Southern Domain Research Articles

Terrane geochemistry contrasts across the Iapetus Suture in Ireland

AbstractIn the Irish Caledonides, volcanism has been significant in terrane identification and in reconstructions of the Appalachian/Caledonian orogen. Crucial to these reconstructions is the recognition of ocean margins using obducted ocean floor relics (ophiolites) and supra-subduction zone volcanic assemblages. The volcanic rocks provide much evidence for the affinity of a terrane, however, by analogy with present day examples, the ocean floor sediments may provide the best way of tracing both ocean-floor magmatic activity, and continental source areas. This investigation shows that the Irish Lower Palaeozoic volcanogenic terranes can be discriminated in terms of their shale geochemistry, which also gives information on their provenance and environment of deposition. South Mayo shales are dominated by volcaniclastic material derived both from both an arc and from an ophiolitic source. The Northern and Central belts of the Central Terrane show very similar lithogeochemistries, apparently derived in part from intermediate to silicic volcanic complexes. The Ordovician-Silurian inliers that straddle the Suture Zone, here termed the Southern Domain, show a chemistry close to that of the Leinster Terrane, which, coupled with a greater degree of sea-floor weathering, suggests a terrane with sediment of both volcanic and continental provenance being deposited in deeper water further from land. Across the suture the Leinster Terrane shows a mature chemistry which clearly suggests a continental provenance, together with a volcanogenic input from supra-subduction volcanism. This maturity is probably due to slower rates of sedimentation with longer residence times for volcanic detritus, plus the existence of a deeply weathered continental basement.

Kinematics of arc‐continent collision in the eastern Central Range of Taiwan inferred from geomorphic analysis

Active arc‐continent collision in Taiwan is an example of a typical collisional orogen, with a deformed continental margin (Foothills Belt, Hsueshan Range, and Central Range) separated from an accreted island arc (Coastal Range) by a collapsed forearc basin (Longitudinal Valley). The geomorphology of the eastern Central Range reflects spatial and temporal variations in uplift rate and pattern along the orogen during late Quaternary time. Digitized drainage basin perimeters, stream channels, and the mountain‐piedmont junction of a 200‐km‐long segment of the eastern Central Range provide data for calculating quantitative geomorphic parameters. Statistical analyses of these parameters define four domains of distinct landscape character. The northern domain extends northward along the coast from the northern end of the Longitudinal Valley (LV) and has landscape characteristics suggesting uplift near the mountain front. The central two domains are adjacent to the LV and show evidence of uplift concentrated in the interior of the range, away from the front. The boundary between the two central domains occurs near the middle of the LV. The southern domain extends southward along the coast from the southern end of the LV and has characteristics suggesting a transient response to increased uplift rate. Analysis of fluvial terrace profiles in the Central Range adjacent to the southern half of the LV indicates a change in uplift pattern through time, with migration of maximum uplift rate from the mountain front to the interior of the range. This change probably occurred just prior to late Pleistocene time.

Phyllosilicate preferred orientation in relation to strain path determination in the lower Paleozoic Stavelot-Venn Massif (Ardennes, Belgium)

Phyllosilicate preferred orientation, measured by means of an X-ray pole figure goniometer, allows a quantitative exploration of the slaty cleavage development. The relationship between phyllosilicate preferred orientation and strain, as demonstrated in the March model, furthermore enables the application of phyllosilicate pole figures in a strain determination in slate belts. Both applications are illustrated in this paper, in which the results are presented of a crystallographic fabric analysis of argillites in the southern domain of the lower Paleozoic Stavelot-Venn Massif (Belgium). In these argillites the development of a phyllosilicate preferred orientation is primarily defined by the low-grade metamorphism and the deformation history. The metamorphism, causing recrystallisation to be active during cleavage development, assures the development of a high degree of preferred orientation. On the other hand, the deformation history defines the type of phyllosilicate pole figure pattern. Based on the distinction of different pole figure patterns, a possible strain path for the argillites in the southern domain of the Stavelot-Venn Massif is proposed. An axially symmetric pole figure pattern indicates a Caledonian cleavage development in a pure flattening regime. The Variscan cleavage development, on the other hand, is characterised by an additional extensional component in the cleavage plane. If the primary phyllosilicate fabric, due to a pre-deformational compaction or a Caledonian cleavage development, underwent a complete reorientation caused by syntectonic recrystallisation, a typical orthorhombic pole figure pattern is formed. Otherwise a transition pole figure pattern develops, reflecting the superposition of the Variscan cleavage development on a pre-existing Caledonian cleavage fabric. The general deformation history of the southern domain of the Stavelot-Venn Massif differs substantially from that described for the northern domain. Contrary to the northern domain the southern domain seems to have undergone considerable Variscan deformation.

