Hydrothermal alteration and tectonic evolution of an intermediate- to fast-spreading back-arc oceanic crust: Late Ordovician Solund-Stavfjord ophiolite, western Norway

Abstract The Solund‐Stavfjord ophiolite complex (SSOC) in western Norway represents a remnant of the Late Ordovician oceanic lithosphere, which developed in an intermediate‐ to fast‐spreading Caledonian back‐arc basin. The internal architecture and magmatic features of its crustal component suggest that the SSOC has a complex, multistage sea floor spreading history in a supra‐subduction zone environment. The youngest crustal section associated with the propagating rift tectonics consists of a relatively complete ophiolite pseudostratigraphy, including basaltic volcanic rocks, a transition zone between the sheeted dyke complex and the extrusive sequence, sheeted dykes, and high‐level isotropic gabbros. Large‐scale variations in major and trace element distributions indicate significant remobilization far beyond that which would result from magmatic processes, as a result of the hydrothermal alteration of crustal rocks. Whereas K₂O is strongly enriched in volcanic rocks of the extrusive sequence, Cu and Zn show the largest enrichment in the dyke complex near the dyke–volcanic transition zone or within this transition zone. The δ¹⁸O values of the whole‐rock samples show a general depletion structurally downwards in the ophiolite, with the largest and smallest variations observed in volcanic rocks and the transition zone, respectively. δ¹⁸O values of epidote–quartz mineral pairs indicate 260–290°C for volcanic rocks, 420°C for the transition zone, 280–345°C for the sheeted dyke complex and 290–475°C for the gabbros. The ⁸⁷Sr/⁸⁶Sr isotope ratios show the widest range and highest values in the extrusive rocks (0.70316–0.70495), and generally the lowest values and the narrowest range in the sheeted dyke complex (0.70338–0.70377). The minimum water/rock ratios calculated show the largest variations in volcanic rocks and gabbros (approximately 0–14), and generally the lowest values and range in the sheeted dyke complex (approximately 1–3). The δD values of epidote (?1 to ?12‰), together with the δ¹⁸O calculated for Ordovician seawater, are similar to those of present‐day seawater. Volcanic rocks experienced both cold and warm water circulation, resulting in the observed K₂O‐enrichment and the largest scatter in the δ¹⁸O values. As a result of metal leaching in the hot reaction zone above a magma chamber, Zn is strongly depleted in the gabbros but enriched in the sheeted dyke complex because of precipitation from upwelling of discharged hydrothermal fluids. The present study demonstrates that the near intact effect of ocean floor hydrothermal activity is preserved in the upper part of the SSOC crust, despite the influence of regional lower greenschist facies metamorphism.     

Ophiolites of the Eastern Peninsulas zone (Eastern Kamchatka): Age, composition, and geodynamic diversity

Abstract   The geological, geochemical and mineralogical data of dismembered ophiolites of various ages and genesis occurring in accretionary piles of the Eastern Peninsulas of Kamchatka enables us to discriminate three ophiolite complexes: (i) Aptian–Cenomanian complex: a fragment of ancient oceanic crust, composed of tholeiite basalts, pelagic sediments, and gabbroic rocks, presently occurring in a single tectonic slices (Afrika complex) and in olistoplaques in Pikezh complex of the Kamchatsky Mys Peninsula and probably in the mélange of the Kronotsky Peninsula; (ii) Upper Cretaceous complex, composed of highly depleted peridotite, gabbro and plagiogranite, associated with island arc tholeiite, boninite, and high-alumina tholeiitic basalt of supra-subduction origin; and (iii) Paleocene–Early Eocene complex of intra-island arc or back-arc origin, composed of gabbros, dolerites (sheeted dykes) and basalts produced from oceanic tholeiite melts, and back-arc basin-like dolerites. Formation of the various ophiolite complexes is related to the Kronotskaya intra-oceanic volcanic arc evolution. The first ophiolite complex is a fragment of ancient Aptian–Cenomanian oceanic crust on which the Kronotskaya arc originated. Ophiolites of the supra-subduction zone affinity were formed as a result of repeated partial melting of peridotites in the mantle wedge up to the subduction zone. This is accompanied by production of tholeiite basalts and boninites in the Kamchatsky Mys segment and plagioclase-bearing tholeiites in the Kronotsky segment of the Kronotskaya paleoarc. The ophiolite complex with intra-arc and mid-oceanic ridge basalt geochemical characteristics was formed in an extension regime during the last stage of Kronotskaya volcanic arc evolution.     

