Narrow Oceanic Basin Research Articles

Late Jurassic magmatism in the Ligurian‐Piedmont Ocean constrained by zircon ages of mafic and felsic meta‐intrusives

AbstractPaleogeographic reconstructions show that the Ligurian‐Piedmont Ocean (LPO) was a relatively narrow oceanic basin but the actual amount of oceanic lithosphere generated and the timing of magmatic accretion are still subjects of debate. New U–Pb dating of zircon from two pairs of FeTi‐oxide metagabbro and metaplagiogranite s.l. of the Susa and Lanzo Valleys Ophiolites (Western Alps) yields Late Jurassic magmatic ages (~150 Ma), 10–15 Ma younger than most of the magmatism in the LPO. The potential significance of the geochronological data presented are discussed in relation to the structural architecture of the LPO, emphasizing that the investigated ophiolites represent the youngest oceanic lithosphere accreted in the Western Alps.

Jurassic-Paleogene spreading history of the northern Indian Ocean as a constraint on the evolution of the north Australia continental margin

The spreading history of the northern Indian Ocean is re-examined with the aim of interpreting the late Mesozoic-Paleogene evolution of the northern Australian continental margin in eastern Indonesia and Timor-Leste. The earliest seafloor spreading (‘Argo phase’) developed between approximately 155-131Ma (Kimmeridgian, Late Jurassic to Valanginian, Early Cretaceous). A clockwise pole of rotation relative to Australia (‘Argo pole’) is interpreted at 7°09’S, 133°07’E, a present-day location near the Tanimbar islands in eastern Indonesia. A second ‘Gascoyne’ phase of spreading, between 131-100Ma (late Early Cretaceous), had an interpreted clockwise rotation pole at 3°40’N, 125°00’E, a present-day location between the islands of Mindanao (southern Philippines) and Sulawesi (eastern Indonesia). A third ‘Wharton’ phase of spreading, commencing at the beginning of the Late Cretaceous (~100Ma) had a more distant pole (clockwise pole at 5°S, 171°E: Jacob et al., 2014), although more local rotations are interpreted for the Greater Sula Spur continental terrane, about a pole estimated at 9°24’S, 134°40’E, close to the earlier Argo pole.The motions of two continental terranes are inferred from the spreading history. The Argoland Terrane, which is identified in this study as eastern Sundaland (Borneo and Java), would not have entirely detached from Australia before the mid Cretaceous due to the relatively proximal locations of the Argo and Gascoyne rotation poles. In this scenario Argoland could not have collided with Eurasia in the Cretaceous, as suggested by several previous studies. The Greater Sula Spur Terrane (several continental fragments in present-day eastern Indonesia), is repositioned in the initial reconstructions immediately north of Timor, and did not move northward to its late Paleogene (pre-Neogene collision) location until the Wharton phase of rifting that commenced in the Late Cretaceous. This northward motion of the Greater Sula Spur was accommodated by the development of a narrow oceanic basin (the Banda Embayment).The reconstructions suggest that the Argoland and Greater Sula Spur continental terranes formed a continuous crustal connection between SE Asia and northern Australia throughout the late Mesozoic and early Paleogene, and that the Indian Ocean developed entirely within the body of the disrupting Gondwanaland supercontinent, rather than linking eastwards into the Pacific Ocean.

Pre- to post-collisional Ediacaran magmatism in the western portion of the Borborema Province (NE Brazil): Petrogenesis and insights for the crustal evolution during the Brasiliano Orogeny

The Central subprovince of the Borborema Province (NE Brazil) is characterized by a voluminous and diversified late-Neoproterozoic magmatism (ca. 640 to 540 Ma) that resulted in plutons with significant correlations among their U-Pb ages, TDM Nd model ages, εNd(t) values, geochemical signatures, and petrology, evidencing the importance of their study to the understanding of the Brasiliano/ Pan-African evolution in the region. The Jardim Mirim Suite (630 Ma) comprises magmatic-epidote bearing calc-alkaline to slightly high-K calc-alkaline tonalitic and granodioritic rocks characterized by εNd(t) values ranging from −3.0 to 0.8 and Mesoproterozoic TDM Nd model ages (1.4 to 1.1 Ga). The Colina do Horto Pluton (594 Ma) is composed of shoshonitic monzodiorites, monzonites, and granites that exhibit, in general, strongly negative εNd(t) values (−18.2 to −16.5) and Paleoproterozoic TDM Nd model ages (1.9 to 2.1 Ga). The Cana Brava (572 Ma) and Poço plutons consist of alkaline ferroan granites composed of Fe-rich mafic minerals and characterized by strongly negative εNd(t) values (−15.3 to −14.8, respectively), Paleoproterozoic TDM Nd model ages (2.0 Ga), and geochemical signatures similar to high Ba-Sr granites. The study of these plutonic bodies indicates three main magmatic events, developed in pre-collisional (probably associated with the closure of a narrow oceanic basin) to post-collisional (associated with lithospheric delamination process) settings. The contrasting features of the studied rocks reflects changes in magmatic sources in the region. Whereas the origin of pre-collisional magmas is associated with the mixing of juvenile Brasiliano source with Cariris Velhos rocks (ca. 1000–870 Tonian crustal rocks from the Borborema Province), post-collisional rocks were generated by underplating-driven partial melting of a mafic to felsic Paleoproterozoic crust.

