Crustal Transect Research Articles

Extension Dynamics of the Northern Fonualei Rift and Spreading Center and the Southern Mangatolu Triple Junction in the Lau Basin at 16°S

AbstractDue to the complexity of 2D magnetic anomaly maps north of 18°S and the sparsity of seismic data, the tectonic evolution of the northern Lau Basin has not yet been unraveled. We use a multi‐method approach to reconstruct the formation of the basin at ∼16°S by compiling seismic, magnetic, gravimetric and geochemical data along a 185 km‐long crustal transect. We identified a crustal zonation which preserves the level of subduction input at the time of the crust's formation. Paired with the seafloor magnetization, the crustal zonation enabled us to qualitatively approximate the dynamic spreading history of the region. Further assessment of the recent tectonic activity and the degree of tectonic overprinting visible in the crust both suggest a complex tectonic history including a dynamically moving spreading center and the reorganizing of the local magma supply. Comparing the compiled data sets has revealed substantial differences in the opening mechanisms of the two arms of the Overlapping Spreading Center (OSC) that is made up by the northernmost tip of the Fonualei Rift and Spreading Center in the east and the southernmost segment of the Mangatolu Triple Junction in the west. The observed transition from a predominantly tectonic opening mechanism at the eastern OSC arm to a magmatic opening mechanism at the western OSC arm coincides with an equally sharp transition from and strongly subduction influenced crust to a crust with virtually no subduction input. The degree of subduction input alters the geochemical composition, as well as the lithospheric stress response.

Structural analysis and measured extension in fault complexes along the Lofoten‐Vesterålen margin, offshore Norway, in the context of crustal‐scale rifting towards breakup in NE Atlantic

AbstractLate Cretaceous–Palaeocene continental extension within the Lofoten‐Vesterålen margin is investigated by integrating 2D‐3D seismic and potential field datasets, together with updated crustal transects. Most of that deformation is recorded by two low‐angle detachment structures named West Røst High Fault Complex (WRHFC) and North Utrøst Ridge Fault Complex (NURFC) located at the southern and central‐northern portions of the studied area, respectively. Multiple extensional episodes of various intensities were mapped as different fault stages, including one Albian‐Cenomanian phase, four early Late Cretaceous phases, three Late Cretaceous to latest Cretaceous–Palaeocene phases, and one Palaeocene phase. The WRHFC is narrower in extent and has accommodated a relatively greater amount of localized extension (ca. 18–19 km), whereas the NURFC occupies a wider area with widespread extension intensity (ca. 6–8 km). In comparison, the total across‐margin average extension within the southern, central, and northern portions of the Lofoten‐Vesterålen and NE Greenland conjugate margins are ca. 192, 221, and 266 km, respectively. Such results indicate an apparent extension discrepancy between derived extension from measured fault‐block geometries within the fault complexes and the whole conjugate margin system, with only ca. 11% and 13% of the extension seen on the studied seismic profiles on the WRHFC and NURFC, respectively. The corrected maximum extension for purely sub‐seismic resolution faulting on both the WRHFC and NURFC fault complexes is ca. 164 and 46 km, respectively. Finally, both WRHFC and NURFC structures provide key evidence for a ductile mode of deformation towards breakup that is expressed through shear zones‐rift topography interactions with overlaying listric/detachment faults. These features reflect the resulting multiphase tectonic evolution across the asymmetric Lofoten‐Vesterålen and NE Greenland conjugate margins, and the obliquity in the breakup axis location along them. The study outcomes are pertinent and applicable to understand the breakup evolution of the northern NE Atlantic and its vicinity.