Zircon sensitive high mass-resolution ion microprobe U–Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications

Abstract   The Late Miocene–Pliocene Taitao ophiolite is composed of a complete sequence of classic oceanic lithosphere and is exposed approximately 50 km southeast of the Chile triple junction, where the Chile Ridge subducts beneath the South American Plate. Gabbros and ultramafic rocks are folded into a complex pattern, but only evidence for block rotation has been reported in the overriding sheeted dyke complex. In the present study, sensitive high mass-resolution ion microprobe U–Pb and fission-track dating methods were applied to zircon crystals separated from gabbros and sheeted dykes. Two sets of radiometric ages of gabbros range between 5.9 ± 0.4 and 5.6 ± 0.1 Ma. These ages coincide within their error ranges and imply rapid intrusion and cooling of gabbros. The U–Pb age of a dacite dyke intruded into the sheeted dyke complex was determined to be 5.2 ± 0.2 Ma. These data indicate that the magmas of the Taitao ophiolite were formed during the 6 Ma Chile Ridge collision event and emplaced in a shorter period than previously thought. A short segment of the Chile Mid-oceanic Ridge must have been emplaced during the 6 Ma event.     

Metal-depleted root zones of the Troodos ore-forming hydrothermal systems, Cyprus

The cupriferous pyrite deposits of Cyprus were precipitated from hydrothermal solutions derived by interaction of contemporaneous seawater with hot mafic rock at the ancient Troodos spreading centre. Here we identify the zones in which this interaction took place. The zones occur in the lower part of the sheeted dyke complex, and within them 30–50% of the rock is made up of epidosite, an epidote-quartz rock, replacing the dykes as sheets and pipes. The epidosites contain abundant fluid inclusions, which give trapping temperatures of 350–400°C or even higher, and contain water normally near seawater in salinity. Zones of epidosite are elongate parallel to the strike of the sheeted dykes, and are up to 1 km wide. The rocks throughout these zones are strongly depleted in Cu and Zn, and the metals removed are sufficient to supply the ore deposits. In fact several large ore deposits lie along strike from zones of epidosite. All of these features support the identification of the epidosites as the hydrothermal reaction zones.The location of the epidosite zones immediately above the gabbros of the plutonic complex supports the hypothesis that the heat to drive the ore-forming systems came from the underlying magma, as is also likely for modern black smoker springs.     

Geology, magmatic affinity and geotectonic environment of some Caledonian ophiolites in Norway

Ophiolites within the Norwegian Caledonides were generated during at least two distinct periods, i.e. Tremadocian-Arenigian and Ashgillian. The older generation show a long-lived magmatic development (ca 500-470 Ma) with rocks that range from MORB, IAT, boninites, calc-alkaline to alkaline basalt in geochemical affinity. This development is compatible with generation in a subduction-influenced environment, and a plausible modern equivalent may be the ensimatic arc-basin evolution in the western Pacific.The metabasalts of the younger (Ashgill, 443±3 Ma) ophiolite occurrence are characterized by a dominance of N- to E-MORB compositions with subduction zone influence. The volvanics of the complex are intercalated with, and overlain by, continentally-derived sediments. Calc-alkaline as well as alkaline lavas and volcanic-lastics apparantly define late magmatic products. Associated with the youngest ophiolite is a sedimentary melange with olistoliths of mature island arc-derived volcanics, as well as clasts with MORB-IAT affinity, chert and sandstone, the latter probably derived from an accretionary prism. An Andaman Sea model appears most applicable to the youngest ophiolite/associated sediments assemblage.Contemporaneously with the renewed, Ashgillian spreading episode, widespread deposition of clastic sequences of Ashgillian age took place unconformably on the Lower Ordovician ophiolite complexes in fault controlled basins.     

An island arc origin for the Canyon Mountain ophiolite complex,eastern Oregon,U.S.A.

