Oceanic Fracture Zones Research Articles

Zona de fratura de Vitória-Trindade no Oceano Atlântico sudeste e suas implicações tectônicas

A integração de dados geológicos e geofísicos permitiu um melhor entendimento da inter-relação entre estruturas oceânicas e continentais. Fenômenos tectônicos e magmáticos do Meso-Cenozóico do Sudeste Brasileiro podem ter sido condicionados pelo prolongamento das zonas de fratura oceânicas em direção à margem continental. O foco deste trabalho é a evolução da Zona de Fratura de Vitória-Trindade (ZFVT). A Zona de Fratura de Vitória-Trindade (ZFVT), localizada na latitude de 18º40' na Cordilheira Mesoatlântica (CMA), foi mapeada utilizando-se sísmica de reflexão, gravimetria e isócronas magnéticas. A ZFVT na CMA corresponde a um deslocamento axial em torno de 6 km, configurando uma descontinuidade de segunda ordem. O segmento assísmico da ZFVT é definido por uma depressão gravimétrica de direção E-W. Sua extensão para oeste corresponde à Cadeia Vitória-Trindade, representada por altos topográficos individualizados, que limitam desníveis crustais de mais de 400 m e províncias sedimentares de características e espessuras distintas. Esta região cortada pela intrusão de diques (alguns aflorando na superfície) e sills, indica instabilidade tectônica com a movimentação de falhas e ocorrência de estruturas transpressivas (estruturas em flor), elevando toda a coluna sedimentar incluindo o assoalho oceânico. Estas observações sugerem reativações recentes da crosta oceânica com esforços alternantes, ora compressivos ora distensivos, associados a vulcanismos e falhamentos normais. Evidências geológicas e geofísicas, além do mapeamento do alinhamento de epicentros de terremotos, sugerem que o prolongamento da ZFVT está relacionado ao Alto de Vitória e ao tectonismo e magmatismo na porção emersa, representado pelo Lineamento Sismo-Magmático Alcalino Trindade-Paxoréu / Alto do Paranaíba. Propõe-se que a ZFVT atuou e continua atuando como conduto para o magmatismo da Pluma Mantélica de Trindade.

SHALLOW GEOLOGICAL STRUCTURES OF THE SOUTH SHETLAND TRENCH, ANTARCTIC PENINSULA

In order to study the differential passive subduction among the three oceanic segments of the Phoenix Plate remnant, the R/V OGS-Explora acquired new geophysical data in the South Shetland Trench (offshore to the Antarctic Peninsula). An integrated geophysical dataset was acquired in the trench across the incoming oceanic plate segmented by the “D” and “E” oceanic Fracture Zones, and along the frontal part of the Antarctic Peninsula accretionary prism. We present here a high resolution multibeam bathymetry map, Chirp sub-bottom profiles and marine magnetic data to study changes in the deformational style along strike and to better understand features and processes of passive subduction. The new multibeam image and Chirp data display in detail shallow structures affecting the seafloor as a response to crustal deformation. Normal faults, bounding horst and graben structures cutting the incoming plate, are detected and characterized by variable orientation with respect to the inferred “D” and “E” Fracture Zones. Normal faults, width and depth of the trench and morphology of the frontal prism are related to bending and roll-back of the Phoenix Plate, and to inherited structural discontinuities. Some of these oblique faults offset the seafloor, with variable throws and with local dip changes. The orientation of the oblique faults, the different depth of the three oceanic segments and narrowing and deepening of the trench indicate that differential coupling between the Hero, Shackleton, “E” and “D” Fracture Zones may locally modify the regional stress field orientation and the sinking of each oceanic segment, during bending and roll-back.