Petrogenesis of Early Permian Intrusive Rocks from Southeastern Inner Mongolia, China: Constraints on the Tectonic Framework of the Southeastern Central Asian Orogenic Belt

AbstractThe late Paleozoic tectonic framework of the southeastern Central Asian Orogenic Belt is key to restricting the accretion orogeny between the Siberia Craton and the North China Craton. To clarify the framework, petrogenesis of early Permian intrusive rocks from southeastern Inner Mongolia was studied. Zircon U‐Pb dating for bojite and syenogranite from Ar‐Horqin indicate that they were emplaced at 288–285 Ma. Geochemical data reveal that the bojite is highly magnesian and low‐K to middle‐K calc‐alkaline, with E‐MORB‐type REE and IAB‐like trace element patterns. The syenogranite is a middle‐K calc‐alkaline fractionated A‐type granite and shows oceanic‐arc‐like trace element patterns, with depleted Sr‐Nd‐Hf isotopes, (87Sr/86Sr)I = 0.7032–0.7042, εNd(t) = +4.0 to +6.6 and zircon εHf(t) = +11.14 to +14.99. This suggests that the bojite was derived from lithospheric mantle metasomatized by subducted slab melt, while the syenogranite originated from very juvenile arc‐related lower crust. Usng data from coeval magmatic rocks from Linxi–Ar‐Horqin, the Ar‐Horqin intra‐oceanic arc was reconstructed, i.e., initial transition in 290–280 Ma and mature after 278 Ma. Combined with regional geological and geophysical materials in southeastern Inner Mongolia, an early Permian tectonic framework as ‘one narrow ocean basin of the PAO’, ‘two continental marginal arcs on its northern and southern’ and ‘one intra‐oceanic arc in its southern’ is proposed.

Superimposed deformation of the Solonker Belt and nearby regions in western Inner Mongolia, China

AbstractThe deformation of the Solonker Belt and nearby regions is helpful for understanding the tectonic evolution of the Central Asian Orogenic Belt. This study carried out structural analysis in the Mandula and Ganqi areas of western Inner Mongolia, including the Solonker Belt, the Southern Orogenic Belt and the northern Yinshan Belt (Langshan range). Our results reveal that the Solonker Belt, the Southern Orogenic Belt and the northern Yinshan Belt underwent two stages (D1 and D2) of deformation during the Mesozoic period. The D1 stage produced the NNE-directed thrusts and asymmetric folds, indicating a NNE–SSW contraction. The northern Yinshan Belt, the Southern Orogenic Belt and the Solonker Belt formed as a series of NNE-verging tectonic nappes. The D2 stage developed the NE-trending folds and the SE- or NW-directed thrusts that cross-cut the D1 structures. The two events of nearly orthogonal or oblique shortening gave rise to the superimposed structures (e.g. fold interference patterns). The quartz veins that filled the fractures of the D1 deformation contain zircons of Middle Triassic U–Pb ages. The new dating data, along with the regional sedimentary hiatus, led us to infer that the D1 stage of deformation occurred in Middle Triassic time and the D2 stage occurred in Late Jurassic time. We consider that the D1 stage of deformation resulted from a convergent event, which might be related to the closure of the Palaeo-Asian Ocean or limited, narrow ocean basins; and the D2 stage of deformation was the far-field result of subduction of the Palaeo-Pacific Ocean and the closure of the Mongol-Okhotsk Ocean.