Quantification and Restoration of the Pre‐Drift Extension Across the NE Atlantic Conjugate Margins During the Mid‐Permian‐Early Cenozoic Multi‐Rifting Phases

AbstractThe formation of the NE Atlantic conjugate margins is the result of multiple rifting phases spanning from the Late Paleozoic and culminating in the early Eocene when breakup was accompanied with intense magmatic activity. The pre‐breakup configuration of the NE Atlantic continental margins is controlled by crustal extension, magmatism, and sub‐lithospheric processes, all of which need to be quantified for the pre‐breakup architecture to be restored. Key parameters that need to be extracted from the analysis of crustal structures and sediment record include stretching factors, timing of rifting phases, and nature of the deep crustal structures. The aim of this study is to quantify the pre‐drift extension of the NE Atlantic conjugate margins using interpreted crustal structure and forward basin modeling. We use a set of eight 2D conjugate crustal transects and corresponding stratigraphic models, constrained from an integrated analysis of 2D and 3D seismic and well data. The geometry and thickness of the present‐day crust is compared to a reference thickness which has experienced limited or no crustal extension since Permian time allowing the quantification of crustal stretching. Based on the eight conjugate crustal transects, the total pre‐drift extension is estimated to range between 181 and 390 km with an average of 270–295 km. These estimates are supported by the results of forward basin modeling, which predict total extension between 173 and 325 km, averaging 264 km. The cumulative pre‐drift extension estimates derived from basin modeling are in turn used to calculate the incremental crustal stretching factors at each of the three main rifting phases between the conjugate Greenland‐Norwegian margins. The mid‐Permian early Triassic rifting phase represents 32% of the total extension, while the equivalent values are 41% for the mid‐Jurassic to mid‐Cretaceous and 27% for the Late Cretaceous‐Paleocene rifting phases. These values are used to establish and present at first, a full‐fit palinspastic plate kinematic model for the NE Atlantic since the mid‐Permian and will be the base for future work on more elaborated models in order to build accurate paleogeographic and tectonic maps.

Brown Amphibole as Tracer of Tectono-Magmatic Evolution of the Atlantis Bank Oceanic Core Complex (IODP Hole U1473A)

Abstract Brown amphibole is a minor but common mineral component in lower oceanic crust. It is generally interpreted as products of migrating SiO2 and H2O-rich fluids or melts, which can be either residual melts from advanced magmatic differentiation of Mid-Ocean Ridge Basalt (MORB), or hydrothermal fluids including a seawater component. Within the lower oceanic crust exhumed at the Atlantis Bank Oceanic Core Complex, along the ultraslow Southwest Indian Ridge, brown amphibole is ubiquitous in all lithologies from olivine- to oxide-gabbros and diorites, including both undeformed and plastically deformed varieties. We here show the results of a systematic petrological study conceived to unravel the nature of the H2O-rich component recorded in brown amphiboles and document: (i) the evolution of migrating melts during the magmatic stage and (ii) different extents of melt-bearing deformation events recorded throughout the entire crustal transect. The low Cl contents and the light over heavy rare earth elements (LREE/HREE) ratios and high Ti contents in brown amphiboles indicate they crystallized from melts with a magmatic hydrous component. Consistently, their δ18O values are in equilibrium with Mid-Ocean Ridge Basalt (MORB) composition, except for diorite amphiboles that possibly record the local assimilation of altered minerals. In undeformed olivine gabbros, interstitial pargasite crystallized at hypersolidus conditions (~1000°C) from the melt residual after late stages of MORB differentiation. We speculate that before the olivine gabbro crystal mush reached fully solid state, some aliquots of residual melts were extracted and accumulated within discrete intervals. There, ferrobasaltic melts differentiated through the early crystallization of Fe-Ti oxides and clinopyroxene as liquidus phases, ultimately forming the oxide gabbros. This process promoted rapid Si enrichment and depletion in Fe, Ti, V in the residual melt, later extracted to form the crosscutting diorite veins. The mylonitic olivine gabbros record high-temperature plastic deformation (~900°C ± 50°C) under hypersolidus conditions, involving melts residual from previous crystallization of the gabbroic rock. Further solid-state plastic deformation led to substantial grain size reduction and, consequently, to an increase in porosity. This created pathways for subsequent melt focusing, which likely represent late-stage differentiated melts migrating throughout the lower crustal section. This study shows that brown amphibole in the Atlantis Bank lower oceanic crust is the crystallization product of melts residual from advanced magmatic differentiation, which are also locally involved in the plastic deformation events during crustal accretion.

