Pre-existing Oceanic Crust Research Articles

The ephemeral history of Earth’s youngest supra-subduction zone type ophiolite from Timor

Ophiolites occur widely in orogenic belts, yet their origins remain controversial. Here we present a modern example with a geodynamic model from Timor, eastern Indonesia, where Earth’s youngest supra-subduction zone (SSZ)-type ophiolitic fragments are exposed. Zircon U-Pb ages and geochemical data indicate a short timespan (~10 to 8 Ma) for the magmatic sequence with boninitic and tholeiitic arc compositions. We interpret the Timor ophiolite as part of the infant Banda arc-forearc complex, which formed with the opening of the North Banda Sea and subsequent arc-continent collision along the irregular Australian continental margin. Our study connects the occurrence of small, short-lived ocean basins in the western Pacific with orogens around the globe where ephemeral SSZ-type ophiolites occur. These orogenic ophiolites do not represent preexisting oceanic crust, but result from upper-plate processes in early orogenesis and thus mark the onset of collision zone magmatism.

The role of immiscible sulfides for sulfur isotope fractionation in arc magmas: Insights from the Talkeetna island arc crustal section, south-central Alaska

While sulfur isotopes have proven powerful tracers of sulfur (re)cycling within subduction zones, the origin of the 34S-enrichment seen in arc magmas remains a subject of debate, with competing hypotheses implicating both mantle and crustal processes. Herein, we investigate these competing models through the study of the Early–Middle Jurassic Talkeetna Arc section exposed in the Chugach Mountains, south-central Alaska, reporting the sulfur isotope systematics of rocks spanning a depth transect that extends from the upper mantle (∼0.9–1.2 GPa, ∼30–35 km) up into the mid-crust (∼0.2–0.5 GPa, ∼5–15 km). Marked by an increase in δ34S values from −1.28 to +5.61‰, these data reveal a significant isotope effect associated with melt differentiation, with 32S being preferentially sequestered from primitive basaltic melts into ultramafic and mafic cumulates, yielding progressively more evolved melts enriched in 34S. Leveraging a quantitative petrological model that constrains the liquid line of descent and sulfide saturation as a function of sulfur content, oxygen fugacity, pressure and temperature, we then demonstrate that, in the presence of dissolved oxidized sulfur species, the saturation of immiscible magmatic sulfides is capable of generating the observed sulfur isotope fractionation. While fluid saturation is unlikely to occur at lower crustal depths, ascending melts will almost certainly experience fluid saturation and degassing. As a result, sulfide immiscibility and magma degassing are not mutually exclusive and may, instead, represent complementary processes that combine to explain the observed range of positive δ34S compositions. In Talkeetna, crustal assimilation is unlikely to have played a role to increase δ34S values as there was little to no assimilation of pre-existing oceanic crust containing seawater sulfate during the formation of the Talkeetna Arc section. Furthermore, our model suggests that primitive melt in isotopic equilibrium with the most primitive ultramafic cumulates have mantle-like δ34S values between ∼0 and +1.1‰, which suggest limited input of 34S-enriched slab-derived sulfur into the sub-arc mantle. Collectively, these findings emphasize that mantle-like δ34S values for primitive arc melt may be common, while revealing the underappreciated importance of deep crustal crystallization of immiscible magmatic sulfides to generate positive δ34S compositions in erupted arc magmas.

Seismic Structure, Gravity Anomalies and Flexure Along the Emperor Seamount Chain

AbstractThe Hawaiian‐Emperor seamount chain in the Pacific Ocean has provided fundamental insights into hotspot generated intraplate volcanism and the long‐term strength of oceanic lithosphere. However, only a few seismic experiments to determine crustal and upper mantle structure have been carried out on the Hawaiian Ridge, and no deep imaging has ever been carried out along the Emperor seamounts. Here, we present the results of an active source seismic experiment using 29 Ocean‐Bottom Seismometers (OBS) carried out along a strike profile of the seamounts in the region of Jimmu and Suiko guyots. Joint reflection and refraction tomographic inversion of the OBS data show the upper crust is highly heterogeneous withPwave velocities <4–5 km s−1, which are attributed to extrusive lavas and clastics. In contrast, the lower crust is remarkably homogeneous with velocities of 6.5–7.2 km s−1, which we attribute to oceanic crust and mafic intrusions. Moho is identified by a strongPmParrival at offsets of 20–80 km, yielding depths of 13–16 km. The underlying mantle is generally homogeneous with velocities in the range 7.9–8.0 km s−1. The crust and mantle velocity structure has been verified by gravity modeling. While top of oceanic crust prior to volcano loading is not recognized as a seismic or gravity discontinuity, flexural modeling reveals a ∼5.0–5.5 km thick preexisting oceanic crust that is overlain by a ∼8 km thick volcanic edifice. Unlike at the Hawaiian Ridge, we find no evidence of magmatic underplating.

