Northern Craton Research Articles

Lithospheric structure of the western Borborema Province from receiver functions and surface-wave dispersion: Implications for basin inversion

The Proterozoic Borborema province of NE Brazil was under extensional stresses during the Mesozoic, causing stretching and thinning of the lithosphere. During Cenozoic times, compression associated with the development of the South Atlantic and collision along the Andean front dominated throughout the Province. One of the main consequences of such compression might have been the formation of the Chapada de Araripe, an inverted rift basin located in the Western Province. We undertook a passive seismic experiment along a NS profile crossing the Borborema province and a short EW profile across the Chapada de Araripe, with the goal of providing constraints on the origin of the Chapada de Araripe. A variety of mechanisms can contribute to basin uplift, so understinding basin inversion is in turn important for understanding the tectonic evolution and geodynamics of NE Brazil. Through joint inversion of receiver functions and surface-wave dispersion, we show that the crust is typically 34–38 km thick under the western Province; in contrast, the crust is 43–46 km thick under the northern craton. No apparent crustal thinning is observed under the Araripe Basin, due to the presence of a relatively thick (10–14 km) high-velocity (4.1–4.3 km/s) lowermost crust. Our velocity models also image a drop in S-velocity (from 4.6 km/s to 4.3 km/s) at 135–145 km depth under the southern half of the profile, which we interpret as a mid-lithospheric discontinuity, and at 120–140 km depth under the Araripe Basin, likely representing the lithosphere-asthenosphere boundary. Our findings confirm the presence of thin lithosphere under the Araripe Basin - originally postulated by a collocated MT survey - that might have facilitated tectonic inversion of this basin through regional stress concentration and thermal buoyancy. These findings also suggest additional buoyancy contributed by a mafic lowermost crust right under the basin.

Diamond ages from Victor (Superior Craton): Intra-mantle cycling of volatiles (C, N, S) during supercontinent reorganisation

The central Superior Craton hosts both the diamondiferous 1.1 Ga Kyle Lake and Jurassic Attawapiskat kimberlites. A major thermal event related to the Midcontinent Rift at ca. 1.1 Ga induced an elevated geothermal gradient that largely destroyed an older generation of diamonds, raising the question of when, and how, the diamond inventory beneath Attawapiskat was formed. We determined Re–Os isotope systematics of sulphides included in diamonds from Victor by isotope dilution negative thermal ionisation mass spectrometry in order to obtain insights into the age and nature of the diamond source in the context of regional tectonothermal evolution. Regression of the peridotitic inclusion data (n=14 of 16) yields a 718 ± 49 Ma age, with an initial 187Os/188Os ratio of 0.1177 ± 0.0016, i.e. depleted at the time of formation (γOs −3.7 ± 1.3). Consequently, Re depletion model ages calculated for these samples are systematically overestimated. Given that reported 187Os/188Os in olivine from Attawapiskat xenoliths varies strongly (0.1012–0.1821), the low and nearly identical initial Os of sulphide inclusions combined with their high 187Re/188Os (median 0.34) suggest metasomatic formation from a mixed source. This was likely facilitated by percolation of amounts of melt sufficient to homogenise Os, (re)crystallise sulphide and (co)precipitate diamond; that is, the sulphide inclusions and their diamond host are synchronous if not syngenetic.The ∼720 Ma age corresponds to rifting beyond the northern craton margin during Rodinia break-up. This suggests mobilisation of volatiles (C, N, S) and Os due to attendant mantle stretching and metasomatism by initially oxidising and S-undersaturated melts, which ultimately produced lherzolitic diamonds with high N contents compared to older Kyle Lake diamonds. Thus, some rift-influenced settings are prospective with respect to diamond formation. They are also important sites of hidden, intra-lithospheric volatile redistribution that can be revealed by diamond studies. Later emplacement of the Attawapiskat kimberlites, linking the carbon cycle to the surface, was associated with renewed disturbance during passage of the Great Meteor Hotspot. Lherzolitic diamond formation from oxidising small-volume melts may be the expression of an early and deep stage of the lithospheric conditioning required for the successful eruption of kimberlites, which complements the late and shallow emplacement of volatile-rich metasomes after upward displacement of a redox freezing front.