Divergent behaviour of Th and U during anatexis: Implications for the thermal evolution of orogenic crust

AbstractMobilization and migration of the heat‐producing elements (HPE) during anatexis is a critical process in the development of orogenic systems, the evolution of continental crust and the stabilization of cratons. In many crustal rocks the accessory minerals are the dominant hosts of Th and U, and the behaviour of these minerals during partial melting controls the concentrations of these elements in draining melt and residue. We use phase equilibrium modelling to evaluate if loss of melt saturated in the essential structural constituents of the accessory minerals can explain the concentrations of Th and U in residual metasedimentary migmatites and granulites along two well‐characterized crustal transects in the Ivrea zone, Italy and at Mt Stafford, Australia. While an equilibrium model of accessory mineral breakdown and melt loss approximates the depletion of U in the residual crust along both transects, it does not explain the relative enrichment of Th. We propose that the high Th concentrations in residual crust may be explained by either inhibition of monazite dissolution by kinetic factors or near‐peak growth of new high Th grains and overgrowth rims on undissolved monazite due to migration of melt through the orogenic crust. Retention of the HPE in the middle and deep orogenic crust may allow metasedimentary granulites to overcome the enthalpy barrier of melting to achieve ultrahigh temperature conditions and may be partly responsible for the slow cooling of many granulite terranes. Lastly, although the mantle was warmer and crustal heat production was higher in the past, peak temperatures and apparent thermal gradients of high‐temperature (HT)–ultrahigh temperature (UHT) granulite terranes have not decreased significantly since the Neoarchean. However, the pressure of HP granulite facies metamorphism has increased gradually from the Archean to the Phanerozoic, which suggests that the lithosphere became stronger as secular cooling of the mantle enabled plate collisions to form thicker orogens. Thus, as the lithosphere became stronger, the proportion of HT–UHT metamorphism associated with thin lithosphere and mantle heat has decreased, whereas the proportion associated with the formation of thick crust and radiogenic heat has increased.

Circum-Arctic Lithosphere Evolution (CALE) Transect C: displacement of the Arctic Alaska–Chukotka microplate towards the Pacific during opening of the Amerasia Basin of the Arctic

Abstract This paper synthesizes the framework and geological evolution of the Arctic Alaska–Chukotka microplate (AACM), from its origin as part of the continental platform fringing Baltica and Laurentia to its southward motion during the formation of the Amerasia Basin (Arctic Ocean) and its progressive modification as part of the dynamic northern palaeo-Pacific margin. A synthesis of the available data refines the crustal identity, limits and history of the AACM and, together with regional geological constraints, provides a tectonic framework to aid in its pre-Cretaceous restoration. Recently published seismic reflection data and interpretations, integrated with regional geological constraints, provide the basis for a new crustal transect (the Circum-Arctic Lithosphere Evolution (‘CALE’) Transect C) linking the Amerasia Basin and the Pacific margin along two paths that span 5100 km from the Lomonosov Ridge (near the North Pole), across the Amerasia Basin, Chukchi Sea and Bering Sea, and ending at the subducting Pacific plate margin in the Aleutian Islands. We propose a new plate tectonic model in which the AACM originated as part of a re-entrant in the palaeo-Pacific margin and moved to its present position during slab-related magmatism and the southward retreat of palaeo-Pacific subduction, largely coeval with the rifting and formation of the Amerasia Basin in its wake.

Extension, hyperextension and mantle exhumation offshore Norway: a discussion based on 6 crustal transects

Extension, hyperextension and mantle exhumation offshore Norway: a discussion based on 6 crustal transects

Reactive flow as dominant evolution process in the lowermost oceanic crust: evidence from olivine of the Pineto ophiolite (Corsica)

The Jurassic Pineto ophiolite from Corsica exposes a ~1-km-thick troctolite–olivine-gabbro sequence, interpreted to represent a lowermost sector of the gabbroic oceanic crust from a (ultra-)slow spreading system. To constrain the petrogenesis of the olivine-gabbros, minor and trace element analyses of olivine (forsterite = 84–82 mol%) were carried out. Olivine from the olivine-gabbros is depleted in incompatible trace elements (Sc, V, Ti, Y, Zr and heavy rare earth elements) with respect to olivines from associated troctolites. Depleted incompatible element compositions are also shown by olivine (forsterite = 86 mol%) from a clinopyroxene-rich troctolite. The incompatible element compositions of olivine argue against a petrogenetic process entirely driven by fractional crystallization. We propose that melts migrating through an olivine–plagioclase crystal mush chemically evolved by reaction with the existing minerals, changing in composition as it flowed upward. The melt residual from these interactions led to partial dissolution of preexisting olivine and to crystallization of clinopyroxene, generating olivine-gabbro bodies within a troctolite matrix. Reactive flow was the major evolution process active in the ~1-km crustal transect exposed at the Pineto ophiolite, producing lithological variations classically attributed to fractional crystallization processes.