OCCURRENCE AND SIGNIFICANCE OF SERPENTINITE-HOSTED, TALC- AND AMPHIBOLE-RICH FAULT ROCKS IN MODERN OCEANIC SETTINGS AND OPHIOLITE COMPLEXES: AN OVERVIEW

This paper reviews the occurrence and significance of talc- and amphibole-rich fault rocks developed in mafic-ultramafic sequences and evaluates their role in deformation and alteration of the oceanic lithosphere from different tectonic settings (from spreading mid-ocean ridges up to orogenic belts). Recently, talc and amphibole-rich fault rocks have been sampled and studied from detachment fault surfaces along slow and ultra-slow spreading mid-ocean ridges, and constraining the conditions of deformation and strain localization during the evolution of oceanic core complexes. These rocks are documented not only in oceanic core complexes, but also in other oceanic fracture zones where ultramafic rocks are exposed on the seafloor, while only few occurrences have been reported in ophiolite sequences. Samples recovered in situ in oceanic settings record heterogeneous deformation (crystal-plastic to cataclastic) under greenschist- facies conditions and are commonly restricted to localized shear zones (< 200 m) and are associated with intense talc-amphibole metasomatism. The presence of mechanically weak minerals, such as talc, serpentine and chlorite, may be critical to the development of such fault zones and may enhance unroofing of upper mantle peridotites and lower crustal gabbroic rocks during seafloor spreading. Talc in particular may be influential in lubricating and softening mylonitic shear zones and can lead to strain localization and focused hydrothermal circulation along such faults. The rheology of these rocks, and its evolution during dehydration reactions could play an important role also in subduction-zone processes and during the formation of ultramafic orogenic belts. Here, we review the occurrence and significance of talc- and amphibole-rich fault rocks in different tectonic settings on the seafloor and evaluate their role in deformation and alteration of the oceanic lithosphere.

Petrological and geochemical study of the Late Cretaceous ophiolite of Khoy (NW Iran), and related geological formations

This paper, based on 113 new whole rock analyses and about 3500 electron microprobe analyses of the mineral phases, is dedicated to the petrography and geochemistry of the Khoy ophiolites and related formations, NW Iran. It is complementary to a previous paper published in this Journal, where we gave a detailed description of the geology of the Khoy area, including various geological field sections, two geological maps in colour, new micropaleontological data and 27 new 40K– 40Ar datings (Khalatbari-Jafari, M., Juteau, T., Bellon, H., Whitechurch, H., Cotten, J., Emami, H., 2004. New geological, geochronological and geochemical investigations on the Khoy ophiolites and related formations, NW Iran. J. Asian Earth Sci. 23, 507–535). Our conclusions are: (a) The petrographic study confirms the field data showing the existence of two ophiolite complexes in the region of Khoy. (b) The Late Cretaceous ophiolitic lavas of the Khoy region exhibit very homogeneous T-MORB-type multi-element plots, suggesting that they were formed at oceanic spreading centers, by partial melting of a depleted mantle source, probably contaminated by one or several regional mantle plumes, responsible for their moderate enrichment in LREE. They do not show any negative anomaly for Nb, Zr or Ti, which allows us to exclude a genesis in a ‘supra-subduction’ environment. (c) The Late Cretaceous ophiolite of Khoy was created at a slow-spreading oceanic ridge. (d) Cryptic variations along extrusive and layered gabbros sections suggest frequent replenishment and magma mixing events in the magma chambers. (e) The ‘supra-ophiolitic turbiditic series’ overlying the Late Cretaceous ophiolite was accumulated in a subduction trench running along the northwestern margin of the Iran Block. This trench was fed with detrital volcanic fragments from both sides: T-MORB basalt fragments from the ocean-side, and arc-type basalts from the continent-side. (f) The meta-ophiolites of Khoy probably also represent slow-spreading conditions, and the porphyroclastic to mylonitic tectonites preserved in these metamorphic slices attest to extreme conditions of ductile shearing, characteristic of oceanic fracture zones.