The Canyon Mountain ophiolite, Oregon, is exceptional in lacking sheeted dikes, basaltic pillow lavas, and sediments that are characteristic of many other ophiolites. Instead, the uppermost portion of the complex consists of a significant volume of plagiogranites, which, in addition to minor basalts, intrude a large section of keratophyres believed to be of volcanic origin. The trend of intrusive rocks and of bedding in the keratophyres is mostly parallel to layering in the underlying gabbroic cumulates and to contacts between units in the remainder of the ophiolite. It is suggested that the plagiogranites, basalts, and keratophyres comprise a sill complex. Both the plagiogranites and the keratophyres are similar, respectively, to low-K₂O plutonic and extrusive rocks of island arcs. The mineralogy and penetrative deformation structures of the ultramafic and some of the gabbroic rocks of the ophiolite indicate greater depth of formation, related to magmatism and diapirism above a Benioff zone. Radiometric age dates of plagiogranites confine the minimum age of the complex to the Early Permian. The Canyon Mountain ophiolite may thus be correlative with other fragments of a Lower Permian arc terrane throughout northeastern Oregon which were chaotically mixed during renewed subduction in middle to late Triassic time.     

Amphibolite facies conditions in the oceanic crust: example of amphibolitized flaser-gabbro and amphibolites from the Chenaillet ophiolite massif (Hautes Alpes,France)

Petrological and mineralogical data on amphibolitized gabbros from an Alpine ophiolite massif (Chenaillet Massif, France) are presented. Comparison with metagabbros dredged from the ocean floor shows that synkinematic amphibolite facies conditions may be reached in gabbros after their initial crystallization in the vicinity of the ridge. It is suggested that sub-horizontal plastic flow took place in the gabbroic layer near the axis of a slowly spreading ocean ridge before the intrusion of diabase dykes. This thermo-tectonic regime which at the Chenaillet produced flaser-gabbros and layers of foliated amphibolites with brown hornblende and pargasite, probably also affected most of the other ophiolitic gabbros of the Piemont zone prior to the low-temperature/high-pressure Alpine metamorphism.     

The palaeorotation of the troodos microplate,cyprus, in the late mesozoic-early cenozoic plate tectonic framework of the Eastern Mediterranean

The Troodos ophiolite formed during the Late Cretaceous (Turonian) in a spreading setting above a northerly-dipping subduction zone. Palaeomagnetic data establish that the ophiolite experienced a 90° anticlockwise palaeorotation that occurred during the Late Cretaceous-Early Eocene interval During this period, the ophiolite was stranded adjacent to a generally south-facing continental margin, the remnants of which are preserved in south-west Cyprus (Mamonia Complex), southern Cyprus (Moni melange) and in northern Cyprus (Kyrenia Range). A review of field evidence from these Mesozoic basin margin units shows that strike-slip played a critical role in their Late Cretaceous-Early Tertiary deformation while magnetic and gravity surveys support the existence of major lineaments preserved at depth beneath these areas. Thus, in agreement with available palaeomagnetic data from mainland Turkey and Africa, the rotated crustal unit is believed to be relatively small with its boundaries probably located in the vicinity of Cyprus. In the favoured palaeorotation model, a small supra-subduction zone oceanic crustal unit was rotated about a pole located close to the exposed ophiolite complex. Fragments of the northern continental margin became attached to the rotating microplate along strike-slip lineaments and were then carried southward to their present position. In the light of the regional tectonic setting, it was probably the oblique consumption of crust beneath the ophiolite and/or a collision outside the present area of Cyprus that provided the necessary driving force for the palaeorotation.     

Dykes and structures of the NE rift of Tenerife, Canary Islands: a record of stabilisation and destabilisation of ocean island rift zones