The Kibaran of southern Africa: Tectonic evolution and metallogeny

The tectono-magmatic and metallogenic evolution of the ∼ 1100 Ma (Late Kibaran) high-grade Namaqua-Natal metamorphic belt is reviewed and compared with that of the Grenville Province and the Kibaran Belts of central and eastern Africa. In Namaqualand, 1300–1600 Ma supracrustal rocks are recognised, including arc-related sequences in the east and extensional foreland basin sequences in the west. These were laid down either on a heterogeneous basement of Eburnian (∼ 2000 Ma) age or on juvenile oceanic crust, prior to Late Kibaran deformation, metamorphism and multiple plutonic intrusion. The pre-Namaqua basement infrastructure has been recognised largely in the eastern and NW parts of the orogen, adjacent to areas of exposed Eburnian crust and probably only the Areachap Terrane in the east comprises juvenile Kibaran crust. In Natal, ∼1400 Ma, predominantly metavolcanic supracrustal gneisses were extensively intruded by voluminous plutonic batholiths during the ∼ 1050 Ma tectonothermal event. The northern terrane is interpreted as an ophiolitic assemblage, obducted to the NE onto the Kaapvaal Craton, whereas the southern domains are predominantly arc-related. No older basement is recognised and the entire belt comprises juvenile crustal material, none of which is apparently older than ∼ 1400 Ma. The kinematic evolution of the entire Namaqua-Natal Belt is comparable. In both areas, early SW- and NE- d directed thrusting and nappe emplacement during collision, were followed by pervasive steep transcurrent ductile shearing. In Natal. the later shearing event gave rise to major WNW-trending oblique sinistral transcurrent “steep belts”, whereas the same event in Namaqualand is manifested as major, subvertical, NW-trending dextral transcurrent shear zones. The difference in dominant sense of movement in the two regions can be attributed to changes in orientation of the adjacent Archaean craton margin, within a progressive, consistently oriented, NE-SW directed zone of plate convergence. In eastern Namaqualand and Namibia, the transcurrent shearing gave rise to a north-propagating stricke-slip rift system in which the volcano-sedimentary Koras-Sinclair-Ghansi sequences were deposited. Mineral cooling ages of ∼ 950 Ma, reported from throughout the belt, are considered to signal the end of the Kibaran cycle in southern Africa. The metallogenic evolution is discussed in the light of the tectono-magmatic model presented. Economically important stratiform volcanic and sedimentary exhalative base-metal occurrences are extensively associated with early supracrustal sequences throughout the belt. Other metal deposits and occurrences are predominantly related to the voluminous granitoid and more restricted metabasic suites, which characterise the younger rocks in both Natal and Namaqualand. The metallogenesis of the dominantly convergent-collisional Namaqua-Natal Belt is similar to that seen in the Grenville Province, but distinct from, and younger than, the lower grade, strike-slip basin-related Kibaran Belt of central and eastern Africa.