Aspects of the Cenozoic deformational history of the Northeast Faroe–Shetland Basin, Wyville–Thomson Ridge and Hatton Bank areas

The nature and age of the Cenozoic compressional/transpressional deformation within the NE Faroe–Shetland Basin, the Wyville–Thomson Ridge and Hatton Bank areas have been investigated, primarily using seismic reflection data. In all three areas, the folds reach approximately 2 to 4k min amplitude and 40k min wavelength. Early and mid-Eocene compressional/transpressional deformation affected the Hatton Bank and Wyville–Thomson Ridge areas, and folding was locally active even earlier, during Paleocene/Cretaceous times. However, the main Cenozoic compressional/transpressional tectonism that affected the Hatton Bank area was coeval with development of the regional Late Eocene Unconformity (C30), and with changes in spreading geometries and a phase of accelerated subsidence in the Rockall Basin. Within the NE Atlantic margin, WNW-to NW-trending lineaments/transfer zones and associated oceanic fracture zones facilitate significant structural segmentation. Offsets in the continent–ocean boundary along Hatton Bank probably reflect inherited basin architecture, and many Cenozoic folds in the Hatton Bank, Wyville–Thomson Ridge and NE Faroe–Shetland Basin areas are considered to mainly reflect compressional buttressing against pre-existing structures. However, relatively small lateral displacements probably occurred along some reactivated transfer zones following continental break-up. Paleocene–Eocene compressional/transpressional deformation may have affected parts of the Faroe–Shetland Basin, but seismic resolution of this is largely masked by pervasive polygonal faulting. Significant, early to mid-Miocene compressional/transpressional deformation is recorded in the NE Faroe–Shetland Basin, and may also have exerted a major influence on the Wyville–Thomson Ridge and surrounding area. In particular, mid-Miocene growth of the Faroe Bank Channel syncline may have resulted in major changes in northern hemisphere deep-ocean circulation with associated impact on global climate. Compressional/transpressional deformation appears to have continued into Pliocene– ?Recent times and resulted in the development of features such as the Pilot Whale Anticline and associated mud volcanoes/diapirs.

Geochemistry of Nevado de Longaví Volcano (36.2°S): a compositionally atypical arc volcano in the Southern Volcanic Zone of the Andes

The Quaternary Nevado de Longavi volcano of the Andean Southern Volcanic Zone (SVZ) has erupted magmas that range in composition from basalt to low-silica dacite, although andesites are the dominant erupted magma type. Amphibole is a common phenocryst phase in andesites throughout the volcano, and it is the dominant mafic phase in Holocene dacites and their included mafic enclaves. Compositions of magmas erupted at Longavi volcano define arrays that diverge from trends delineated by neighboring frontal-arc volcanoes. Although mafic compositions at Longavi are broadly similar to basalts at other SVZ centers, Longavi intermediate and evolved magmas have systematically lower abundances of incompatible major (K 2 O, P 2 O 5 ) and trace elements (Rb, Zr, Nb, REE, Th, etc), as well as high Ba/Th, Sr/Y, and La/Yb ratios. Longavi volcano magmas define two differentiation series with regard to enrichments of Rb (and other incompatible elements) with increasing silica. A high-Rb series that includes the oldest units of the volcano comprises basalts to andesites dominated by anhydrous mineral assemblages with chemical compositions similar to other SVZ magmatic series. The series with low Rb, on the other hand, includes the Holocene units that evolved from basaltic andesites to dacites by means of fractional crystallization wherein amphibole and calcic plagioclase dominate the mineral assemblage. Magmas parental to low-Rb series are interpreted to be high-degree mantle melts, highly hydrous and oxidized, formed as a response to high fluid inputs into the subarc mantle. Enhanced water transport to the subarc mantle is a plausible effect of the subduction of the oceanic Mocha Fracture Zone that projects beneath Nevado de Longavi. Volcanoes located over oceanic fracture zones further south along the SVZ have erupted hornblende-bearing magmas that share some chemical similarities with Longavi volcano magmas.

Curvilinear patterns of oceanic fracture zones

The goal of this paper is to determine the geometry of the Earth's oceanic fracture zones (FZs) on a worldwide scale and to describe their geometrical patterns. For this, 214 FZs were digitized in 11 digital images (Warnken, 2001). The curvatures of each FZ were then measured and categorized. The overall result is that the majority of FZs (89.3%) share the same geometrical behavior referred to as “curve pattern” in this work. On the Northern hemisphere, most of the FZs have their first curve pointing to the right and on the Southern hemisphere to the left, when looking from a FZ's starting point on an ocean ridge axis towards its end point. Exceptions to this mirror-like curve pattern are either located within ±10 degree latitude of the Earth's equator or seem to be influenced by other factors, such as proximity to a continental crust or truncation by lineaments.