Crustal Formation on a Spreading Ridge Above a Mantle Plume: Receiver Function Imaging of the Icelandic Crust

AbstractIceland sits astride a mid‐ocean ridge underlain by a mantle hot spot. The interplay of these two geological processes has the potential to generate a complex and laterally variable crustal structure. The thickness of the Icelandic crust is a long running and controversial debate, with estimates ranging from a thin 20‐km crust to a thick 40‐km crust. We present new images of the first‐order seismic discontinuity structure of the Icelandic crust based on a joint inversion of receiver function and ambient noise‐derived surface wave dispersion data. Inversion results are validated through comparison to receiver functions multiphase common conversion point stacks across the densely instrumented Northern Volcanic Zone. We find a multilayered crustal structure consisting of a 6‐ to 10‐km‐thick upper crust underlain by either one or two discontinuities. The shallower discontinuity is found at depths of ≈20 km throughout Iceland. The deeper discontinuity is only present in some regions, defining the base of a lens‐like lower layer with maximum depths of 44 km above the center of the mantle plume. Either of these two discontinuities could be interpreted as the seismic Moho, providing an explanation why previous estimates of crustal thickness have diverged. Such structure may form via underplating of a preexisting oceanic crust as has been hypothesized in other ocean island plume settings. However, we demonstrate with a simple petrological model that variability in seismic discontinuity structure can also be understood as a consequence of compositional variation in melts generated with distance from the plume center.

Review of Late Jurassic-early Miocene sedimentation and plate-tectonic evolution of northern California: illuminating example of an accretionary margin

Production of voluminous igneous arc rocks, high-pressure/low-temperature (HP/LT) metamafic rocks, westward relative migration of the Klamath Mountains province, and U–Pb ages of deposition, sediment sources, and spatial locations of Jurassic and younger, detrital zircon-bearing clastic rocks constrain geologic development of the northern California continental edge as follows: (1) At ~175 Ma, transpressive plate underflow began to generate an Andean-type Klamath-Sierran arc along the margin. (2) Oceanic crustal rocks were metamorphosed under HP/LT conditions in an inboard, east-inclined subduction zone from ~170-155 Ma. Except for the Red Ant blueschists, such lithologies remained stored at depth; most HP/LT mafic tectonic blocks returned surfaceward only during mid- and Late Cretaceous time, chiefly entrained in circulating, buoyant Franciscan mud-matrix melange. (3) By ~165 Ma and continuing to ~150-140 Ma, erosion supplied volcanogenic debris to proximal Mariposa-Galice ± Myrtle overlap strata. (4) At ~140, immediately prior to the onset of paired Franciscan and Great Valley Group (GVG) + Hornbrook deposition, the Klamath salient was deformed and displaced ~100–150 km westward relative to the Sierran arc, stranding pre-existing oceanic crust on the south as the Coast Range Ophiolite (CRO). (5) After the end-of-Jurassic seaward step-out of the Farallon-North American convergent plate junction, terrigineous debris began to be deposited in the outboard Franciscan trench and intervening Great Valley forearc. (6) Voluminous sedimentation and accretion of Franciscan Eastern + Central belts and GVG detritus took place during paroxysmal igneous activity and rapid, nearly orthogonal plate convergence at ~125-80 Ma. (7) Sierran arc volcanism-plutonism ceased by ~80 Ma in northern California, signaling a transition to shallow, nearly subhorizontal eastward plate underflow attending Laramide orogeny far to the east. (8) Presently exposed Paleogene-lower Miocene Franciscan Coastal Belt sedimentary strata were deposited in a tectonic realm unaffected by HP/LT subduction. (9) Grenville-age detrital zircons are absent from the post-120 Ma Franciscan section. (10) Judging from petrofacies and zircon U–Pb data, the Franciscan Eastern Belt contains debris derived principally from the Sierra Nevada and Klamath ranges; detritus from the Idaho Batholith as well as Sierra Nevada Batholith may be present in some Central Belt sandstones, whereas clasts from the Idaho Batholith, Challis volcanics, and Cascade Range appear in progressively younger Paleogene-lower Miocene Coastal Belt sediments. (11) Gradual NW dextral offset of the Franciscan trench deposits of as much as ~1,600 km may have occurred relative to the native GVG forearc and basement terranes of the American Southwest.