The classic Wilson cycle revisited

Abstract In the first application of the developing plate tectonic theory to the pre-Pangaea world 50 years ago, attempting to explain the origin of the Paleozoic Appalachian–Caledonian orogen, J. Tuzo Wilson asked the question: ‘Did the Atlantic close and then reopen?’. This question formed the basis of the concept of the Wilson cycle: ocean basins opening and closing to form a collisional mountain chain. The accordion-like motion of the continents bordering the Atlantic envisioned by Wilson in the 1960s, with proto-Appalachian Laurentia separating from Europe and Africa during the early Paleozoic in almost exactly the same position that it subsequently returned during the late Paleozoic amalgamation of Pangaea, now seems an unlikely scenario. We integrate the Paleozoic history of the continents bordering the present day basin of the North Atlantic Ocean with that of the southern continents to develop a radically revised picture of the classic Wilson cycle The concept of ocean basins opening and closing is retained, but the process we envisage also involves thousands of kilometres of mainly dextral motion parallel with the margins of the opposing Laurentia and Gondwanaland continents, as well as complex and prolonged tectonic interaction across an often narrow ocean basin, rather than the single collision suggested by Wilson.

Detrital zircon U-Pb-Hf isotopes and provenance of Late Neoproterozoic and Early Paleozoic sediments of the Simao and Baoshan blocks, SW China: Implications for Proto-Tethys and Paleo-Tethys evolution and Gondwana reconstruction

Early Paleozoic evolution of the northern Gondwana margin is interpreted from integrated in situ U-Pb and Hf-isotope analyses on detrital zircons that constrain depositional ages and provenance of the Lancang Group, previously assigned to the Simao Block, and the Mengtong and Mengdingjie groups of the Baoshan Block. A meta-felsic volcanic rock from the Mengtong Group yields a weighted mean 206Pb/238U age of 462±2Ma. The depositional age for the previously inferred Neoproterozoic Lancang and Mengtong groups is re-interpreted as Early Paleozoic based on youngest detrital zircons and meta-volcanic age. Detrital U-Pb zircon analyses from the Baoshan Block define three distinctive age peaks at older Grenvillian (1200–1060Ma), younger Grenvillian (~960Ma) and Pan-African (650–500Ma), with εHf(t) values for each group similar to coeval detrital zircons from western Australia and northern India. This suggests that the Baoshan Block was situated in the transitional zone between northeast Greater India and northwest Australia on the Gondwana margin and received detritus from both these cratons. The Lancang Group yields a very similar detrital zircon age spectrum to that of the Baoshan Block but contrasts with that for the Simao Block. This suggests that the Lancang Group is underlain by a separate Lancang Block. Similar detrital zircon age spectra suggest that the Baoshan Block and the Lancang Block share common sources and that they were situated close to one another along the northern margin of East Gondwana during the Early Paleozoic. The new detrital zircon data in combination with previously published data for East Gondwana margin blocks suggests the Early Paleozoic Proto-Tethys represents a narrow ocean basin separating an “Asian Hun superterrane” (North China, South China, Tarim, Indochina and North Qiangtang blocks) from the northern margin of Gondwana during the Late Neoproterozoic-Early Paleozoic. The Proto-Tethys closed in the Silurian at ca. 440–420Ma when this “Asian Hun superterrane” collided with the northern Gondwana margin. Subsequently, the Lancang Block is interpreted to have separated from the Baoshan Block during the Early Devonian when the Paleo-Tethys opened as a back-arc basin.

Epiphytic Microalgal Species Composition and Dynamics on Host Green Seaweeds (Ulvaphyceae) on the Northern Coast of Jeddah, Saudi Arabia

Epiphytic microalgae on seaweeds are sessile microscopic plants grown with attached or associated to seaweeds hosts and seaweeds are multicellular and macroscopic macro-algae, which are abundant in intertidal zones of coastal environments. Study on epiphytic microalgae is rare at Jeddah coast of the Red Sea. Thus, an investigation on epiphytic microalgae species composition and dynamics on seaweeds of Ulvaphyceae was carried out at northern coast of Jeddah, the Red Sea. The Red Sea is narrow oceanic basin which is lying between the African and the Asian continental shelves. During the study, 3 species of seaweeds were recorded in Ulvaphyceae which were Chaetomorpha linum, Enteromorpha intestinalis and Ulva lactuca. A total of 70 epiphytic microalgae were identified, including 63 belong to Bacillariophyceae, 5 belong to Cyanophyceae and 2 belong to Dinophyceae. Among the identified epiphytic microalgae of the host seaweeds of Ulvaphyceae, the percent contribution of epiphytic Bacillariophyceae, Cyanophyceae and Dinophyceae were 91.53, 6.55 and 2.61%, respectively. The cell abundance of epiphytes on host seaweeds of Ulvaphyceae varied from 8.00 to 93.00 cells/ cells/100 g of Ulvaphyceae seaweeds. The highest cell abundance was in spring and the lowest was in summer. On host seaweeds of Ulvaphyceae, the epiphytes of Licmophora abbreviata, Gyrosigma fasciola, Leptocylindrus danicus, Navicula distans, Navicula transitans, Pleurosigma angulatum and Pleurosigma normanii, Thalassionema frauenfeldii and Nitzschia hungarica were above 10% among epiphytic microalgae though Amphora spp., and Cocconeis spp., were found throughout the year. This finding could be the important source for future explanation of marine epiphytes and their host seaweed eco-biogeographical phenomena in the Red Sea.