Displacement transfer from borders to interior of a plate: A crustal transect of Iberia

A N-S crustal transect of the whole Iberian Plate is presented. The displacement transfer during the Alpine deformation from the border ranges (Cantabrian and Betic chains) toward the interior ones was analyzed by means of balanced crustal sections. Inconsistencies in shortening values between upper and middle-lower crust in the Cantabrian Mountains (Basque–Cantabrian Zone) imply a mid-crustal detachment transferring orogenic shortening southward toward the Spanish Central System. Depending on the shortening value attributed to the Central System, this mid-crustal detachment could involve the entire Iberian Plate, connecting the Cantabrian Mountains and the Betic Chain. The very small crustal root in the Central System can be explained as a result of the tectonic load caused by the upper crustal shortening that was transferred from the border ranges. We conclude that a simple shear model, rather than a pure shear with lithospheric folding, better explains the whole Iberia deformation. Finally, the total Alpine shortening of the Iberian Plate is estimated between 267 and 292km.

Crustal architecture and tectono-magmatic history of the western offshore of India: Implications on deepwater sub-basalt hydrocarbon exploration

Abstract The western continental margin of India (WCMI) has evolved as a result of two-stage rifting and breakup events, the first one between India and Madagascar during the mid Cretaceous, and the second event between India and Seychelles during the late Cretaceous. Detailed seismic studies by earlier workers in the offshore margin confirm that it is a poly-rift margin with significant volcanic emplacements, and on the basis of these datasets; the western margin was classified as volcanic. In the present study, constrained potential field modeling of four profiles across the western offshore region revealed that the eastern basin comprising of Laxmi and Laccadive basins as well as the adjoining Laxmi and the Laccadive ridges are underlain by the extended continental crust and has been punctured by several magmatic intrusions. The models further suggest that the continent–ocean boundary (COB) lies to the west of Laccadive and Laxmi ridges, and also in the southern part of Laccadive basin. In this paper, we presented high-resolution seismic images at the margin that have revealed the presence of deeper sedimentary strata below the K/T flood-basalts and provided detailed interpretation of few sections belonging to a 3D seismic block in the deepwater Kerala–Konkan offshore. The sections reveal numerous features such as the sag basin, fractures, faults and folding below the basaltic top. Further, significant sedimentary wedges terminating against the volcanic highs are inferred to be the Mesozoic sub-basalt plays. The interpreted crustal transects, the inferred combination of tectono-magmatic processes and the probable occurrence of sub-basalt sediments at the margin are analyzed and encapsulated to provide a basis for assessing deepwater petroleum prospectivity of the entire western margin offshore.

Impact of lithospheric heterogeneities on continental rifting evolution: Constraints from analogue modelling on South Atlantic margins

Abstract Lithospheric-scale experiments integrated with restored crustal transects are used to study the evolution of the Central Segment (confined between the Rio Grande Fracture Zone to the south and the Chain Fracture Zone to the north) of the South Atlantic margin. The presence of crustal inhomogeneities, located within the Brazilian Santos and Campos basins, have been analysed and modelled in order to better understand their effects on the rift evolution and resulting structural architecture of the conjugate rifted margins. The results show that heterogeneities located within the lower crust can have a remarkable impact on the along-margin segmentation promoting articulated basins with horsts and grabens in response to a relative “strong” rheology, and focused and deeper basins related to a relatively “weak” rheology on the equivalent parts of the conjugate pairs. In particular, at the early-stage rift evolution the deformation is concentrated at the inner margin where, in the presence of a weak lower crust rheology, a main deep listric half-graben fault and associated thick and wedge-shaped syn-rift basin sequences are developed. A strong lower crust rheology, instead, gives rise to more planar, rotated, domino-type faulted basins with thinner sequences directly controlled by the individual fault-blocks. At the late-stage rift evolution, once the effects of the initial crustal rheology inhomogeneities are reduced due to the lithosperic thinning process, the outer margin records a late syn-rift sequence which shows comparable thicknesses for both cases of lower crust rheologies. This tectono-stratigraphic evolution of the rifting process gives rise to along-margin alterations in symmetry versus asymmetry of the width and structural architecture. The performed analogue modelling experiments also indicate that during the rifting evolution pieces of brittle mantle are preserved and could be elevated beneath the developed upper crustal structures, giving rise to complicated predictions for the along-margin heat-flow.