A subducted oceanic ridge influencing the Nankai megathrust earthquake rupture

Megathrust earthquake rupture is highly influenced by seafloor bathymetric features such as oceanic fracture zones, seamounts, and ridges in convergent plate margins where these features subduct. Recent seismic reflection and refraction data reveal a trough-parallel subducted oceanic ridge that is attached to the descending Philippine Sea Plate beneath the accretionary wedge of the overriding Eurasian Plate in the eastern Nankai Trough subduction zone consisting of the Tonankai and Tokai segments. The seismic survey data confirm the existence of subducted paleo-Zenisu ridge that was suggested by just magnetic data, and as a result, expands its distribution to the west. The newly constrained subducted ridge that spans the two segments is estimated to be a maximum of ∼2.5 km high, ∼20–30 km wide, and ∼200 km long. Spatial mapping of the ridge shows that it is located roughly at the seaward edge of the coseismic rupture zone of the 1944 Tonankai earthquake ( M=8.1). This ridge appears to be in close contact with the seaward end of the rigid backstop in the Tonankai segment. Both the spatial correlation and the ridge–backstop collision geometry suggest that the subducted ridge might be strongly mechanically coupled and may thus play a significant role as a seaward barrier inhibiting the 1944 earthquake rupture from propagating farther seaward.

Intra-oceanic production of continental crust in a Th-depleted ca. 3.0�Ga arc complex, western Superior Province, Canada

The English Lake magmatic complex in the western Superior Province of Canada represents a fragment of early (3.0 Ga) continental crust exposed in oblique cross section through tonalitic upper levels and subjacent quartz diorite, diorite and gabbro, which are cut by late gabbro, anorthosite and hornblendite dykes. Massive, foliated and gneissic units of tonalitic to gab- broic composition, crystallized over a 10 to 18 m.y. period, bear common geochemical attributes, including negative Th, U and Nb anomalies, and only slight LREE and LILE enrichment on NMORB-normalized trace-element profiles. Epsilon Nd values (+0.1 to +1.7) and d 18 O (+6.7 to +8.0 &) do not co-vary with silica or other crustal contamination indices. High Mg#'s and Ni contents suggest derivation from, or interaction with mantle, and large positive anomalies for Ba, Sr and Pb, as well as high U/Th, suggest metaso- matism by hydrous fluids. Trace-element profiles resemble those of primitive intra-oceanic island arc magmas except for the negative Th-U anomaly, which precludes the involvement of either oceanic (sedimentary or basaltic) or continental crust in the petrogenesis of English Lake magmas. In order to account for the unusual geochemical character of the suite, we postulate that water-rich fluids derived from subducted, sea-floor- altered serpentinite provided the flux for melting a depleted mantle wedge. Contemporaneous, proximal high Th/Nb tonalites suggest that the zone of serpenti- nite subduction occurred within a restricted arc segment possibly due to subduction of either: (a) a seamount chain oriented broadly perpendicular to an arc, or (b) a similarly oriented serpentinite-enclosed oceanic fracture zone or fault.

Early Proterozoic oceanic crust in the northern Colorado Front Range: Implications for crustal growth and initiation of basement faults

Early Proterozoic oceanic crust exposed in a shear zone in the northern Colorado Front Range provides insight into the tectonic evolution of the Colorado province. Tectonically intermingled mylonitic amphibolite (locally pillowed), metagabbro, clinopyroxenite, metachert, and mélange are preserved within the Buckhorn Creek shear zone west of Fort Collins. Whole rock geochemical data support formation of the igneous protoliths in an oceanic environment. Major and trace element data overlap samples from modern oceanic ridges, and are distinct from modern arc trends. Metagabbros and clinopyroxenites preserve a subhorizontal lineation defined by igneous pyroxenes; core and mantle microstructures indicate initial deformation at temperatures &gt;900°C. Secondary amphiboles define both subhorizontal and subvertical lineations. Kinematic analysis indicates that the rocks experienced E‐W strike‐slip deformation prior to and during amphibolite‐facies metamorphism, followed by N‐S shortening. 40Ar/39Ar data from secondary hornblende yield age steps as old as 1.9 Ga. Based on the chemical data and kinematic evidence for strike‐slip deformation, we propose that the Buckhorn Creek rocks originated at an oceanic ridge‐transform intersection. The high‐T pyroxene fabrics formed during ocean floor deformation prior to 1.7 Ga regional deformation of the Colorado province. The Buckhorn Creek fragment likely represents original oceanic basement for 1.8–1.7 Ga arc magmatism and sedimentation that ultimately formed the Colorado province continental crust. Reactivation of original oceanic fracture zones may be responsible for the network of regularly spaced northeast trending shear zones in Colorado. Once formed, subvertical transform faults are likely to survive the transition from oceanic to continental crust and persist as long‐lived crustal discontinuities.

