequence. The zircon populations include a high amount of metamorphic, inherited grains.The second magmatic event, related with an forearc/back-arc extension is represented by E-MORB basalts with negative Nb-Ta anomalies and slightly positive εNd(t) values of +0.78 to +2.71, cogenetic with the Araxá Group. An OIB-like basalt sample is also related to this stage and has a U-Pb age of 819.7 ± 6.3 Ma. εNd(t) values of +1.82 to +2.57, εHf(t) in zircon around-zero and TDM ages from 1.03 to 1.18 Ga.The third event is represented by syn- to late-tectonic OIB-like basaltic/gabbroic magmatism at 651 ± 6 Ma, with positive εNd(t) value of +2.57 and younger TDM age at 1.18 Ga, that can be associated to regional continental collision and closure of the Neoproterozoic ocean.Also, provenance studies into metasedimentary rocks support that passive and active margins could be successively superposed during the main thrusting tectonic event in the Brasiliano orogeny, but slices of different stratigraphic units preserve their depositional isotopic signature.These findings suggest that the occurrence of main mafic magmatic events over a timespan as long as the entire evolution of the Brasília Belt (1.0–0.6 Ma) might be a key to individualize superposed tectonic basins in Central Brazil.