Sicily’s fold–thrust belt and slab roll-back: the SI.RI.PRO. seismic crustal transect

Sicily is a thick orogenic wedge formed by (1) the foreland (African) and its Sicilian orogen and (2) the thick-skinned, Calabrian–Peloritani wedge. The crust under central Sicily, from the Tyrrhenian margin to the coastline of the Sicily Channel, has been investigated by the multidisciplinary (SI.RI.PRO.) research project. The project dealt with the nature and thickness of the crust and depth and geometry of the Moho, which is essential in formulating subduction models and improving the knowledge of African and Tyrrhenian–European lithospheres. The results resolve features such as (1) the main orogenic wedge, (2) the very steep, NW–SE-trending regional monocline suggesting inflection of the foreland crust, (3) the deep Caltanissetta synform imaged, for the first time, to about 25 km, and (4) the top of the crystalline basement and the inferred crust–mantle boundary. The SI.RI.PRO. transect confirmed that the NNW-dipping, autochthonous Iblean platform of SE Sicily and its basement extends all the way into central Sicily. Further NW, towards the NNW end of the transect, a large uplift involves the Iblean platform and its underlying basement. The associated gravity anomaly is interpreted as the southern wedge edge of the Tyrrhenian mantle that splits the subducting Iblean–Pelagian (African) continental slab from an overlying synformal stack of allochthonous thrust sheets. Supplementary materials: Additional data are available at www.geolsoc.org.uk/SUP18594 .

Seismic structure and rheology of the crust under mainland China

The crust and upper mantle in mainland China were relatively densely probed with wide-angle seismic profiling since 1958, and the data have provided constraints on the amalgamation and lithosphere deformation of the continent. Based on the collection and digitization of crustal P-wave velocity models along related wide-angle seismic profiles, we construct several crustal transects across major tectonic units in mainland China. In our study, we analyzed the seismic activity, and seismic energy releases during 1970 and 2010 along them. We present seismogenic layer distribution and calculate the yield stress envelopes of the lithosphere along the transects, yielding a better understanding of the lithosphere rheology strength beneath mainland China. Our results demonstrate that the crustal thicknesses of different tectonic provinces are distinctively different in mainland China. The average crustal thickness is greater than 65km beneath the Tibetan Plateau, about 35km beneath South China, and about 36–38km beneath North China and Northeastern China. For the basins, the thickness is ~55km beneath Qaidam, ~50km beneath Tarim, ~40km beneath Sichuan and ~35km beneath Songliao. Our study also shows that the average seismic P-wave velocity is usually slower than the global average, equivalent with a more felsic composition of crust beneath the four tectonic blocks of mainland China resulting from the complex process of lithospheric evolution during Triassic and Cenozoic continent–continent and Mesozoic ocean–continent collisions. We identify characteristically different patterns of seismic activity distribution in different tectonic blocks, with bi-, or even tri-peak distribution of seismic concentration in South Tibet, which may suggest that crustal architecture and composition exert important control role in lithosphere deformation. The calculated yield stress envelopes of lithosphere in mainland China can be divided into three groups. The results indicate that the lithosphere rheology structure can be described by jelly sandwich model in eastern China, and crème brulee models with weak and strong lower crust corresponding to lithosphere beneath the western China and Kunlun orogenic belts, respectively. The spatial distribution of lithospheric rheology structure may provide important constraints on understanding of intra- or inter-plate deformation mechanism, and more studies are needed to further understand the tectonic process(es) accompanying different lithosphere rheology structures.