Geochemistry and depositional environment of ferromanganoan metasediments on the Island of Kythnos, Cyclades, Greece

Samples of Fe-Mn metasediments (hematite quartzites, mica-bearing quartzites and garnet-bearing amphibole schists) from the lower sub-unit of Kythnos island, part of Attic-Cycladic complex of Alpine age, have been collected and analysed for 45 elements, including rare earth elements. The hematite quartzites and mica-bearing hematite quartzites occur as intercalations in the upper part of a metavolcanic sequence or in the lower part of an overlying metasedimentary sequence of the northern part of Kythnos island. They show trace elements concentration and rare earth element (REE) patterns similar to those of hydrothermal deposits. The garnet-bearing amphibole schists occur in the sedimentary sequence of the southeastern part of the island. They have lower Fe/Mn ratios and higher trace element and REE contents in comparison with the hematite-quartzites and mica-bearing hematite quartzites. According to these geochemical characteristics, the protolith of Fe-Mn metasediments may have been formed under oceanic hydrothermal activity. In this process, circulating heated seawater reacted with basic volcanic rocks to form metal bearing hydrothermal solutions enriched in Fe, Mn and trace elements. These hydrothermal solutions were discharged on the ocean floor, through shear and/or fracture zones in an environment of decreasing temperature. The decreasing negative Eu anomalies from the lower (hematite quartzites) to the upper part (garnet-bearing amphibole schists) of the sequence possibly indicate a change in the condition (T, fO 2 ) of the deposition as well as an increase of the detrital-hydrogenous components in relation to the hydrothermal fluids.

About this title

Intraplate strike-slip deformation belts are common tectonic features, particularly at convergent plate boundaries, where they are produced by both oblique convergence and continental indentation. These lithosphere-scale structures, which also occur in other geodynamic environments such as passive margins, are characterized by complex structural architectures, by the occurrence of large earthquakes, and by the fast uplift and/or subsidence of localized crustal sectors. Intraplate strike-slip belts can also control the ascent and emplacement of deeply sourced magmas. In some cases, intraplate strike-slip belts link with oceanic fracture zones and transform faults, transferring transform shear from the ridges to the interior of the plates. This evidence has an important impact of the classical concept of transform faulting. This volume contains 13 papers from an international field of contributors. Studies of intraplate strike-slip deformation belts from Africa, Antarctica, Eurasia, North America and South America are included.

Intraplate strike-slip tectonics as an alternative to mantle plume activity for the Cenozoic rift magmatism in the Ross Sea region, Antarctica