Kinematic model for the curvature of the northern Subalpine Chain, France

Analyses of cross-sectional shortening, mapped faults and calcite twin strain are used in conjunction with geometric-kinematic modeling to interpret the mechanism of curvature formation in the northern Subalpine Chain of France. There is a 45° change in strike around the Chain that is convex towards the foreland (northwest). Five models for fold-thrust belt curvature are applied to the northern Subalpine Chain. They can reproduce the geometry and the kinematic history of a curved fold-thrust belt and can be used to predict the magnitude and orientations of strains and the three-dimensional geometry of the curvature. The models are: pure bending; radial thrusting; curve-parallel simple shear; uniform displacement-uniform shortening; and transport-parallel simple shear. Two principal criteria for distinguishing among the models geometrically are the horizontal tangential strain (extension or contraction parallel to strike) and cross-sectional shortening. The transport-parallel simple shear model predicts no tangential extension in the central domain and greater radial shortening and significant tangential extension in the northeastern and southern domains. This is the pattern observed in the northern Subalpine Chain. Therefore, the curvature of the northern Subalpine Chain is attributed to a transport-parallel simple shear mechanism.

Reworking of Archaean and Early Proterozoic components during a progressive, Middle Proterozoic tectonothermal event in the Albany Mobile Belt, Western Australia

Isotopic data, derived mainly from UPb zircon ion-microprobe analyses, are presented for each of the three tectonic domains of the Albany Mobile Belt, Western Australia. They show that not only the Northern Domain, but also parts of the Central domain originally crystallised about 3 100 Ma ago; they represent reworked Archaean rocks similar to those of the adjoining Yilgarn Block. On the basis of SmNd model ages, a somewhat younger, less precisely defined crustal formation event of probable Early Proterozoic (∼ 2100 Ma) age can be inferred for the southern Domain and the remainder of the Central Domain. The younger component of the Albany Belt is older than some previous estimates, being of similar initial age to most of the Early Proterozoic orogenic provinces of northern and central Australia. Interpretation of the SmNd data is not strictly based on conventional model ages, for it appears that significant fractionation of the rare-earth elements occurred during Middle Proterozoic crustal melting. Pegmatites and granites dated by UPb on zircon provide a temporal framework for the Middle Proterozoic evolution of the Albany Mobile Belt. Progressive deformation, incorporating foliation development, up to four superimposed fold generations, late-tectonic granite intrusion and conjugate shear zone arrays took place under a consistent orientation of maximum compressive stress over a geologically brief time span, about 1190 Ma ago. Dextral transpression and thrusting at 1190 Ma postdates granulite facies metamorphism and major deformation events in the Fraser Mobile Belt. The widespread 1190 Ma rocks are of comparable age to intrusives rocks emplaced during a similar high-grade metamorphic event in the Bunger Hills of Antarctica (a region that is commonly juxtaposed with the Albany Mobile Belt in Gondwana reconstructions).

Basement reflectors in the Kerguelen Plateau, south Indian Ocean: Indications for the structure and early history of the plateau

The Kerguelen Plateau is a large structure in the south Indian Ocean of which the nature (oceanic or continental) and origin are not known. In this work, multichannel seismic reflection surveys, particularly in the southern domain of the plateau, are used to study its origin. Internal basement reflections characterize most of the investigated area. Basement reflections, together with other geological and geophysical information, indicate that the entire plateau is underlain by a volcanic basement, suggesting that both the northern and the southern domains have an oceanic origin. Basement areas with no internal reflections are associated mostly with basement ridges; basement reflector wedges, associated with volcanic flows from the elevated volcanoes, appear on the flanks of these ridges. We suggest that the basement ridges are the extinct volcanic sources of the Southern Kerguelen Plateau. Typical volcanic morphology is absent in the southern domain, because it was removed during the erosional phase known to have occurred following the emplacement of the plateau and which is apparent in the seismic data. Two types of volcanic sources are identified in the southern domain: isolated volcanoes and linear sources. This pattern is somewhat similar to the volcanic pattern in Iceland, suggesting that the Kerguelen Plateau may have been a similar structure, related to excessive volcanism associated with a hot spot on a mid-ocean spreading ridge in the Early Cretaceous south Indian Ocean.