Many oceanic island rift zones are associated with lateral sector collapses, and several models have been proposed to explain this link. The North–East Rift Zone (NERZ) of Tenerife Island, Spain offers an opportunity to explore this relationship, as three successive collapses are located on both sides of the rift. We have carried out a systematic and detailed mapping campaign on the rift zone, including analysis of about 400 dykes. We recorded dyke morphology, thickness, composition, internal textural features and orientation to provide a catalogue of the characteristics of rift zone dykes. Dykes were intruded along the rift, but also radiate from several nodes along the rift and form en échelon sets along the walls of collapse scars. A striking characteristic of the dykes along the collapse scars is that they dip away from rift or embayment axes and are oblique to the collapse walls. This dyke pattern is consistent with the lateral spreading of the sectors long before the collapse events. The slump sides would create the necessary strike-slip movement to promote en échelon dyke patterns. The spreading flank would probably involve a basal decollement. Lateral flank spreading could have been generated by the intense intrusive activity along the rift but sectorial spreading in turn focused intrusive activity and allowed the development of deep intra-volcanic intrusive complexes. With continued magma supply, spreading caused temporary stabilisation of the rift by reducing slopes and relaxing stress. However, as magmatic intrusion persisted, a critical point was reached, beyond which further intrusion led to large-scale flank failure and sector collapse. During the early stages of growth, the rift could have been influenced by regional stress/strain fields and by pre-existing oceanic structures, but its later and mature development probably depended largely on the local volcanic and magmatic stress/strain fields that are effectively controlled by the rift zone growth, the intrusive complex development, the flank creep, the speed of flank deformation and the associated changes in topography. Using different approaches, a similar rift evolution has been proposed in volcanic oceanic islands elsewhere, showing that this model likely reflects a general and widespread process. This study, however, shows that the idea that dykes orient simply parallel to the rift or to the collapse scar walls is too simple; instead, a dynamic interplay between external factors (e.g. collapse, erosion) and internal forces (e.g. intrusions) is envisaged. This model thus provides a geological framework to understand the evolution of the NERZ and may help to predict developments in similar oceanic volcanoes elsewhere.     

Westward younging disposition of Philippine ophiolites and its implication for arc evolution

Abstract   The different ophiolite complexes in the Philippine island arc system define a progressive younging direction westward. This resulted from the clockwise rotation of the Philippine island arc system during its north-westward translation in the Eocene resulting in its western boundary colliding with the Sundaland–Eurasian margin. As a consequence of this interaction, ophiolite complexes and mélanges accreted into the Philippine island arc system along its western side. A new ophiolite zonation with four belts is proposed that takes into consideration the observed spatial and temporal relationships of the exposed oceanic lithosphere slices. With progressive younging from east to west, Belt 1 corresponds to Late Cretaceous complete ophiolite complexes with associated metamorphic soles along the eastern Philippines, whereas Belt 2 includes Early to Late Cretaceous dismembered ultramafic-mafic complexes with mélanges exposed mainly west of eastern Philippines. Belt 3 is defined by Cretaceous through Eocene to Oligocene ophiolite complexes emplaced along the collision zone between the Philippine Mobile Belt and the Sundaland–Eurasian margin. Belt 4 corresponds to the ophiolite complexes emplaced along continental margins as exposed in the Palawan and Zamboanga–Sulu areas. This proposed zonation hints that the whole Philippine Mobile Belt, except for the strike-slip fault bounded Eocene Zambales ophiolite complex in Luzon, is underlain by Cretaceous proto-Philippine Sea Plate fragments. This is contrary to the previous models that consider only the eastern margin of the Philippines to contain proto-Philippine Sea Plate materials.     

Magma generation and crustal accretion as evidenced by supra-subduction ophiolites of the Albanide–Hellenide Subpelagonian zone

Abstract Ophiolites of the Mirdita–Subpelagonian zone form a nearly continuous belt in the Albanide–Hellenide orogen, including mid‐ocean ridge basalt (MORB) associations in the western Mirdita sector and supra‐subduction zone (SSZ) complexes, with prevalent island arc tholeiitic (IAT) and minor boninitic affinities in the eastern part of the belt (i.e. eastern Mirdita, Pindos, Vourinos). In addition, basalts with geochemical features intermediate between MORB and IAT (MORB/IAT) are found in the central Mirdita and in the Aspropotamos sequence (Pindos). These basalts alternate with pure MORB and are cut by boninitic dykes. The distinctive compositional characteristics of the mafic magmas parental to the different ophiolitic suites can be accounted for by partial melting of mantle sources progressively depleted by melt extractions. Partial melting processes (10–20%) of lherzolitic sources generated pure MORB, leaving clinopyroxene‐poor lherzolite as a residuum. Approximately 10% water‐assisted partial melting of this latter source, in an SSZ setting, may in turn generate basalts with MORB/IAT intermediate characteristics, whereas IAT basalts and boninites may have been derived from 10–20% and 30% partial melting, respectively, of the same source variably enriched by subduction‐derived fluids. In addition, boninites may also have been derived by comparatively lower degrees of hydrated partial melting of more refractory harzburgitic sources. A generalized petrologic model based on mass balance calculations between bulk rock and mineral compositions, indicate that most of the intrusives (from ultramafic cumulates to gabbronorites and plagiogranites), as well as sheeted dykes and volcanics (from basalts to rhyodacites) forming the bulk crustal section of the SSZ ophiolites, may be accounted for by shallow fractional crystallization from low‐Ti picritic parental magmas very similar in composition to IAT picrites from Pacific intraoceanic arcs. The most appropriate tectono‐magmatic model for the generation of the SSZ Tethyan ophiolites implies low velocity plate‐convergence of the intraoceanic subduction and generation of a nascent arc with IAT affinity and progressive slab roll‐back, mantle diapirism and extension from the arc axis to the forearc region, with generation of MORB/IAT intermediate basalts and boninitic magmas.     