Epiphytic Microalgal Dynamics and Species Composition on Brown Seaweeds (Phaeophyceae) on the Northern Coast of Jeddah, Saudi Arabia

Epiphytic microalgae grow on host seaweeds. Seaweeds are multicellular macro-algae, which are abundant in intertidal zones of coastal environments. The report on epiphytic microalgae is rare in the Red Sea. Thus, an investigation on epiphytic microalgae was carried on northern coast of Jeddah, the Red Sea, Saudi Arabia. The Red Sea is narrow oceanic basin which is lying between the African and the Asian continental shelves. During the study, 4 species of seaweeds were recorded in Phaeophyceae which were Laminaria sp., Padina fraseri, Sargassum muticum and Turbinaria ornata. A total of 83 epiphytic microalgae were identified, including 76 belong to Bacillariophyceae, 5 belong to Cyanophyceae, 1 belongs to Chlorophyceae and 1 belongs to Dinophyceae. On seaweeds of Laminaria sp., the dominant epiphytes were Leptocylindrus danicus, Licmophora flabellata and Navicula ramosissma. On seaweeds Padina fraseri, the dominant epiphytes were Cylindrotheca closterium, Navicula sp., Nitzschia sp. and Prorocentrum lima and Cocconeis lineatus. On seaweeds of Sargassum muticum, the dominant epiphytes were Navicula distans, Nitzschia socialis and Navicula vanhoeffeni. On seaweeds Turbinaria ornata, the dominant epiphytes were Leptocylindrus minimus, Bacillaria paxillifer, Leptocylindrus danicus, Nitzschia hungarica and Thalassionema frauenfeldii. The host seaweeds influenced the associated diatoms mainly through its morphology and surface texture and roughness to provide a point of attachment and shelter for host-adapted species. Most of the dominant epiphytes were in smaller cell size diatoms, except Cylindrotheca closterium. The findings of the study focused on individual species of benthic marine diatoms associated with specific seaweeds species even in different regions of the Red Sea in different seasons of Saudi Arabia. These findings could be the important source for future explanation of marine eco-biogeographical phenomena in the Red Sea.

Geodynamic significance of preserved Carboniferous subduction complex in Atbashi range (South Tianshan, Kyrgyzstan) and inferences for crustal-scale structure of north Tarim-Tibet orogenic system

A subduction complex of carboniferous age is preserved in the South Tianshan belt of Kyrgyzstan. It is made of a LP-LT accretionnary prism comprising an obducted ophiolite thrusted by a HP complex made of a sedimentary channel including eclogites boudins and a continental unit. Its structure and metamorphic history are investigated to reconstruct the geodynamic evolution of the northern Tarim in the Upper Palaeozoic. This study gives insights into the crustal-scale structure of this mountain belt, currently intensely reactivated by the India-Asia collision. Eclogites boudins have a N-MORB type composition similar to the unmetamorphosed obducted ophiolite sequence. Evidence for eclogite facies in both acidic and mafic lithologiesand geological structure are in agreement with a deep subduction channel mainly comprised of metasediments. Eclogites boudins are investigated for a preliminary PT study. Prograde stage (I) begins in blue-schist/eclogite facies transition at 520 ± 30°C - 17 ± 1 kbar. Condit ions of peak metamorphism (II) in eclogite facies range from 550 ± 30°C - 18.5 ± 1 kbar to 540- 595°C - 21 kbar. Retrograde stage (III) is also in the eclogite facies conditions at 515 ± 30°C - 16.7 ± 1 kbar. The geological structure and metamorphic conditions of this suture zone implies the subduction of a narrow oceanic basin in a south-dipping subduction zone, while another north-dipping subduction was active below Middle Tianshan. Final stacking of Middle and South Tianshan occurred at 320-310 Ma. These antithetic subduction zones are still reflected in the main structures of Tianshan. Reactivation of the south­ dipping structures since 30 Ma is ascribed to explain the current localisation of uplift and deformation.