Magmatic and geotectonic evolution of a Proterozoic oceanic basin system: the Cape Smith Thrust-Fold Belt (New-Quebec)

The Cape Smith Thrust-Fold Belt (1999-1840 Ma, Parrish, 1989) is divided into two tectonostratigraphic domains separated by the major east-west Bergeron fault. In the southern domain, the Povungnituk Group lies on the Archean craton of the Superior Province. It contains a sedimentary sequence (Lamarche sub-group) grading to a continental LREE-enriched tholeiitic basaltic sequence (Beauparlant sub-group, MgO < 10%, TiO2 = 1.2–3.6%) that is locally overlain by phyllites and alkaline volcanic rocks. The Chukotat Group to the north includes several sequences evolving from olivine-phyric komatiitic basalts (MgO = 19−11 %, TiO < 0.9%) to pyroxene-phyric tholeiitic basalts (MgO = 12.5 − 7%, TiO2 = 0.8–1.1%) with moderately enriched to depleted LREE profiles. Finally, in its northernmost part, the Chukotat Group contains several sequences of LREE-depleted tholeiitic plagioclase-phyric basalts (MgO < 8%, TiO2 = 1.3–2.8%). The northern domain presents: (1) a vast dismembered ophiolitic complex (Watts Group) with plutonic peridotitic to gabbroic rocks, sheeted dykes and associated LREE-depleted tholeiitic basalts (MgO = 9.8-5.7%, TiO2 = 0.3–2.2%); and (2) a volcano-sedimentary sequence of shales, siltstones and greywackes intercalated by basaltic to rhyolitic calc-alkaline lavas and volcaniclastites (Parent Group). With respect to their structural, petrological and geochronological characteristics, the two tectono-stratigraphic domains are interpreted as having resulted from a rifting process with (1) the formation of an early oceanic crust (the Purtuniq ophiolite) in the northern domain, and (2) the subsequent opening of the more southern Povungnituk-Chukotat basin. A comparison with present-day rifting processes suggests that the younger Povungnituk-Chukotat basin evolved by an asymmetric rifting model implying: (1) the creation of a fault-bound basins into which shallow water sediments (Lamarche sub-group) accumulated; (2) the formation of an ensialic proto-rift into which LREE-enriched tholeiitic basalts of the Beauparlant sub-group were emplaced, with local emission of alkaline volcanic rocks on volcanic islands; (3) the progressive opening of an oceanic rift and subsequent eruption of weakly LREE-enriched then depleted olivine- and pyroxene-phyric basalts of the Chukotat Group; and (4) the formation of an oceanic crust characterized by the LREE-depleted plagioclase-phyric basalts of the Chukotat Group. Finally, in response to a north-south compression, the northern Chukotat basalts were probably subducted beneath the northern domain, thus initiating the development of a magmatic arc system (Parent Group).

New data and hypothesis on the development of the Tyrrhenian basin

Among those basins which developed during the Neogene and Quaternary at the rear of the perimenditerranean mountain chains, the Tyrrhenian basin is the most recently formed. Comparing the available geological and geophysical data, the Tyrrhenian domain and its neighbourhoods can be divided into more or less homogeneous sectors: namely Northern and Southern Tyrrhenian, Apenninic chain, Maghrebides chain, Apulian and Iblean-African foreland, Ionian foreland. For each sector the different phenomena involved and the crustal characteristics are discussed. The evolution of the domain has been explained by the application of a crustal stretching model developed for continental margins. This approach predicts the formation of basinal areas as a result of the Europe/Africa collisional movements within a plastic-rigid deformation model. Taking into account the mass redistribution after collision, the model explains both the distension developed within the collisional system and the penecontemporaneous development of compressive and distensive phases. As a consequence of the intraplate stresses, horizontal shearings affecting the whole lithosphere at different levels, occur, with delaminations, asthenosphere uprising and gradual collapse in the upper crust. E-W trending and strike-slip dextral faults played a primary role in the evolution of the Tyrrhenian basin inducing the largest tearings at the end of the major transcurrent systems. Such an evolution is complicated by the migrating motion of the shear zone from N to S and by the penecontemporaneous stop in the extension from the northern towards the southern domains. Of outstanding importance is the shear zone which borders the Southern Tyrrhenian enabling the opening of the basin and allowing the development of the major group of en-echelon sub-basins with a NW-SE trend.