Paleo-elevation and subsidence of [formula omitted] Shatsky Rise inferred from CO2 and H2O in fresh volcanic glass

Shatsky Rise, a large Mesozoic oceanic plateau in the northwest Pacific, consists of three massifs (Tamu, Ori, and Shirshov) that formed near a mid-ocean-ridge triple junction. Published depth estimates imply that Shatsky Rise has not subsided normally, like typical oceanic lithosphere. We estimated paleo-eruption depths of Shatsky Rise massifs on the basis of dissolved CO2 and H2O in volcanic glass and descriptions of cores recovered from five sites of Integrated Ocean Drilling Program Expedition 324. Initial maximum elevations of Shatsky Rise are estimated to be 2500–3500 m above the surrounding seafloor and the ensuing subsidence of Shatsky Rise is estimated to be 2600–3400 m. We did not observe the anomalously low subsidence that has been reported for both Shatsky Rise and the Ontong Java Plateau. Although we could not resolve whether Shatsky Rise originated from a hot mantle plume or non-plume fusible mantle, uplift and subsidence histories of Shatsky Rise for the both cases are constrained based on the subsidence trend from the center of Tamu Massif (∼2600 m) toward the flank of Ori Massif (∼3400 m). In the case of a hot mantle plume origin, Shatsky Rise may have formed on young (∼5 Ma) pre-existing oceanic crust with a total crustal thickness of ∼20 km. For this scenario, the center of Shatsky Rise is subsequently uplifted by later (prolonged) crustal growth, forming the observed ∼30 km thickness crust. For a non-plume origin, Shatsky Rise may have formed at the spreading ridge center as initially thick crust (∼30 km thickness), with later reduced subsidence caused by the emplacement of a buoyant mass—perhaps a refractory mantle residuum—beneath the center of Shatsky Rise.

Crustal structure beneath the Galápagos Archipelago from ambient noise tomography and its implications for plume-lithosphere interactions

[1] To constrain the seismic velocity structure of the crust beneath the Galapagos Archipelago, we conducted a tomographic study using high-frequency Rayleigh waves obtained from cross correlations of ambient noise. We analyzed waves with periods between 5 and 8.5 s, sensitive to shear wave velocity (VS) structure between about 3 and 10 km depth, after accounting for the effect of water depth. Crustal velocities are up to 25% lower than those of very young crust at the East Pacific Rise and are comparable to those of Hawaii. We attribute the lower than normal velocities to the combined effect of heating and the presence of melt in the crust above the Galapagos plume as well as the construction of a highly porous volcanic platform emplaced atop preexisting oceanic crust. On average, VS between 3 and 10 km depth beneath the western archipelago is up to 15% lower than beneath the eastern archipelago. We attribute the west-to-east velocity increase to a decrease in porosity of the volcanic platform and to cooling of the crust after its passage above the Galapagos plume. The results of this study, in combination with previous work, indicate that many of the unusual aspects of the Galapagos Archipelago are the result of variations in the thickness and internal structure of the chemical and thermal lithospheres. Our findings indicate that observed variations in the flexural response to loading observed in the Galapagos cannot be explained by the current thermal state of the lithosphere. Instead, the flexural response likely represents varying elastic strength at the time of loading. We also propose that the northwest and northeast trending alignments of volcanic centers found throughout the archipelago (the Darwinian lineations) may be associated with preexisting zones of weakness in the lithosphere formed during earlier episodes of ridge jumping and ridge propagation that were later reactivated by stresses generated by plume-lithosphere interactions.

Structural evolution of preexisting oceanic crust through intraplate igneous activities in the Marcus‐Wake seamount chain

Multichannel seismic reflection studies and seismic refraction surveys with ocean bottom seismographs in the Marcus‐Wake seamount chain in the northwestern Pacific Ocean reveal P wave velocity structures of hot spot‐origin seamounts and adjacent oceanic crust. Inside the seamounts are central high‐velocity (>6.5 km/s) structures extending nearly to the top that may indicate intrusive cores. Thick sediment layers (up to 4 km) with P wave velocities of 4–5 km/s have accumulated on seafloor that predates seamount formation. Downward crustal thickening of up to 2 km was documented beneath a large seamount cluster, but thickening was not confirmed below a small seamount cluster. Volume ratios of an intrusive core to a seamount body are 15–20%, indicating that most of the supplied magma was consumed in forming the thick sedimentary and volcaniclastic layer constituting the seamount flanks. Underplating and downward crustal thickening may tend to occur when second or later intrusive cores are formed in a seamount. P wave velocities in the lowest crust and in the uppermost mantle below the seamount chain are 0.1–0.2 km/s higher and 0.3–0.5 km/s lower, respectively, than velocities below oceanic crust. We explain this difference as a result of sill‐like intrusion of magma into the lower crust and uppermost mantle. Reflected waves observed at offsets >200 km are from mantle reflectors at depths of 30–45 km and 55–70 km. The shallower reflectors may indicate structures formed by intraplate igneous activities, and the deeper reflectors may correspond to the lithosphere‐asthenosphere boundary.