Abstract The West Antarctic Rift System is one of the largest areas of crustal extension in the world. Current interpretations on its driving mechanisms mostly rely on the occurrence of one or more mantle plumes, active during the Cenozoic or the Mesozoic. Recent studies of structural-chronological relationships between emplacement of plutons, dyke swarms, and volcanic edifices since middle Eocene in northern Victoria Land imply that magma emplacement is guided by strike-slip fault systems that dissect the western rift shoulder in Victoria Land. These studies led to a critical re-examination of the arguments used to support plume models. In Victoria Land, the linear geometry of the uplift and the relative chronology of uplift and extension are inconsistent with the traditional concepts of lithospheric evolution above a mantle plume. The geochemical signature of the mafic rocks is equivocal, because both OIB and HIMU features cannot be exclusively interpreted in terms of plume activity. From a thermal point of view, magma production rates are low compared with the core part of plume-related provinces. Additionally, the hot mantle below the West Antarctic Rift System is not documented as deep as expected for mantle plumes and the shape of thermal anomaly is related to lithospheric geometry, being linear rather than having circular symmetry. The lack of any decisive evidence for plume activity is contrasted by evidence that large-scale tectonic features guide magma emplacement: the Cenozoic fault systems reactivated inherited Palaeozoic tectonic discontinuities and their activity is dynamically linked to the Southern Ocean Fracture Zones. As an alternative to both active, plume-driven rifting and passive rifting, we propose that lithospheric strike-slip deformation could have promoted transtension-related decompression melting of a subplate mantle already decompressed and veined during the late Cretaceous amagmatic extensional rift phase. Magma ascent and emplacement occurred along the main strike-slip fault systems and along the transtensional fault arrays departing from the master faults.

Geochemistry and Tectonic Setting of the Ophiolitic Ingalls Complex, North Cascades, Washington: Implications for Correlations of Jurassic Cordilleran Ophiolites

This study synthesizes new and existing chemical data to interpret the tectonic setting of the Late Jurassic Ingalls Complex of the Northwest Cascades, Washington, and to compare it with other Cordilleran ophiolites of similar age. Mafic rocks of the Ingalls Complex represent a spectrum of magma types including normal‐type and enriched mid‐ocean ridge basalt, within‐plate basalt, and island arc basalt. Based on mafic magma chemistry, compositions of Cr‐spinels in associated peridotite, and field relations, the Ingalls Complex is a suprasubduction zone ophiolite formed in a back‐arc basin cut by an oceanic fracture zone. Regional tectonic relations also support this hypothesis. Our synthesis of available chronologic, geochemical, and stratigraphic data for all Jurassic Cordilleran ophiolites shows that the Ingalls Complex has similarities and differences with each. The most compelling likeness is between the Ingalls Complex and Josephine ophiolite, both of which exhibit a wide range of magmatic affinities, Cr‐spinel compositions that are consistent with a suprasubduction zone origin in a back‐arc basin setting, and positions inboard of a coeval Jurassic arc.

Geological mapping in northeastern Somalia (Midjiurtinia region): Field evidence of the structural and paleogeographic evolution of the northern margin of the Somalian plate

A detailed geological investigation of the continental margins of northeastern Somalia, in land areas contiguous to major oceanic structures (e.g. the Alula–Fartaq fracture zone), was carried out from 1988 to 1991, and the results are given in the enclosed 1:200,000 geological map, derived from photointerpretation and fieldwork at the scale 1:50,000. With respect to the rift of the Gulf of Aden, the stratigraphic sequences illustrated in the map have been distinguished as pre- and syn- and post-rift sediments. The pre-rift sediments rest on the low-grade ?pre-Palaeozoic phyllites and pan-African granitic intrusions and consist of sedimentary cover of continental, lagoon and marine facies ranging in age from Dogger to Eocene. Cretaceous–Eocene sediments pass eastward into clastic and marly deposits of the pelagic domain of the Indian Ocean. The syn- and post-rift sequences ranging in age from Oligocene to Miocene crop out only in narrow “en-echelon” basins striking WNW–ESE along the coast of the Gulf of Aden. The deposits belonging to the syn- and post-rift sequences are discordant and transgressive over the Meso–Cenozoic substratum and are composed by organogenic neritic marine, lagoonal and continental deposits. The tectonic setting of the area is characterized by half-graben bordered by faults with displacements of several kilometres striking WNW–ESE. Faults bordering contiguous half-graben systems often dip in opposite directions, so that in the transition area between different ones a “transfer” or “accommodation” zone develops; the zones are on the landward projection of the oceanic fracture zones, whilst the syn-rift basins are on the landward projection of the oceanic ridge. Based on our fieldwork the following reconstruction is put forward: 1. During the Early Oligocene the Afro-Arabian plate underwent a phase of intense faulting which led to the formation of small syn-rift basins elongated in the WNW–ESE direction, separated by transfer zones and structural highs. 2. Progressive crustal extension led to the formation of spreading centres of oceanic crust corresponding to earlier syn-rift basins. The fracture zones linking spreading centres formed in alignment with continental transfer zones.