Volcanic complexes in the eastern ridge of the Canary Islands: the Miocene activity of the island of Fuerteventura

Fuerteventura has been since early stages of its growth the result of three different adjacent large volcanic complexes: Southern, Central and Northern. The definition of these volcanic complexes and their respective growing episodes is based on volcano-stratigraphic, morphological and structural criteria, particularly radial dyke swarms. Each complex has its own prolonged history that might be longer than 10 m.y. During that time, several periods of activity alternating with gaps accompanied by important erosion took place. The evolution of each volcanic complex has been partially independent but all the three are affected by at least three Miocene tectonic phases that controlled considerably their activity. The volcanic complexes are deeply eroded and partially submerged. In the core of the Northern and the Central volcanic complexes there is a set of submarine and plutonic rocks intensely traversed by a dyke swarm, known as the Basal Complex. The Basal Complex has been interpreted in different ways but all previous authors have considered it to be prior to the subaerial shield stage of the island. Here we advance the idea that the Basal Complex represent the submarine growing stage of the volcanic complexes and the hypabyssal roots (plutons and dykes) of their successive subaerial growing episodes.Two seamounts situated nearby, southwest of the island, might be interpreted as remains of two other major volcanoes. These two volcanoes, together with those forming the present emerged island of Fuerteventura, and finally those of Famara and Los Ajaches situated further north on Lanzarote constitute a chain of volcanoes located along a lineation which is subparallel to the northwestern African coastline and which may relate to early Atlantic spreading trends in the area.     

Lead isotopic study of the Xigaze ophiolite (Tibet): the problem of the relationship between magmatites (gabbros,dolerites, lavas) and tectonites (harzburgites)

The Pb isotopic study of the Xigaze ophiolite (Tibet) shows that the tectonic (harzburgites) and magmatic (gabbros, dolerites, lavas) units bear different isotopic signatures, the harzburgitic rocks show much higher²⁰⁷Pb/²⁰⁴Pb values than the samples from the magmatic unit. Analyses of primary magmatic clinopyroxenes separated from the harzburgites confirmed that this difference is real and not due to secondary alteration processes. UPb analysis allowed determination of the crystallization age of the magmatites at 120 ± 10 m.y. This age is compatible with paleontological age determinations and thus demonstrates that UPb behaved more or less as a closed system since formation of the ophiolite. If an exchange with seawater has occurred, it must necessarily have happened shortly after crystallization of the Xigaze oceanic crust. Dolerite dykes crossing the harzburgites show similar Pb isotopic composition as the magmatites, indicating that they were emplaced when the ultramafic rocks were already brittle; however, the time difference of the emplacement was probably not larger than 280 m.y. This makes it evident that two sources with different isotopic signatures, one similar to normal MORB, the other with much higher²⁰⁷Pb/²⁰⁴Pb ratios, were active in the immediate neighborhood. One possible environment which can account for this association is a propagating ridge.     