Microearthquake streaks and seismicity triggered by slow earthquakes on the mobile south flank of Kilauea Volcano, Hawai'i

We perform waveform cross correlation and high precision relocation of both background seismicity and seismicity triggered by periodic slow earthquakes at Kilauea Volcano's mobile south flank. We demonstrate that the triggered seismicity dominantly occurs on several preexisting fault zones at the Hilina region. Regardless of the velocity model employed, the relocated earthquake epicenters and triggered seismicity localize onto distinct fault zones that form streaks aligned with the slow earthquake surface displacements determined from GPS. Due to the unknown effects of velocity heterogeneity and nonideal station coverage, our relocation analyses cannot distinguish whether some of these fault zones occur within the volcanic crust at shallow depths or whether all occur on the decollement between the volcano and preexisting oceanic crust at depths of ∼8 km. Nonetheless, these Hilina fault zones consistently respond to stress perturbations from nearby slow earthquakes.

Oxygen isotope composition of xenoliths from the oceanic crust and volcanic edifice beneath Gran Canaria (Canary Islands): consequences for crustal contamination of ascending magmas

Xenolith samples of marine terrigenous sediments and altered Jurassic MORB from Gran Canaria (Canary Islands) represent samples of sub-island oceanic crust. These samples are postulated to define end-members for crustal contamination of basaltic and felsic ocean island magmas. The meta-igneous rocks show great heterogeneity in oxygen isotope compositions ( δ 18O 3.3–8.6‰), broadly correlating with their stratigraphic position in the oceanic crust. Gabbros interpreted as fragments of oceanic crust layer 3 have δ 18O values of 3.3–5.1‰, which is lower than MORB (5.7–6.0‰). Layer 2 lavas and dykes show a broader range of δ 18O of 4.1–8.6‰. Therefore, high-temperature metamorphism seems to have been the dominant process in layer 3, while both high- and low-temperature alteration have variably affected layer 2 rocks. Siliciclastic sediments have high δ 18O values (14.1–16.4‰), indicating diagenesis and low-temperature interaction with seawater. The oxygen isotope stratigraphy of the crust beneath Gran Canaria is typical for old oceanic crust and resembles that in ophiolites. The lithologic boundary between older oceanic crust and the igneous core complex at 8–10 km depth—as postulated from geophysical data—probably coincides with a main magma stagnation level. There, the Miocene shield phase magmas interacted with preexisting oceanic crust. We suggest that the range in δ 18O values (5.2–6.8‰) [Chem. Geol. 135 (1997) 233] found for shield basalts on Gran Canaria, and those in some Miocene felsic units (6.0–8.5‰), are best explained by assimilation of various amounts and combinations of oceanic and island crustal rocks and do not necessarily reflect mantle source characteristics.

Chlorine–potassium variations in melt inclusions from Raivavae and Rapa, Austral Islands: constraints on chlorine recycling in the mantle and evidence for brine-induced melting of oceanic crust

Chlorine abundance variations in oceanic basalts can provide insights into the degassing and volatile recycling history of the mantle as well as shallow melt/hydrosphere interaction. We have examined major, trace and volatile element abundances in olivine-hosted melt inclusions from the islands of Raivavae and Rapa in the Austral Island chain. The island of Raivavae sits atop a pre-existing fracture zone and thus provides the opportunity to examine the relationship between melt/hydrosphere interaction and local lithospheric structure. The majority of inclusions from both Raivavae and Rapa have well correlated chlorine and potassium concentrations consistent with a source Cl/K2O ratio of ∼0.04, similar to that of uncontaminated mid-ocean ridge and ocean island basalts. The similarity of chlorine/potassium ratios in mid-ocean ridge basalts, Austral Islands basalts, and basalts from many other ocean islands suggests that chlorine/potassium does not significantly vary in the mantle. Because the plume sources of many ocean island chains contain varying types and quantities of recycled oceanic crust and sediments, this indicates that most of the chlorine added to oceanic crust during seafloor alteration is removed during subduction and is not recycled into the deep mantle. High chlorine contents (up to 0.14 wt%) and chlorine/potassium ratios in melt inclusions from an early-erupted Raivavae lava derive from assimilation of Cl-rich brines or brine-impregnated oceanic crust. A small subset of inclusions from the same lava show more extreme chlorine enrichment (up to 2.5 wt%), are depleted in incompatible trace elements relative to normal inclusions, and show extreme fractionation of high-field-strength elements (HFSEs) relative to large-ion-lithophile or rare-earth elements. These latter inclusions derive from partial melting of the pre-existing oceanic crust under brine-saturated conditions. HFSE depletions in these inclusions reflect the stabilization of a HFSE-bearing phase in the lower crust, probably due to high chlorine fugacity. HFSE anomalies are also associated with high chlorine content in mid-ocean ridge basalts. We suggest that these anomalies are also generated by the stabilization of HFSE-bearing phases in high-chlorine-activity melts or fluids. This process may also provide a means of stabilizing rutile in the sub-arc mantle wedge.