Construction of the granitoid crust of an island arc part I: geochronological and geochemical constraints from the plutonic Kohistan (NW Pakistan)

We present major and trace element analyses and U–Pb zircon intrusion ages from I-type granitoids sampled along a crustal transect in the vicinity of the Chilas gabbronorite of the Kohistan paleo-arc. The aim is to investigate the roles of fractional crystallization of mantle-derived melts and partial melting of lower crustal amphibolites to produce the magmatic upper crust of an island arc. The analyzed samples span a wide calc-alkaline compositional range (diorite–tonalite–granodiorite–granite) and have typical subduction-related trace element signatures. Their intrusion ages (75.1 ± 4.5–42.1 ± 4.4 Ma) are younger than the Chilas Complex (~85 Ma). The new results indicate, in conjunction with literature data, that granitoid formation in the Kohistan arc was a continuous rather than punctuated process. Field observations and the presence of inherited zircons indicate the importance of assimilation processes. Field relations, petrographic observations and major and trace element compositions of the granitoid indicate the importance of amphibole fractionation for their origin. It is concluded that granitoids in the Kohistan arc are derivative products of mantle derived melts that evolved through amphibole-dominated fractionation and intra crustal assimilation.

Crustal structure beneath the Iberian Peninsula and surrounding waters: A new compilation of deep seismic sounding results

We present here an updated compilation of crustal parameters beneath the Iberian Peninsula and surrounding waters, inferred from extensive deep seismic exploration performed in the last three decades. Firstly, the more relevant experiments developed in both terrestrial and marine domains are revisited, together with the corresponding seismic velocity–depth models published that reveal the characteristics of the different tectonic domains sampled, including oceanic zones, continental margins, orogenic and rift belts. The results are summarized in three crustal transects sketched along representative directions. Secondly, we compiled the Moho depths along all the available seismic profiles to produce a geo-referred database. This database has then been interpolated using kriging algorithms to obtain a continuous Moho depth model, in the form of a regular grid file. This integrative Moho map for a key area in the Western Mediterranean documenting the interaction between European and African plates can contribute to constrain further regional studies, either at crustal, lithospheric or mantle scales.

Crustal transect across the North Atlantic

Two dimensional crustal models derived from four different ocean bottom seismographic (OBS) surveys have been compiled into a 1,580 km long transect across the North Atlantic, from the Norwegian More coast, across the extinct Aegir Ridge, the continental Jan Mayen Ridge, the presently active Kolbeinsey Ridge north of Iceland, into Scoresby Sund in East Greenland. Backstripping of the transect suggests that the continental break-up at ca. 55 Ma occurred along a west-dipping detachment localized near the western end of a ca. 300 km wide basin thinned to less than 20 km crustal thickness. It is likely that an east-dipping detachment near the present day Liverpool Land Escarpment was active during the late stages of continental rifting. A lower crustal high-velocity layer (7.2–7.4 km/s) interpreted as mafic intrusions/underplating, was present beneath the entire basin. The observations are consistent with the plume hypothesis, involving the Early Tertiary arrival of a mantle plume beneath central Greenland and focused decompression melting beneath the thinnest portions of the lithosphere. The mid-Eocene to Oligocene continental extension in East Greenland is interpreted as fairly symmetric and strongly concentrated in the lower crustal layer. Continental break-up which rifted off the Jan Mayen Ridge, occurred at ca. 25 Ma, when the Aegir Ridge became extinct. The first ca. 2 m.y. of oceanic accretion along the Kolbeinsey Ridge was characterized by thin magmatic crust (ca. 5.5 km), whereas the oceanic crustal formation since ca. 23 Ma documents ca. 8 km thick crust and high magma budget.