P−T conditions during emplacement of the Bay of Islands ophiolite complex

A high-temperature contact is described between the basal pargasite-bearing spinel-lherzolites of the Bay of Islands ophiolite complex and underlying garnet-granulite facies metagabbros of its dynamothermal aureole. Three distinct high-temperature hydrous assemblages occur in the basal mylonites of the peridotite, and spinel- and garnet-bearing corona textures indicative of increase in pressure under constant or increasing temperature conditions are described for the first time from the uppermost part of the aureole. On the basis of garnet-clinopyroxene geothermometry and garnet-forming reactions in metabasic rocks, P?T conditions of 7–11 kbar, 750–850°C are estimated for rocks on both sides of the contact. Steep inverted gradients in both temperature and pressure of equilibration occur in the aureole, which most likely represents a thinned, overturned and metamorphosed section through an ophiolite sequence. It is proposed that the aureole formed in a low-angle shear zone cutting the oceanic crust and upper mantle.Age data shows that the Bay of Islands Complex was 30–40 Ma old and therefore relatively cold at the time of formation of the aureole. Prolonged (> 1 Ma) shear heating must therefore have occurred at high shear stresses and movement rates (≥ 1 kbar, 10 cm/yr) to produce the high contact temperatures. The displacement surface probably initiated as a discrete fault, evolving into a viscous shear zone with time. Downward movement of the locus of shearing into weaker lithologies and finally thrusting of the ophiolite-aureole complex over cold sediments accounts for the preservation of steep metamorphic gradients in the aureole.The observed pressures at the ophiolite-aureole contact are 3–7 kbar in excess of the expected load pressure from the present thickness of the ophiolite. The cause of the pressure excess was removed before formation of lower-grade parts of the aureole. Possible explanations are tectonic thinning of the ophiolite during displacement or more likely emplacement of nappes on top of the ophiolite before formation of the aureole. A model involving detachment of the ophiolite slice from below a subduction zone can account for the high pressures, rapid uplift and erosion during displacement, and the coincidence of K-Ar ages of amphiboles from the aureole and the sheeted dyke complex of the ophiolite.     

The dyke swarm of Mount Calanna (Etna, Italy): an example of the uppermost portion of a volcanic plumbing system

A new multidisciplinary study, combining geology, petrography, and geochemistry, on the rocks of the isolated hill of Mount Calanna (Mount Etna, Italy) has provided evidence for the existence of a dyke swarm, formed by more than 200 dykes distributed over an area of ~0.7 km², with an intensity of intrusion up to 40%. All bodies are deeply altered, and the geological and mesostructural surveying of 132 dykes revealed that they intruded in E–W direction, with an average dip of 60°. The faults affecting the outcrop have in general an E–W strike and dip of ~55°: these have all normal motion and have been interpreted as coeval with the dykes. This interpretation contrasts with the previous hypothesis that considered Mount Calanna as a thrust resulting from compressive deformation resulting from the gravitational spreading of the volcanic edifice. Mount Calanna is here interpreted as the uppermost portion of a vertically extensive magmatic plexus that fed the eruptive activity of one (or more) eruptive center/s sited in the Valle del Bove area. Measurements of the apparent densities on 23 dykes and host rock samples give an average value of 2,420 kg/m³ for the entire complex, ~15% lower than the density expected for hawaiitic magma, placing an important constraint on the geophysical identification of similar structures. Considering that Mount Etna is not an old eroded edifice but an active and growing volcano, the exposure of this subvolcanic structure can be regarded as exceptional. Its geometry and physical characteristics can be thus regarded as an interesting example of the present-day shallow plumbing system of Mount Etna as well as of other basaltic volcanoes.     

A Cretaceous to Paleocene-Eocene South China sea basin origin for the Zambales Ophiolite Complex,Luzon, Philippines?

Abstract The Eocene Zambales Ophiolite Complex that exhibits transitional mid-ocean ridge basalt-island arc tholeiite (MORB-IAT) characteristics was formed in a subduction-related marginal basin. The different surrounding marginal basins of the Philippines, namely, the South China Sea, Sulu Sea Basin, Celebes Basin and the West Philippine Basin have all been modeled to be of probable provenance of this ophiolite complex. Certain information (e.g. age, rock geochemistry, paleomagnetic rotations) and limitations, nevertheless, are inconsistent with the ophiolite complex being generated in these regions. Recent geophysical evidence suggests that the southwest sub-basin of the South China Sea Basin is probably Cretaceous to Paleocene-Eocene in age. This makes it possible to speculate that the Zambales Ophiolite Complex could have come from this sub-basin. The present day rifting of the southern Izu-Mariana arc can be taken as a modern day analog of this type of ophiolite generation.     