Crustal construction of a volcanic arc, wide‐angle seismic results from the western Alaska Peninsula

Results from the 1994 Aleutian Seismic experiment delineate basic oceanic arc crustal architecture, constrain magmatic flux rates and bulk arc composition, and address questions of continental crustal genesis. Here we present results from a transect across protocontinental crust of the westernmost Alaska Peninsula (line A3) and compare this structure to a purely oceanic arc transect farther west. Arc crustal structure is similar along these two transects. Magmatic accretion occurs at the top and bottom of preexisting oceanic crust as a 5‐ to 10‐km‐thick upper crustal carapace of low velocity (2–5.8 km s−1) volcaniclastics, flows and small plutons, and a mafic lower crustal underplate (∼7.0 km s−1) of variable thickness, for a maximum arc crust thickness of ∼25–30 km. Lateral lower crustal velocity gradients and high velocities (>7.5 km s−1) beneath the forearc suggest dominantly vertical lower crustal accretion above a focused melt source and a forearc underlain by little magmatic crust but rather partially intruded and/or serpentinized mantle. The ratio of upper to lower crustal volume is ∼1, and the total arc crust volume implies a magmatic flux of ∼67 km3 km−1 m.y.−1, more than twice previous estimates for this arc and global productivity. The crust is thinner and more mafic than continental crust, and it lacks a massive tonalitic upper crust characteristic of the continents. An interpreted accumulation of upper crustal carapace material at midcrustal depths on line A3 has a velocity of ∼6.4 km s−1, suggesting an intermediate composition. Accretionary complex terranes consisting of accumulations of this type material would thus have bulk compositions similar to continental crust.

Lithospheric structure of the Costa Rican Isthmus: Effects of subduction zone magmatism on an oceanic plateau

We present the results of a multidisciplinary geophysical study, conducted to investigate the lithospheric structure of the Costa Rican Isthmus. The physical properties of the lithosphere are resolved by three‐dimensional (3‐D) simultaneous inversion of velocity anomalies and hypocenter parameters using local earthquakes and 2‐D forward modeling of onshore and offshore seismic refraction and gravity data. According to our results, the northern half of the Costa Rican Isthmus is constituted by a ∼40‐km‐thick crust, with a 6‐ to 7‐km‐thick oceanic crust subducting under it. The uppermost level of the basement and most of the marginal wedge show intermediate velocities and high densities, in good agreement with those described for flood basalts. The midlevel shows velocities and densities representative of oceanic crust. The bottommost level (20–40 km) shows high velocities and densities, typical of mafic rocks, and the upper mantle displays anomalously low densities and velocities. Intracrustal heterogeneities at intermediate wavelengths are indicated by prominent velocity anomalies. These results are consistent with a basement beneath the Costa Rican Isthmus being part of the Caribbean plateau, originated at 85–90 Ma with the onset of the Galápagos hotspot. The upper level corresponds to the flood basalts extruded during this phase, and it includes most of the marginal wedge. The second level represents the preexisting oceanic crust. The mafic lower crust, intracrustal heterogeneities, and anomalous upper mantle are interpreted to be built up by underplating, intrusion, and crystallization of basaltic melts, formed under the influence of subducting lithosphere dehydration.

Gabbroic xenoliths from La Palma, Tenerife and Lanzarote, Canary Islands: evidence for reactions between mafic alkaline Canary Islands melts and old oceanic crust