Architecture of volcanic rifted margins: new insights from the Exmouth – Gascoyne margin, Western Australia

Australia's western continental margin formed during Gondwana breakup between Australia – Antarctica and India, leaving behind an evolving plate boundary with significant new magmatic crust accreted to the continental fragments. In this context, the margin segment from the outer Exmouth Plateau to the Gascoyne Abyssal Plain has been repeatedly studied as an example of volcanic margin formation. However, many recent analyses based on selective interpretation of geophysical datasets have come to conflicting conclusions regarding both the tectonic architecture and the age of breakup on this margin. These differences profoundly impact on models for both the global plate circuit and for the timing and extent of magmatism implicated in volcanic breakup processes. We present new interpretations of the distribution of magmatic and pre-rift rock packages in this margin, based on the integrated interpretation of two deep crustal transects with existing seismic-reflection, refraction, gravity, magnetic and geochemical data. Interpretations are constrained by data from sparse Ocean Drilling Program and petroleum-exploration drilling, and dredging. We find evidence for significant accumulation of magmatic rocks and their clastic derivatives infilling extensional fault-controlled basins developed in a broad volcanic-margin transition zone between the outer Exmouth Plateau and true oceanic crust. These rocks have distinctive seismic facies in the form of seaward-dipping reflector sequences, and are dense and magnetised. Most significantly, these packages give rise to potential-field anomalies that have previously been interpreted as due to seafloor spreading. Recognition of these packages in a volcanic-margin transition zone has implications for the recognition of the inboard edge of unequivocal oceanic crust, the oceanic – volcanic-margin boundary. The main locus of igneous activity in the volcanic-margin transition zone off the Exmouth Plateau is spatially offset from a previously recognised high-velocity zone, suggesting that these two phenomena may not be temporally related. Seismically imaged differences in total thinning and partitioning of thinning, between upper and lower crust, provide support for models of depth-dependent thinning previously proposed for this margin.

Trans-Alaska Crustal Transect and continental evolution involving subduction underplating and synchronous foreland thrusting

We investigate the crustal structure and tectonic evolution of the North American continent in Alaska, where the continent has grown through magmatism, accretion, and tectonic under-plating. In the 1980s and early 1990s, we conducted a geological and geophysical investigation, known as the Trans-Alaska Crustal Transect (TACT), along a 1350-km-long corridor from the Aleutian Trench to the Arctic coast. The most distinctive crustal structures and the deepest Moho along the transect are located near the Pacific and Arctic margins. Near the Pacific margin, we infer a stack of tectonically underplated oceanic layers interpreted as remnants of the extinct Kula (or Resurrection) plate. Continental Moho just north of this underplated stack is more than 55 km deep. Near the Arctic margin, the Brooks Range is underlain by large-scale duplex structures that overlie a tectonic wedge of North Slope crust and mantle. There, the Moho has been depressed to nearly 50 km depth. In contrast, the Moho of central Alaska is on average 32 km deep. In the Paleogene, tectonic underplating of Kula (or Resurrection) plate fragments overlapped in time with duplexing in the Brooks Range. Possible tectonic models linking these two regions include flat-slab subduction and an orogenic-float model. In the Neogene, the tectonics of the accreting Yakutat terrane have differed across a newly interpreted tear in the subducting Pacific oceanic lithosphere. East of the tear, Pacific oceanic lithosphere subducts steeply and alone beneath the Wrangell volcanoes, because the overlying Yakutat terrane has been left behind as underplated rocks beneath the rising St. Elias Range, in the coastal region. West of the tear, the Yakutat terrane and Pacific oceanic lithosphere subduct together at a gentle angle, and this thickened package inhibits volcanism.

Rates of continental breakup magmatism and seafloor spreading in the Norway Basin–Iceland plume interaction