Tectonic rotations during the Chile Ridge collision and obduction of the Taitao ophiolite (southern Chile)

Abstract The < 6 Ma young Taitao ophiolite, exposed at the westernmost promontory of the Taitao Peninsula, is located approximately 40 km southeast of the Chile triple junction and consists of a complete sequence of oceanic lithosphere. Systematic sampling for paleomagnetic study was performed to understand the complex obduction processes of the ophiolite onto the forearc of the South American Plate. Two representative demagnetization paths of remanent magnetization vectors were observed. One is characterized by stable univectorial demagnetization paths and was observed in volcaniclastic rocks and dyke complexes. Orientations of their remanent magnetization vectors indicate various degrees of counterclockwise rotations. The other is characterized by multivectorial demagnetization paths and was observed in the plutonic units (gabbros and ultramafic rocks). From these, two distinct stable remanent magnetization vectors were isolated; one has high coercivity and the other has low coercivity along the demagnetization paths with little influence of viscous magnetizations. This suggests that the complex deformation history involved at least two rotational events. The clockwise rotation, inferred from high coercivity remanent magnetization vectors, was attributed to a ridge collision event and the counterclockwise rotation, inferred from the low coercivity remanent magnetization vectors, was attributed to an accommodation phase into the South American forearc during obduction and final emplacement of the ophiolite. Folds developed during this period. Paleomagnetic restorations of the internal structures of the plutonic units and dyke complexes suggest that they probably originated in a mid‐oceanic ridge environment near a transform fault. The counterclockwise rotation of the plutonic and dyke complex units during the obduction generated tectonic gaps between these and the basement. The volcaniclastic rocks must have been deposited at nearly their present location, filling the tectonic gaps, as less effect of tectonic rotation was identified on these rocks.     

Ophiolite detachment related to syn-collision thrusting

Many major ophiolite bodies can best be explained by detachment and initiation of subduction at a spreading axis in a narrow oceanic basin bordered on the external side by a passive continental foreland margin, followed by subduction of the remnant ocean basin and syn-collision emplacement of the ophiolite and overlying arc system onto the foreland. Evidence from Burma and the Philippines suggests that detachment and subduction at a spreading axis were related to regional compressive stress within an earlier collision belt on the internal side of the ophiolite. In Burma, detachment of a Jurassic ophiolite was in response to foreland thrusting in a Triassic collision belt to the east, while in the western Philippines, detachment of a Palaeocene ophiolite can most easily be explained as a response to back-thrusting in a late Cretaceous collision belt in Mindanao.     

The Troodos ophiolitic complex was probably formed in an island arc

The Troodos ophiolitic complex in Cyprus has been widely regarded as a fragment of oceanic crust that was created in a mid-oceanic ridge. However, about one-third of the analyzed rocks of the lower pillow lavas and sheeted complex in it follows a calc-alkalic trend. This strongly suggests that the massif was created as a basaltic volcano in an island arc with a relatively thin oceanic-type crust rather than in a mid-oceanic ridge. Other chemical features and the structure of the massif are also consistent with an island arc origin. Some other ophiolitic complexes also appear to have been formed in island arcs.     

The long-term growth of volcanic edifices: numerical modelling of the role of dyke intrusion and lava-flow emplacement

The contribution of intrusive complexes to volcano growth is attested by field observations and by the monitoring of active volcanoes. We used numerical simulations to quantitatively estimate the relative contributions to volcano growth of elastic dislocations related to dyke intrusions and of the accumulation of lava flows. The ground uplift induced by dyke intrusions was calculated with the equations of Okada (Bull. Seismol. Soc. Am., 75 (1985) 1135). The spreading of lava flows was simulated as the flow of a Bingham fluid.With realistic parameters for dyke statistics and lava-flow rheology we find the contribution of dyke intrusions to the growth of a basaltic shield archetype to be about 13% in terms of volume and 30% in terms of height. The result is strongly dependent on the proportion of dykes reaching the surface to feed a lava flow. Systematic testing of the model indicates that edifices tend to be high and steep if dykes are thick and high, issued from a small and shallow magma chamber, and if they feed lava flows of high yield strength.The simulation was applied to Ko'olau (O'ahu Is., Hawai'i) and Piton de la Fournaise (Réunion Is.) volcanoes. The simulation of Ko'olau with dyke parameters as described by Walker (Geology, 14 (1986) 310; U.S. Geol. Surv. Prof. Pap., 1350 (1987) 961) and with lava-flow characteristics collected at Kilauea volcano (Hawai'i Is.) results in an edifice morphology very close to that of the real volcano. The best fit model of the Piton de la Fournaise central cone, with its steep slope and E–W elongation, is obtained by the intrusion of 10 000 short and thick dykes issued from a very small and shallow magma chamber and feeding only 700 low-volume lava flows. The same method may be applied to the growth of basaltic shields and other volcano types in different environments, including non-terrestrial volcanism.