Seismic evidence for the presence of Jurassic oceanic crust in the central Gulf of Cadiz (SW Iberian margin)

We investigate the crustal structure of the SW Iberian margin along a 340 km-long refraction and wide-angle reflection seismic profile crossing from the central Gulf of Cadiz to the Variscan continental margin in the Algarve, Southern Portugal. The seismic velocity and crustal geometry model obtained by joint refraction and reflection travel-time inversion reveal three distinct crustal domains: the 28–30 km-thick Variscan crust in the north, a 60 km-wide transition zone offshore, where the crust abruptly thins ~ 20 km, and finally a ~ 7 km-thick and ~ 150 km-wide crustal section that appears to be oceanic in nature. The oceanic crust is overlain by a 1–3 km-thick section of Mesozoic to Eocene sediments, with an additional 3–4 km of low-velocity, unconsolidated sediments on top belonging to the Miocene age, Gulf of Cadiz imbricated wedge. The sharp transition between continental and oceanic crust is best explained by an initial rifting setting as a transform margin during the Early Jurassic that followed the continental break-up in the Central Atlantic. The narrow oceanic basin would have formed during an oblique rifting and seafloor spreading episode between Iberia and Africa that started shortly thereafter (Bajocian) and lasted up to the initiation of oceanic spreading in the North Atlantic at the Tithonian (late Jurassic-earliest Cretaceous). The velocity model displays four wide, prominent, south-dipping low-velocity anomalies, which seem to be related with the presence of crustal-scale faults previously identified in the area, some of which could well be extensional faults generated during this rifting episode. We propose that this oceanic plate segment is the last remnant of an oceanic corridor that once connected the Alpine-Tethys with the Atlantic ocean, so it is, in turn, one of the oldest oceanic crustal fragments currently preserved on Earth. The presence of oceanic crust in the central Gulf of Cadiz is consistent with geodynamic models suggesting the existence of a narrow, westward retreating oceanic slab beneath the Gibraltar arc-Alboran basin system.

Carbon release by off-axis magmatism in a young sedimented spreading centre

Continental rifting creates narrow ocean basins, where coastal ocean upwelling and enhanced silicate weathering remove carbon dioxide from the atmosphere. Evidence from seismic data, sonar backscatter and seafloor images, and geochemical water analyses suggest that in young sedimented rifts, active magmatism occurs in a broader region than appreciated and releases carbon from the sediments.

Structure and tectonics of lower crustal and upper mantle rocks in the Jurassic Mirdita ophiolites, Albania

The Jurassic Mirdita ophiolite crops out in the western branch of the Hellenide‐Dinaride ophiolite belt in the Balkan Peninsula; it represents a remnant of the Tethyan oceanic lithosphere developed between the Apulian and Pelagonian microcontinents. Emplaced during the Cretaceous on the western margin of Pelagonia, the Mirdita ophiolite was involved in southwestward thrusting of the Hellenide‐Dinaride tectonic units during the Eocene alpine tectonics. It was little affected, however, by these alpine events as marked by nearly horizontal Cretaceous limestone deposits overlying the ophiolite. A continuous section of Middle Jurassic oceanic crust, including thin gabbros, a N‐S‐trending sheeted dike complex, and extrusive rocks, is exposed in the central part of the ophiolite and thickens eastward within a regional, N‐S‐trending synform. Peridotite massifs exposed along the western and eastern edges of this synform show major structural and petrological differences. The eastern ultramafic domain has a harzburgitic mantle exhibiting a high‐temperature asthenospheric foliation dipping steeply to moderately to the west. Major chromite deposits are restricted to these eastern ultramafic massifs. The western ultramafic domain has a zoned mantle, with asthenospheric harzburgite exposed at its western margin, progressively replaced eastward by plagioclase‐peridotites that were highly strained at low temperatures, and that are bounded by amphibole‐peridotites along with crustal rocks. The occurrence of plagioclase is ascribed to melt impregnation processes that occurred immediately before or during intense lithospheric deformation. The gabbros in the eastern massifs are thicker (1–2 km and layered, whereas they are highly thin and discontinuous in the western massifs. Mantle peridotites of the western massifs locally are in direct contact with the overlying diabase and volcanic rocks along ductile shear zones. Development of epidote‐amphibolite facies metamorphism in upper‐crustal rocks was produced by hydrothermal alteration in the oceanic realm, and was intense and widespread in the western domain. The Mirdita ophiolite likely originated in a short‐lived, narrow ocean basin, which was closed during the Middle Jurassic by eastward overthrusting of the western domain, and westward subduction of the eastern domain. This constrictional phase was followed by dextral oblique convergence between the Pelagonian and Apulian microcontinents. This model implies a parautochtonous origin of the Mirdita ophiolite between these two microcontinents.