Meso-Neoproterozoic petroleum systems of the Eastern Siberian sedimentary basins

The elements of the petroleum systems of the Siberian platform have been formed in the course of geological evolution in Meso- and Neoproterozoic time and correspond to the supercontinent stage from assembly of Paleo-Mesoproterozoic Columbia to break-up of Neoproteorzoic Rodinia. Several large sedimentary basins were established within the Siberian craton during the Riphean. A number of passive continental margin basins (the Turukhansk, Cis-Patom basins and others) developed on the southern, western, and, probably, northern craton peripheries. Several other basins, such as the Irkineeva-Vanavara, Kotuy, Anabar-Kureika, and Udzha basins, were entirely intracratonic. The structural patterns of these basins and their petroleum systems were essentially affected by Baikalian orogeny events with intensive dislocations and erosion of Riphean series, especially along the margins of the continent. As a result, hydrocarbon accumulations formed by the source rocks, which have realized their potential in Riphean within the margin basins, were, most likely, completely destroyed. Vendian and Lower Cambrian deposits were the main source for present day oil fields of the central and northern parts of the Nepa-Botuoba Anteclise and the Turukhansk Uplift. The Upper Riphean source rocks of the intracratonic basins, on the contrary, are unlikely to be affected by the significant pre-Vendian erosion and, thus, could be a source for the oil fields of the Yurubchen-Tokhomo Zone and the Katanga Saddle. Such difference in petroleum system evolution for different basins is also supported by results of oil–source rock correlations based on biomarker studies.A new mega-cycle of formation of the sedimentary cover started on the Siberian platform in the Vendian (Ediacaran). The initial fabric of all the main structural elements of the platform was established at this time. The large Vendian depressions (the “prototypes” of Paleozoic Kureika and Cis-Sayan-Yenisei syneclises and Cis-Patom Trough) were areas of mainly marine sedimentation where the high potential hydrocarbon source series could be deposited. Large elevated areas served as a terrigenous provenance in the Early Vendian resulting in a large amount of clastic material deposited along their flanks. The most promising areas for new discoveries of hydrocarbon fields are in the slopes of the Baikit, Nepa-Botuoba and Anabar anticlises faced towards the Kureika Syneclise.

Géochimie et cadre géodynamique du volcanisme néoprotérozoïque terminal (vendien) du Haut Atlas occidental, Maroc

Late Neoproterozoic Vendian volcanic and volcaniclastic rocks are widely distributed in the western High Atlas. They are located north of the Tizi n'Test Fault, separating the West African Craton from a northerly adjacent craton. These volcanic rocks overlie a semipelitic formation, which represents the equivalent of the Tidilline and Anzi Formations of the Anti-Atlas. The geochemical characteristics of these volcanic rocks suggest a calc-alkaline active margine environment associated with the post Pan-African tectonics. They differ from those of the Anti-Atlas by their lower content of K 2O. The later rock type was generated by a melting process of the crust subducted beneath the northern craton. A carbonate-shale unit, which contains examples of interstratified calc-alkaline dacite, overlies the volcanic succession, demonstrating that the volcanic activity continued sporadically until Early Cambrian times.

Historical biogeography of the cicadas of Wallacea, New Guinea and the West Pacific: a geotectonic explanation

The present-day distribution patterns of the cicadas of Wallacea, New Guinea, and the West Pacific reflect the extremely complex geotectonic history of that area. The patterns found in two unrelated monophyletic groups of cicadas (Insecta, Homoptera, Cicadoidea), the subtribe Cosmopsaltriaria and the tribe Chlorocystini with its sister tribe Prasiini, are analysed and related to data from the geological literature. The two groups have a comparable distributional range over Sulawesi, Maluku, New Guinea, the Bismarck Archipelago, and the East-Melanesian archipelagos, to Tonga and Samoa. The occurrence of endemic species, species groups, and genera in congruently delimited parts of this range made us recognize 14 areas of endemism. The areas of endemism coincide with geological entities, which have been isolated during a considerable period of time. Most of the areas originated from oceanic island arcs, which developed as a result of subduction along the southern and western edges of the Pacific tectonic plate, others are rifted microcontinents. Concentrations of species and especially endemic species of the various cicada genera in different areas of endemism suggest that each of these genera evolved in isolation on one of the afore mentioned geological entities. These entities can therefore be regarded as source areas for the genera. The island arc fragmented as a result of collisions first with the Asian- and later with the Australian tectonic plate and several of its parts reamalgamated at the northern craton of the Australian plate, where they now form the greater part of the island of New Guinea. After reamalgamation of the arc fragments the cicadas of the respective genera that had evolved on these fragments could disperse into adjoining areas, but since cicadas are poor dispersers the source areas of the genera can still be recognized in present-day distribution patterns. The generic relationships in the two groups of cicadas under study indicate area relationships that comply with the latest palaeogeographic reconstructions. The main vicariant speciation events in the cicada cladograms correspond with the presumed sequences of fragmentation of the island arc. Cicada phylogeny and distribution combined reflect the historic area relationships rather than the present ones.