Lateral variations in tectono-magmatic style along the Lofoten–Vesterålen volcanic margin off Norway

The ∼400km-long passive continental margin west of the Lofoten–Vesterålen archipelago, off northern Norway, links the volcanic rifted Vøring margin and the sheared W Barents Sea margin. Multi-channel seismic reflection profiles, supplemented with crustal velocity, gravity and magnetic anomaly data are used to outline the regional setting and main tectono-magmatic features. A well-defined along-strike margin segmentation comprising three segments characterized by distinct crustal properties, structural and magmatic styles, sediment thickness, and post-opening history of vertical motion is revealed. The margin segments are governed by changes in fault polarity on Late Jurassic–Early Cretaceous border faults and are separated by coeval cross-margin transfer zones which acted as persistent barriers to rupture propagation and reflect the trend and character of older structural heterogeneities. The transfer zones spatially correlate to small-offset, early opening oceanic fracture zones, implying a structural inheritance from one rift episode to another culminating with lithospheric breakup at the Paleocene–Eocene transition. The pre-seafloor spreading margin structural evolution is governed by the older, predominantly Late Jurassic–Early Cretaceous structural framework. However, the margin also provides evidence for mid- and Late Cretaceous extension events that are poorly understood elsewhere off Norway. Furthermore, the Lofoten–Vesterålen post-breakup subsidence history contrasts with the adjacent margins reflecting breakup in thicker crust and a diminishing volume of high-velocity lower crust emplaced during breakup.

Computer technologies used to obtain new information on crustal structure in oceanic fracture zones: A case study on the active segment of Sao Paulo Fracture Zone, Central Atlantic

Computer technologies used to obtain new information on crustal structure in oceanic fracture zones: A case study on the active segment of Sao Paulo Fracture Zone, Central Atlantic

Mud diapirism in front of the Barbados accretionary wedge: the influence of fracture zones and North America–South America plate motions

Mud diapirism in sediments on the ocean floor ahead of the Barbados accretionary wedge, previously believed to be a consequence of fluid overpressure and fluid migration caused by the advance of the accretionary wedge, is shown to have been initiated at about 3.4Ma by a change in the motion between the North and South American plates across fracture-zones. Neither the deposition of the Orinoco Fan nor the advance of the Barbados accretionary wedge played a part in triggering this Pliocene mud diapirism, although the former was important in creating the conditions that favoured diapirism, and the latter has sustained and reactivated diapirs to the present day. Active mud diapirism occurs from the deformation front of the accretionary wedge to at least as far as 23km ahead of it, in linear diapir fields that are aligned WNW–ESE, along the northern sides of north-facing basement scarps flanking oceanic fracture zones. The basement troughs on the northern sides of the basement ridges flanking the oceanic fracture zones contain increased thicknesses of low-permeability pelagic–hemipelagic sediments, which are prone to overpressuring and undercompaction, and are therefore a likely diapiric source. There is evidence of Plio-Pleistocene diapirism in one of the fracture-zone troughs, nearly 350km east of the accretionary wedge. During most of the Tertiary, compression and transpression across the fracture zones tectonically thickened the already thicker accumulation of sediment in the fracture-zone troughs, increasing its diapiric potential, which was increased further by the deposition of overlying Orinoco submarine fan sediments from 12Ma. Pliocene diapirism was triggered by a change in the relative plate motion between North and South America from compression to dextral strike–slip movement, between 3 and 5Ma. While fluid expelled from beneath the accretionary wedge, travelling through faults opened up in the lower part of the sediment fill of the fracture-zone troughs, cannot be excluded as the mechanism driving the Pliocene diapirism, the very long distance required for transmission of fluid favours alternative explanations. One of these alternatives is that mud was mobilised by the water generated by the smectite–illite transformation in sediment heated by fluid flow in the oceanic crust beneath the fracture zones. This flow may have resulted from a reorganisation of the fluid-flow regime of the igneous oceanic crust, in response to the change of stress and strike–slip faulting along the fracture zones.