Structure and evolution of the eastern Gulf of Aden conjugate margins from seismic reflection data

The Gulf of Aden is a young and narrow oceanic basin formed in Oligo-Miocene time between the rifted margins of the Arabian and Somalian plates. Its mean orientation, N75°E, strikes obliquely (50°) to the N25°E opening direction. The western conjugate margins are masked by Oligo-Miocene lavas from the Afar Plume. This paper concerns the eastern margins, where the 19-35 Ma breakup structures are well exposed onshore and within the sediment-starved marine shelf. Those passive margins, about 200 km distant, are non-volcanic. Offshore, during the Encens-Sheba cruise we gathered swath bathymetry, single-channel seismic reflection, gravity and magnetism data, in order to compare the structure of the two conjugate margins and to reconstruct the evolution of the thinned continental crust from rifting to the onset of oceanic spreading. Between the Alula-Fartak and Socotra major fracture zones, two accommodation zones trending N25°E separate the margins into three N110°E-trending segments. The margins are asymmetric: offshore, the northern margin is narrower and steeper than the southern one. Including the onshore domain, the southern rifted margin is about twice the breadth of the northern one. We relate this asymmetry to inherited Jurassic/Cretaceous rifts. The rifting obliquity also influenced the syn-rift structural pattern responsible for the normal faults trending from N70°E to N110°E. The N110°E fault pattern could be explained by the decrease of the influence of rift obliquity towards the central rift, and/or by structural inheritance. The transition between the thinned continental crust and the oceanic crust is characterized by a 40 km wide zone. Our data suggest that its basement is made up of thinned continental crust along the southern margin and of thinned continental crust or exhumed mantle, more or less intruded by magmatic rocks, along the northern margin. [ABSTRACT FROM AUTHOR]

Proterozoic geochronology and tectonic evolution of southern Africa

Abstract The Precambrian foundation of southern Africa consists of Archaean cratonic nuclei surrounded by belts formed during three separate Proterozoic orogenic episodes. Crust that formed or was reworked at 2.05–1.8 Ga is represented by orogenic belts (e.g. Kheis-Magondi, Ubendian-Usagaran) that partly wrap the ancient craton margins or form extensive basement in younger belts. Orogenic belts formed at 1.35–1.0 Ga record arc magmatism and collisional events during assembly of the Rodinia supercontinent. The Namaqua-Natal Belt defines a major convergent plate boundary active at this time along the southern margin of the Archaean Kaapvaal Craton, and the western part of this belt is inferred to link with a largely buried, NE-trending orogen present in the Kalahari region. Orogenesis in the same time frame farther north is recorded by the Kibaran and Irumide Belts, which are separated by the Palaeoproterozoic Bangweulu Block but are inferred to have undergone a linked tectonic evolution. East of the Irumide Belt, extensive Mesoproterozoic arc crust occurs in Malawi and Mozambique, although strong Neoproterozoic overprinting of the arc rocks makes their original relations unclear. Breakup of Rodinia is signalled by widespread Neoproterozoic alkaline and bimodal magmatism associated with rift zones which, in some cases, evolved into major ocean basins. Subsequent amalgamation of Gondwana led to collisional orogenesis culminating at 575–505 Ma in the Mozambique and West Congo-Kaoko-Gariep-Saldania Belts along the present eastern and western margins of southern Africa. Coeval deformation in the interconnecting, transcontinental Damara-Lufilian-Zambezi Orogen may reflect destruction of linked rifts and narrow ocean basins driven by farfield stresses from the collisional plate margins.