Gabbroic and hornblendite xenoliths from La Palma, Tenerife and Lanzarote fall into three main groups based on petrography and chemistry. One group (comprising all xenoliths from Lanzarote and some from La Palma) consists of highly deformed orthopyroxene-bearing gabbroic rocks that show a strong affinity to N-MORB and oceanic gabbro cumulates in terms of mineral chemistry and REE relations. However, they show mild enrichment in the most incompatible elements (particularly Rb+Ba±K) relative to intermediate and heavy REE, and their Sr–Nd isotope ratios fall within or close to the N-MORB field. The second group (60% of the xenoliths from La Palma) are gabbroic cumulates with zoned clinopyroxenes (Ti–Al-poor cores, Ti–Al-rich rims) and reaction rims of hornblende, biotite and clinopyroxene on other phases. Their trace-element and Sr–Nd isotope relations are in general transitional between N-MORB cumulates and Canary Islands alkali basalts, but they show strong enrichment in Rb, Ba and K relative to other strongly incompatible elements. The third group (comprising some xenoliths from La Palma and all those from Tenerife) are undeformed gabbroic and hornblendite rocks in which hornblende and biotite appear to belong to the primary assemblage. These rocks show strong affinities to Canary Islands alkali basaltic magmas with respect to mineral, trace-element, and Sr–Nd isotope chemistry. The first two groups are interpreted as fragments of old oceanic crust which have been mildly to strongly metasomatized through reactions with Canary Islands alkaline magmas. The reaction process is a combination of enrichment in elements compatible with biotite (and hornblende), and simple mixing between N-MORB cumulates and trapped alkaline magmas. The third group represents intrusions/cumulates formed from mafic alkaline Canary Islands magmas. Modeling indicates that locally up to 50% new material has been added to the old oceanic crust through reactions with ocean island basalts. Reactions and formation of cumulates do not represent simple underplating at the mantle/crust boundary, but have taken place within the pre-existing oceanic crust, and are likely to have significantly thickened the old oceanic crust.

Crustal structure transition from oceanic arc to continental arc, eastern Aleutian Islands and Alaska Peninsula

The Aleutian island arc crosses from the Pacific Ocean to the North-American continent at the island of Unimak. 3-D finite-difference traveltime inversion of our onshore–offshore seismic reflection/refraction data gives a velocity model of the crust and uppermost mantle. The arc crust is on average 30 km thick, but thickens to almost 40 km under the western Alaska Peninsula. The transition from oceanic arc to continental arc is characterised by a decrease in average velocity in the upper crust from about 6.5 km/s to less than 6.0 km/s, with no systematic change in the velocity of the lower crust. Throughout our study area, in the upper 15 km of the crust the highest velocities are observed in the fore-arc just south of the volcanic line. In the lower crust, the lowest velocities of just 6.2 km/s are found close to the volcanic line. The uppermost mantle is quite heterogeneous with velocities ranging from 7.6 to 8.2 km/s, in part due to the thermal gradient from cold fore-arc to hot back-arc. Whereas the Aleutian oceanic (fore-)arc has higher seismic velocities than average continental crust throughout the crust, the Peninsula section is close to the continental average in the upper c. 20 km of the crust. We infer that repeated episodes of arc magmatism can produce a felsic-to-intermediate upper crust as is observed in the continents, but arc magmatism produces a thicker mafic lower crust than the average continent retains. Some of the excess mafic material in the island–arc crust can be attributed to pre-existing oceanic crust, which is less evident or absent in a continental arc.

A trapped Philippine Sea plate origin for MORB from the inner slope of the Izu–Bonin trench

Basement outcrops sampled by submersible and dredge from the inner slope of the Izu–Bonin trench at 32°N and 6200–6700 m water depth have a distinct mid-ocean ridge basalt (MORB) chemistry unlike any other rocks previously sampled in the Izu–Bonin arc. They are low K tholeiites with moderate TiO 2 (0.7–1.8 wt%), extremely low Ba (1.5–7 parts per million), low Ba/La (1.2–3) and are depleted in light rare-earth elements. These samples could represent either an accreted piece of subducting Pacific plate or a trapped remnant of Philippine Sea plate on which the Izu–Bonin arc was built. Although their major and trace element chemistry do not help to distinguish their source, the Sr, Nd and Pb isotopes clearly support a Philippine Sea plate origin. The isotopic signature of the inner trench slope samples matches that of Philippine Sea plate lavas, with 87Sr/ 86Sr=0.70321–0.70373, 143Nd/ 144Nd=0.513057–0.513077 and 206Pb/ 204Pb=18.2–18.5. The samples have elevated 207Pb/ 204Pb (15.3–15.5) and 208Pb/ 204Pb (38.0–38.2) values compared to the Northern Hemisphere reference line (NHRL) and their isotopic signature is distinct from the Mesozoic Pacific MORB being subducted. These are the first samples of trapped Philippine Sea oceanic crust discovered in the Izu–Bonin–Mariana arc. They require that models for the formation of intra-oceanic arc crust account for pre-existing oceanic crust and that estimates of arc magma production rates are lowered accordingly.