Compositional Variations and Tectonosedimentary Conditions of the Atokan Deltaic Sediments, Arkansas: ABSTRACT

ABSTRACT Detailed petrographic studies were conducted to determine the compositional variations and the influences of tectonics on the deposition of the Pennsylvanian Atokan sediments in Arkansas. The rocks have a composition of Q82-90F3-10R5-19 and are classified as sublitharenites. Quartz is dominated by monocrystalline types (82-71%) with minor amounts of polycrystalline variety (3-7%). Feldspars constitute a minor proportion (K-feldspar 2-5%; plagioclase feldspar 0-4%); and lithics (5-12%) are composed of metamorphic and sedimentary fragments. Paleocurrent data and high percentage of quartz suggest that the sediments were primarily derived from the northern craton. However, metamorphic and sedimentary rock fragments imply the Appalachians as a secondary source area. Point counting via the Gazzi-Dickinson technique indicates that all rocks are of a recycled orogen while the sandstone composition shows cratonic derivation. The high quartz content, and a dearth of feldspars indicate that warm and humid climate and low relief extensively altered the primary mineralogic signatures in the source area. Compositional patterns of the Late Paleozoic sandstones with known tectonic origins suggest that the Dickinsonian diagrams are, in this case, insensitive to the true regional tectono-sedimentary scenario. Compositional variations and paleocurrent data suggest that the lithic and feldspathic Stanley sandstones were derived from the uplifted passive margin/craton which lay to the south of the Ouachita basin, while the more quartzose sandstones of the Jackfork and the Atoka were derived from the northern craton. In spite of deposition in a foreland basin adjacent to a thrust belt, the Stanley sandstone compositions plot on the continental block provenance, the Jackfork sandstones cluster around apex Q indicating a cratonic block provenance, and the Atoka sandstones with northern cratonic derivation plot on the recycled orogen field.

Origins of upper Paleozoic quartzose sandstones, American southwest

In the latter half of the Paleozoic, all margins of the North American craton were orogenic highlands except for the Cordilleran miogeocline. Rivers that drained cratonic areas northwest of the Transcontinental arch probably flowed to the topographically low Cordilleran miogeocline, but little direct sedimentary record of these ancient fluvial systems remains. The voluminous upper Paleozoic quartzose sandstone deposits of the American west are indirect evidence for these systems. Northern and central portions of the miogeocline were flooded with quartzose sand in the Pennsylvanian, and by mid-Early Permian, littoral and wind processes were transporting large volumes of quartzose sand southward, along the miogeoclinal/cratonic hingeline. Winds drove some of these sands inland, onto the disrupted western end of the Transcontinental arch, where they mixed with locally derived detritus and collected in great ergs. Calcareous siltstones that were deposited downwind of major ergs probably represent desert loess accumulations. Some eolian sands spilled across the Transcontinental arch to interfinger with carbonate and evaporite deposits in basins of the southwestern United States. Most sandstones deposited by these ancient littoral and eolian systems are fine- to very fine-grained, potash-feldspar subarkoses. Bimodal sandstones are common. The distribution, composition and volume of these sandstones suggest that the northern craton was the major source area and cratonic uplifts of the Ancestral Rockies were a secondary source. The fine grain size, high degree of sorting, and grain-size bimodality suggest that these sands were the most mobile sediments in littoral and eolian settings. A perplexing aspect of these deposits is the lack of compositionally unstable silicates, such as plagioclase, and the lack of phyllosilicate clays. Possibly this reflects great amounts of physical abrasion and sediment winnowing during protracted transport in littoral and eolian systems, and it may partially reflect weathering processes in arid coastal settings. The petrology therefore reinforces the model of a northern cratonic source and protracted southerly transport, but the degree to which the petrology reflects provenance and various physico-chemical processes remains unresolved.

Mid-Devonian to early Permian volcanism: Old World

Summary Three major episodes of activity are distinguished in each of three volcanotectonic provinces. The provinces are represented by a craton to the N and the Moldanubian zone (including central Iberia) in the S separated by the orogen which combines the inner Saxothuringian-Ossa Morena zone with the outer Rhenohercynian-S Portuguese zone. During the first episode (380–325 Ma) activity did not begin on the craton until early Carboniferous and was then continental, alkaline and mainly basaltic; in the orogen, however, bimodal mainly tholeiitic activity assumed to be back-arc in character continued from early in the Devonian, while the southern province was experiencing calc-alkaline volcanism probably related to northwards subduction of the proto-Tethys ocean crust. The second episode saw the alkaline activity of the craton becoming volumetrically reduced and largely more phreatic, while the orogen became volcanologically inactive and calc-alkaline activity probably continued in the S, although the evidence (distal ash-fall farther N) is indirect. In the third episode the orogen, now accreted to the northern craton, experienced, with the craton, renewed and more silica-undersaturated alkaline activity related to newly formed fracture patterns, while in the southern province there was a resurgence of calcalkaline activity.