Detrital fission track thermochronology of Upper Cretaceous syn‐orogenic sediments in the South Carpathians (Romania): inferences on the tectonic evolution of a collisional hinterland

ABSTRACT The tectonic evolution of a collisional hinterland sourcing the Ha?eg Basin, a Late Cretaceous syn‐orogenic sedimentary basin in the South Carpathians (Romania), is revealed through fission track thermochronology of detrital apatite and zircon grains. This basin formed on the upper plate (Getic unit) in response to Late Cretaceous collision with the lower plate (Danubian unit), an allochtonous continental block of the Moesian Platform, upon closure of a narrow oceanic basin (Severin Basin). The fission track results suggest that Turonian to lower Maastrichtian sediments of the Ha?eg Basin have been dominantly derived from pre‐Late Cretaceous sources. The age components they contain relate to pre‐Cretaceous tectonothermal events such as the Variscan orogenic cycle, Jurassic rifting and Severin Basin formation, and to Early Cretaceous compressional tectonics. These results are compatible with the tectonic evolution of the upper plate that is identified as the primary source. From the onset of sedimentation (late Albian) until the early Campanian the Ha?eg Basin resembles a piggy‐back basin formed on the upper plate concomitant with underthrusting and internal stacking of the lower plate. In contrast, important tectonic subsidence during the late Campanian and early Maastrichtian reflects a shift to extensional tectonics causing the unroofing of the collision zone and the exhumation of lower plate rocks back to the surface. Our fission track data place important constraints on the timing of lower plate erosion that must have commenced during the late Maastrichtian, as documented by the completely reset Late Cretaceous age component within upper Maastrichtian sediments (Sînpetru Formation). Late Maastrichtian uplift of the basin and the formation of positive relief at the site of the collision zone is an expression of continuous convergence. The mismatch between the amount of denudation and the amount of sediments trapped in the Ha?eg Basin underlines the importance of concomitant extensional unroofing.

Unravelling the pre-Variscan evolution of the Habach terrane (Tauern Window, Austria) by U-Pb SHRIMP zircon data

The U-Pb SHRIMP age determinations of zircons from the Habach terrane (Tauern Window, Austria) reveal a complex evolution of this basement unit, which is exposed in the Penninic domain of the Alpine orogen. The oldest components are found in zircons of a metamorphosed granitoid clast, of a migmatitic leucosome, and of a meta-rhyolitic (Variscan) tuff which bear cores of Archean age. The U-Pb ages of discordant zircon cores of the same rocks range between 540 and 520 Ma. It is assumed that the latter zircons were originally also of Archean origin and suffered severe lead loss, whilst being incorporated into Early-Cambrian volcanic arc magmas. The provenance region of the Archean (2.64–2.06 Ga) zircons is assumed to be a terrane of Gondwana affinity: i.e., the West African craton (Hoggar Shield, Reguibat Shield). The Caledonian metamorphism left a pervasive structural imprint in amphibolite facies on rocks of the Habach terrane; it is postdated by discordant zircons of a migmatitic leucosome at <440 Ma (presumably ca. 420 Ma). Alpine and Variscan upper greenschist- to amphibolite-facies conditions caused partial lead loss in zircons of a muscovite gneiss ('white schist') only, where extensive fluid flow and brittle deformation due to its position near a nappe-sole thrust enhanced the grains' susceptibility to isotopic disturbance. The Habach terrane – an active continental margin with ensialic back-arc development – showed subduction-induced magmatic activity approx. between 550 and 507 Ma. Back-arc diorites and arc basalts were intruded by ultramafic sills and subsequently by small patches of mantle-dominated unaltered and (in the vicinity of a major tungsten deposit) altered granitoids. Fore-arc (shales) and back-arc (greywackes, cherts) basin sediments as well as arc and back-arc magmatites were not only nappe-stacked by the Caledonian compressional regime closing the presumably narrow oceanic back-arc basin and squeezing mafic to ultramafic cumulates out of high-level magma chambers (496–482 Ma). It also induced uplift and erosion of deeply rooted crystalline complexes and triggered the development of a successor basin filled with predominantly clastic greywacke-arkosic sediments. The study demonstrates that the basement rocks exposed in the Habach terrane might be the 'missing link' between similar units of the more westerly positioned External domain (i.e., Aar, Aiguilles Rouges, Mont Blanc) and the Austroalpine domain to the east (Oetztal, Silvretta).

The Middle Jurassic radiolarites and pelagic limestones of the Nieves unit (Rondaide Complex, Betic Cordillera): basin starvation in a rifted marginal slope of the western Tethys