In year 2000, an ocean bottom seismometer (OBS) profile was acquired across the Møre margin to the Aegir Ridge, an extinct seafloor spreading axis. The margin is an early Eocene volcanic passive margin, located between the Faeroe‐Iceland Ridge (FIR) and the East Jan Mayen Fracture Zone (EJMFZ). The P wave data were modeled by ray tracing to give a crustal transect showing a 10–11 km thick igneous crust created by breakup magmatism, tapering off to magma‐starved seafloor spreading by C23 time (51.4 Ma). The location of the EJMFZ was reinterpreted from a satellite derived gravity map, and spreading direction in the Norway Basin reevaluated. No other fracture zones were confirmed, and both thin oceanic crust (4–5 km) and lack of fracture zones resemble ultraslow spreading on the Arctic Gakkel Ridge. Magnetic seafloor spreading anomalies were identified from the magnetic track recorded with the OBS profile, and half spreading rates were derived. Early seafloor spreading was slow (15–32 mm yr−1), approaching ultraslow (6–8 mm yr−1) by C20 time (42.7 Ma). A V‐shaped pattern seen in the gravity field located only around the northern part of the Aegir Ridge corresponds to increased crustal thickness in the seismic model, recording northeast transport (3–6 mm yr−1) of more melt‐fertile asthenosphere zones. The magma‐starved character of the Norwegian Basin seen also during slow seafloor spreading may be the result of depletion of the asthenosphere when the Iceland plume constructed the FIR to the south, as the asthenosphere is subsequently transported into the Norway Basin.

Lithosphere structure and tectonic evolution of the Alpine arc: new evidence from high-resolution teleseismic tomography

Abstract Several continental and oceanic plates and/or terranes amalgamated during the formation of the tectonically complex Alpine arc. Reliable knowledge of the present structure of the lithosphere-asthenosphere system throughout the Alpine arc from the Western through the Central to the Eastern Alps is crucial for understanding the evolution of this orogen and the current interaction of lithospheric blocks, and additionally, for assessing the amount and orientation of lithosphere subducted in the geological past. We have compiled results from earlier geophysical studies and reinterpretations of existing seismic and geological data for the Alpine crust and Moho. High-resolution teleseismic tomography was used to produce a detailed 3D seismic model of the lower lithosphere and asthenosphere. The combination of these techniques provides new images for the entire lithosphere-asthenosphere system, showing significant lateral variations to depths of 400 km. Over the years the crustal structure has been determined extensively by active seismic techniques (deep seismic sounding) with laterally variable coverage and resolution. For a closer view three international seismic campaigns, using mainly near-vertical reflection techniques in the Western, Central and Eastern Alps, were carried out to assess the crustal structure with the highest possible resolution. The synoptic reinterpretation of these data and an evaluation of existing interpretations have allowed us to construct four detailed deep crustal transects across the Alps along the ECORS-CROP, NFP-20/EGT and TRANSALP traverses. In addition, contour maps of the Moho for the wider Alpine region and of the top of the lower crust were compiled from existing seismic refraction, near-vertical and wide-angle reflection data. Substantial structural differences in the structure of the deep crust appear between the Western, Central and Eastern Alps: doubling of European lower crust in the west resulted from collision with the Ivrea body; indentation of lower Adriatic crust between European lower crust and Moho occurred in the Central Alps; and a narrow collision structure exists under the transitional area between the western and eastern subduction regime under the Tauern Window of the Eastern Alps, where the crustal structure resembles a large-scale flower structure. Most recently, high-resolution teleseismic tomography based on the a priori known 3D crustal structure and compilation of a high-quality teleseismic dataset was successfully developed and applied to derive reliable detailed images of the lower lithosphere. Along strike of the Alps a fast slab-like body is revealed which in the western part is subducted beneath the Adriatic microplate. In the Western Alps detachment of parts of the lower continental slab occurred, possibly induced by the Ivrea body, which acted as a buttress in the collision process of the European and Adriatic plates. The generally SE-directed subduction of the European continental lithosphere changes gradually from west to east to almost vertical under the westernmost part of the Eastern Alps (western Tauern Window and Giudicarie lineament). Unexpectedly, some 50 km further east the subducted continental lower lithosphere is now part of the Adriatic lithosphere and dips NE beneath the European plate. Our tomographic image documents clear bipolar slab geometries beneath the Alpine orogen. The depth extent of the subducted continental lithospheric slab agrees rather well with estimates of post-collisional crustal shortening for the Western and Central Alps. This kinematic control on amounts of lateral motion of the collision zone in the west also allows estimates of the subduction and collision process in the Eastern Alps. The new 3D lithospheric picture for the wider Alpine region to 400 km depth demonstrates the clear connection and interaction between the deep structure of the lithosphere-asthenosphere system and near-surface tectonic features as seen today. It provides new and unexpected evidence for the entire Alpine tectonic evolution, a process which obviously changes significantly from west to east.