Crustal structure of the continent-ocean transition off the Côte d'Ivoire-Ghana transform margin: implications for thermal exchanges across the palaeotransform boundary

Crustal structure of the continent–ocean transition off the Côte d'Ivoire–Ghana transform margin is determined by 2‐D modelling of wide‐angle seismic data and gravity data. The resulting models, together with geological data recently collected by drilling on the margin, are compared with available thermomechanical models of transform margins. In particular, we test the predicted effects on the transform margin crust of continental heating by the adjacent oceanic lithosphere. These effects include: (i) ductile flow of the lower continental crust either parallel to the transform direction or perpendicular to it, from the continent towards the ocean; and (ii) thermal uplift of the transform margin. In the region of the Côte d'Ivoire–Ghana transform margin, the seismic and gravity data indicate that the continental crust remains uniform in thickness as the continent–ocean boundary is approached. The interpreted models lack a transitional domain between continental and oceanic crusts, and abnormally thin oceanic crust is found along the margin. These results exclude large‐scale ductile flow of lower continental crust during the margin formation. The geological data are, moreover, not consistent with thermal uplift of the margin during the period of contact between the transform margin and the spreading centre. Our geophysical and geological data thus infer only moderate heating of the transform continental margin by the adjacent oceanic lithosphere. Moderate heating of the Côte d'Ivoire–Ghana transform margin during margin formation implies that the thermal exchange between the transform margin and the adjacent spreading centre were lower than predicted by available thermal models. Two main factors may have contributed to reduce thermal exchanges across the continent–ocean boundary. The first factor is that the oceanic edge may have been colder than assumed by thermal models. An abnormally cold spreading axis along the transform margin when the oceanic crust was emplaced is supported by the atypical structure of the oceanic crust along the Côte d'Ivoire–Ghana transform margin. The oceanic crustal structure is similar to that found along oceanic fracture zones, where oceanic crust is also emplaced near a ridge–transform intersection. In both cases, abnormally low thermal gradients at the spreading centre along the transform margin may be explained by the cumulative effect of: (i) a cold edge effect of the adjacent continental lithosphere; (ii) accretion at the end of a ridge segment, away from the main magma supply point along the spreading axis; and (iii) an efficient upward cooling of the oceanic lithosphere owing to deep hydrothermal circulation within the faulted lithosphere. The second factor causing moderate thermal exchanges is the possibility of large amounts of water in oceanic lithospheric faults, which reduces the lateral thermal conduction in the lithosphere, since water is much less conductive than rock. During the margin formation, the oceanic edge thus seems to have acted as a thermal buffer between the cold continental margin and the hot spreading centre. We propose that this thermal buffer hypothesis may be generalized to most transform margins, implying a lower amount of continental heating than assumed by the available thermomechanical models.

Land–offshore tectonic links in western Norway and the northern North Sea

Aeromagnetic and marine gravity data sets for the northern North Sea and western Norway have been interpreted in terms of land to offshore tectonic links. The data allow identification of linking-structures both in crystalline basement and overlying sedimentary rocks and consequently facilitate a more comprehensive interpretation of regional structural features and trends. With the help of bathymetric data in the inshore region, major fault structures can be traced between western Norway and the offshore region west of the Øygarden Fault Zone. In this way spatial and chronological links have been established between tectonic events recognized on land and the development of the North Sea Basin. An interpretation of basement geology is proposed for the region between western Norway and the Uer Terrace. This is complemented with a depth to top crystalline basement map covering the North Sea Basin between the Uer terrace and the East Shetland Basin. Two regional NW–SE lineaments were recognized offshore western Norway and given the names Marflo Lineament and Troll Lineament. The Marflo Lineament is interpreted to be a Caledonian or older zone of weakness that accommodated development of a 20–30 km offset in the Jurassic Viking Graben axis. The lineament can be traced into the North Atlantic where it parallels oceanic fracture zones and transform faults. The Troll Lineament relates to a deep geological discontinuity crossing the Uer and Lomre Terraces. A grouping of earthquake epicentres along part of the lineament suggests that the discontinuity is tectonically active.