The Dir-Utror metavolcanic sequence, Kohistan arc terrane, northern Pakistan

The Dir-Utror volcanic series forms a NE–SW trending belt within the northwestern portion of the Kohistan island arc terrane in the western Himalayas of northern Pakistan. The Kohistan arc terrane comprises a diverse suite of volcanic, plutonic, and subordinate sedimentary rocks of late Mesozoic to Tertiary age, developed prior to and after suturing of the Indo-Pakistan and Asiatic continental blocks. The Dir-Utror volcanic series near Dir is dominated by basaltic-andesite and andesite, with subordinate basalt, high-MgO basalt, dacite, and rhyolite. Porphyritic textures are dominant, with less common aphyric and seriate textures. Plagioclase is the dominant phenocryst in mafic to intermediate rocks, K-feldspar and quartz phenocrysts predominate in the dacites and rhyolites. Chlorite, epidote, albite, and actinolite are the most common metamorphic phases; blue-green amphibole, andesine, muscovite, biotite, kaolinite, sericite, carbonate, and opaques are widespread but less abundant. Phase assemblages and chemistry suggest predominant greenschist facies metamorphism with epidote-amphibolite facies conditions attained locally. Whole rock major element compositions define a calc-alkaline trend: CaO, FeO, MgO, TiO 2, Al 2O 3, V, Cr, Ni, and Sc all decrease with increasing silica, whereas alkalis, Rb, Ba, and Y increase. MORB-normalized trace element concentrations show enrichment of the low-field strength incompatible elements (Ce, La, Ba, Rb, K) and deep negative Nb, P, and Ti anomalies—patterns typical of subduction related magmas. Mafic volcanic rocks plot in fields for calc-alkaline volcanics on trace element discrimination diagrams, showing that pre-existing oceanic crust is not preserved here. All rocks are LREE-enriched, with La=16–112×chondrite, La/Lu=2.6–9.8×chondrite, and Eu/Eu*=0.5–0.9. Dacites and rhyolites have the lowest La/Lu and Eu/Eu* ratios, reflecting the dominant role of plagioclase fractionation in their formation. Some andesites have La/Lu ratios which are too high to result from fractionation of the more mafic lavas; chondrite-normalized REE patterns for these andesites cross those of the basaltic andesites, indicating that these lavas cannot be related to a common parent. The high proportion of mafic lavas rules out older continental crust as the main source of the volcanic rocks. The scarcity of more evolved felsic volcanics (dacite, rhyolite) can be explained by the nature of the underlying crust, which consists of accreted intra-oceanic arc volcanic and plutonic rocks, and is mafic relative to normal continental margins. Andesites with high La, La/Lu, K 2O, and Rb may be crustal melts; we suggest that garnet-rich high-pressure granulites similar to those exposed in the Jijal complex may be restites formed during partial melting of the crust.

Structure of an island‐arc: Wide‐angle seismic studies in the eastern Aleutian Islands, Alaska

New seismic wide‐angle data from the eastern Aleutian Islands show a mafic composition and a 30‐km‐thick island‐arc crust. Traveltimes of P and S refracted arrivals and prominent crustal and mantle reflectors observed to offsets of over 300 km were used to derive velocity models for the eastern Aleutian Arc between the islands of Atka and Unimak using a three‐dimensional finite difference modeling and tomography code. We interpret the highest crustal P wave velocities of 7.2–7.4 km/s between about 12 and about 22 km depth to the south and north of the main volcanic line as remainders of preexisting oceanic crust into which arc magma is intruded. Apart from the pieces of oceanic crust, the velocities suggest an overall mafic composition for the arc, composed mainly of metabasalts, diorite and diabase in the upper crust, and garnet‐granulite or amphibolite‐to‐hornblendite in the lower crust. Reflected arrivals from the subducting Pacific plate at depths of 45–55 km beneath the fore‐arc, together with Pn, show a mantle wedge with P wave velocities as low as 7.4 km/s, increasing with depth to about 8.1 km/s with an average of about 7.7 km/s. A mantle composition that grades from mainly pyroxenite (probably ultramafic cumulates) near the Moho to dunite at greater depths best explains these observed velocities.