Middle Jurassic radiolarites and associated pelagic limestones occur in the Rondaide Nieves unit of the Betic Cordillera, southern Spain. The Rondaide Mesozoic includes: (a) a thick succession of Triassic platform carbonates, comparable to the Alpine Hauptdolomit and Kossen facies; (b) Lower Jurassic pelagic limestones comparable to the Alpine Hierlatz and Adnet facies; (c) the Middle Jurassic Parauta Radiolarite Formation, described herein; and (d) a thin Upper Jurassic–Cretaceous condensed limestone succession. The Parauta Radiolarite Formation and associated limestones were studied with respect to stratigraphy, petrography, micropalaeontology (radiolarians, calcareous nanno- and microfossils) and facies. Radiolarite sedimentation occurred in the Middle Bathonian in a restricted and dysoxic deep Nieves basin, perched in the distal zone of a continental margin fringing the Tethyan ocean. This margin was adjacent to a young narrow oceanic basin between the South-Iberian margin and a continental block called Mesomediterranean Terrane. The Nieves basin was part of a marine corridor between the Proto-Atlantic and Piedmont-Ligurian basins of the Alpine Tethys. The regional tectonic position, the stratigraphical evolution since the Triassic, the age and the nature of the Mesozoic facies and the palaeogeographic relations to adjacent domains show striking analogies between the Betic Rondaide margin and coeval units of the Alps.

Magmatic constraints on geodynamic models of subduction in the East Carpathians, Romania

The East Carpathian volcanic arc is the youngest region of calc-alkaline magmatic activity in Eastern Europe. A general age progression of the onset and cessation of magmatic activity occurs along the East Carpathian arc from older volcanic structures (ca. 12 Ma) in the NW to the youngest (<1 Ma) in the SE. Magmatism continued into the Plio-Pleistocene, significantly later than the end of basin closure and the onset of continental collision along the Inner Carpathian arc that is thought to have taken place during the Miocene (9–5 Ma). Migration of magmatic activity from NW to SE along the arc may be explained by a corresponding migration of the magma-generating zone at mantle depths. Major and trace element characteristics of the erupted products are typical of subduction-related magmas and suggest an input of fluids from a dehydrating subducting slab into their mantle source region. Subduction of a narrow oceanic basin is considered to be the most probable cause of the East Carpathian magmatism and its migration. As thick continental crust began to enter the northern part of the trench at around 9 Ma, slab breakoff began although subduction of the detached slab continued at depth. As breakoff progressed from north to south, a rupture or tear propagated along the slab, causing termination of volcanism as the slab sank out of the magma-generation zone. Breakoff of the slab occurred at progressively shallower levels, southward along the arc, causing the volume of erupted arc magmas to diminish. Some unusual geological features at the southern end of the volcanic arc (e.g. contemporaneous eruption of alkaline and calc-alkaline magmas; extreme enrichment in K and other large ion lithophile elements in the arc magmas) may be accounted for by asthenospheric mantle upwelling into the void left behind by the broken slab and increased efficiency of dehydration of the remnants of the slab under the higher thermal regime.

Regional seismic phases across the Ligurian Sea: Lg blockage and oceanic propagation

The Ligurian Sea (West Mediterranean) is an example of a narrow oceanic basin which stops Lg wave propagation. Locating the exact point of Lg extinction was one of the goals of the SISBALIG II seismological experiment, which deployed both inland stations in Provence and Corsica and Ocean Bottom Seismometers (OBS) in the Ligurian Sea. A precise analysis of all available regional seismological data using energy diagrams reveals the role of the Provence margin in the Lg phase extinction for seismic events occurring in the southwestern Alps. The extinction is not a progressive phenomenon along oceanic travel path but occurs in a narrow zone of about 20 km width in the vicinity of the Provence margin. The extinction is accompanied by the generation of two types of diffracted waves: the mantle phase Sn ( V s > 3.5 km s −1), and very low velocity S waves appearing both near the Provence margin and in the middle of the Ligurian basin. We performed numerical simulation of SH wave propagation in two-dimensional (2D) media to investigate the mechanisms of Lg extinction. The simulations confirm that simple crustal thinning at the continent-ocean boundary alone cannot cause the observed extinction of Lg waves, although the change in Moho depth does induce the diffraction of a part of the incident Lg wavetrain into mantle waves, giving rise to the Sn phase on the other side of the basin. The existence of soft oceanic sediments is a very important factor in explaining the extinction, as a large part of the Lg energy is transferred into slow S waves trapped in the sediments. Moreover, comparison of synthetic seismograms and observed data shows that the observed two packets of low-velocity waves are an indication of the asymmetry of the Ligurian basin, with the existence of two shallow basins with very low S velocity (1.0 km s −1) near the Provence margin and in the middle of the Ligurian Sea. However, the attenuation of Lg waves in the vicinity of the Provence margin is more abrupt in the data than in the simulations. This discrepancy could result from the complex geometry of the continent-ocean transition zone and its possibly very diffractive nature, two factors that could add to Lg extinction at the margin and are impossible to take into account in models.