Propagating rift tectonics of a Caledonian marginal basin: Multi-stage seafloor spreading history of the Solund-Stavfjord ophiolite in western Norway

The Late Ordovician Solund-Stavfjord ophiolite in western Norway represents a remnant of the Iapetus oceanic lithosphere that developed in a Caledonian marginal basin. The ophiolite contains three structural domains that display distinctively different crustal architecture that reflects the mode and nature of magmatic and tectonic processes operated during the multi-stage seafloor spreading evolution of this marginal basin. Domain I includes, from top to bottom, an extensive extrusive sequence, a transition zone consisting of dike swarms with screens of pillow breccias, a sheeted dike complex, and plutonic rocks composed mainly of isotropic gabbro and microgabbro. Extrusive rocks include pillow lavas, pillow breccias, and massive sheet flows and are locally sheared and mineralized, containing epidosites, sulfide-sulfate deposits, Fe-oxides, and anhydrite veins, reminiscent of hydrothermal alteration zones on the seafloor along modern mid-ocean ridges. A fossil lava lake in the northern part of the ophiolite consists of a >65-m-thick volcanic sequence composed of a number of separate massive lava units interlayered with pillow lavas and pillow breccia horizons. The NE-trending sheeted dike complex contains multiple intrusions of metabasaltic dikes with one- and two-sided chilled margins and displays a network of both dike-parallel normal and dike-perpendicular oblique-slip faults of oceanic origin. The dike-gabbro boundary is mutually intrusive and represents the root zone of the sheeted dike complex. The internal architecture and rock types of Domain I are analogous to those of intermediate-spreading oceanic crust at modern mid-ocean ridge environments. The ophiolitic units in Domain II include mainly sheeted dikes and plutonic rocks with a general NW structural grain and are commonly faulted against each other, although primary intrusive relations between the sheeted dikes and the gabbros are locally well preserved. The exposures of this domain occur only in the northern and southern parts of the ophiolite complex and are separated by the ENE-trending Domain III, in which isotropic to pegmatitic gabbros and dike swarms are plastically deformed along ENE-striking sinistral shear zones. These shear zones, which locally include fault slivers of serpentinite intrusions, are crosscut by N20°E-striking undeformed basaltic dike swarms that contain xenoliths of gabbroic material. The NW-trending sheeted dike complex in the northern part of Domain II curves into an ENE orientation approaching Domain III in the south. The anomalous nature of deformed crust in Domain III is interpreted to have developed within an oceanic fracture zone or transform fault boundary. REE chemistry of representative extrusive and dike rocks from all three domains indicates N- to E-MORB affinities of their magmas with high Th/Ta ratios that are characteristic of subduction zone environments. The magmatic evolution of Domain I encompasses closed-system fractional crystallization of high-Mg basaltic magmas in small ephemeral chambers, which gradually interconnected to form large chambers in which mixing of primary magmas with more evolved and fractionated magma caused resetting of magma compositions through time. The compositional range from high-Mg basalts to ferrobasalts within Domain I is reminiscent of modern propagating rift basalts. We interpret the NE-trending Domain I as a remnant of an intermediate-spread rift system that propagated northeastwards (in present coordinate system) into a pre-existing oceanic crust, which was developed along the NW-trending doomed rift (Domain II) in the marginal basin. The N20°E dikes laterally intruding into the anomalous oceanic crust in Domain III represent the tip of the rift propagator. The inferred propagating rift tectonics of the Solund-Stavfjord ophiolite is similar to the evolutionary history of the modern Lau back-arc basin in the SW Pacific and suggests a complex magmatic evolution of the Caledonian marginal basin via multi-stage seafloor spreading tectonics.

A seismic study of lithospheric flexure in the vicinity of Tenerife, Canary Islands

Seismic data have been used to determine the crustal and upper mantle structure of Tenerife, Canary Islands, a volcanic island of Tertiary age located on > 140 Ma oceanic crust. Reflection data show that oceanic basement dips gently towards the island, forming a flexural moat which is infilled by 2–3 km of well stratified material. The moat is characterised by a major angular unconformity, which we attribute to volcanic loading of pre-existing oceanic crust and overlying sediments and the subsequent infilling of the flexure by material that was derived, at least in part, from the islands. Refraction data show that the flexed oceanic crust has a mean thickness of 6.41 ± 0.42 km and upper and lower crustal velocities of 4.8–5.4 km s −1 and 6.7–7.3 km s −1 respectively. The flexure, which has been verified by gravity modelling, can be explained by a model in which Tenerife and adjacent islands have loaded a lithosphere with a long-term (> 10 6 yr) elastic thickness of approximately 20 km. Seismic and gravity data suggest that up to 1.5 × 10 5 km 3 of magmatic material has been added to the surface of the flexed oceanic crust which, assuming an age of 6–16 Ma for the shield building stage on Tenerife, implies a magma generation rate of about 0.006 to 0.02 km 3 a −1. This rate is similar to estimates from other African oceanic islands (e.g., Reunion and Cape Verdes), but is significantly less than that which has been calculated at Hawaii. There is no evidence in either the seismic or gravity data that any significant amount of magmatic material has “underplated” the flexed oceanic crust. The crustal and upper mantle structure at Tenerife therefore differs from other oceanic islands such as Hawaii and Marquesas where > 4 km of underplated material have been reported.