Miocene to recent structural development of an extensional accommodation zone, northeastern Baja California, Mexico

The Gulf of California Extensional Province in northeastern Baja California contains a major structural transition from a northern domain of widely spaced basins and ranges to a southern domain of closely spaced, NNW-striking high-angle faults. In the northern domain, the western boundary of the province (the Main Gulf Escarpment) is a single listric normal fault, the San Pedro Mártir fault; in the southern domain it is a 5 km wide zone of numerous E-dipping high-angle normal faults. The western half of this structural transition, studied here, does not occur by a single ‘transfer fault’ but rather by: (1) transfer of displacement from the San Pedro Mártir fault onto multiple fault zones in the footwall, without intervening cross-faults; and (2) increased southward disruption of the hanging wall by WNW-striking strike-slip faults (oblique to the extension direction) and NNW-striking normal faults and extension fractures. This structural transition involves both the hanging wall and the footwall of the escarpment. The lack of a discrete transfer structure is attributed either to insufficient extension (<10%) or to a gradual change in geometry of the basal detachment.

Seismotectonics and present-day relative plate motions in the Tonga-Lau and Kermadec-Havre region

An updated compilation of shallow seismicity in the Tonga-Kermadec arc and back-arc region, including especially 304 focal mechanisms from centroid moment tensor solutions files, provides new insights on the present-day tectonics and allows the relative motions between the Pacific, Indo-Australian and Tonga-Kermadec plates to be quantified. Azimuths of slip vectors along the trench change at 19°S. In the southern domain (35-19°S) the mean azimuth ranges from N280°E to N285°E, while in the northern domain it trends N276–277°E. This 19°S boundary also exits in the back-arc domain. South of 19°S, from 35°S to 20°S the direction of back-arc extension is respectively N135°E, N122°E and N111°E within the southern Havre trough, northern Havre trough and southern Lau basin. In the southern Lau basin the N111°E opening direction is compatible with the trend of the newly mapped Valu Fa ridge. North of 19°S, in the northern Lau basin (16–18°S), a N93°E trending crustal extension occurs along the N135°E Peggy ridge and along an inferred N5–10°E spreading zone. Between these two extensional features a fourth plate, the northern Lau microplate, is present. At 18–19°S a ridge-ridge-ridge triple junction, accommodated by N95°-115°E right-lateral strike-slip fracture zones, is proposed at the complex transitional zone between the southern and the northern parts of the Lau basin. The Fiji-North Tonga fracture zone which bounds northward the Lau back-arc basin is composed of two segments: a N95°E eastern segment extending from the northern end of the Tonga trench to 178°30′W, and a N75°E western segment lying north of the Fiji platform. A ridge-fracture-fracture triple junction is inferred at 178°30′W–15°45′S between the northern end of the Peggy ridge and the two segments of the Fiji-North Tonga fracture zone. Using the model RM-2 of Minster and Jordan (1978) and the directions of back-arc spreading and convergence at the trench, relative plate motions are estimated. The extension rates in the back-arc domain increase northward, and are respectively 0.8, 2.1 and 8 cm/y at 33°S, 28°S and 24°S. Farther north parallelism between directions of different motions does not allow the calculation of any accurate result. Therefore north of 24°S we used a spreading rate of 8 cm/y which corresponds to the 24°S result and also to the value deduced from magnetic anomalies. The consumption rates along the Tonga-Kermadec trench are respectively 7.2, 9.6, 16.4, 17.1 and 17.8 cm/y at 33°S, 28°S, 24°S, 20°S and 17°S. Both the back-arc opening rate and the consumption rate of the Pacific plate sharply increase between the northern Havre trough and the southern Lau basin. This limit coincides with the Louisville ridge-trench junction. Along the N95°E eastern segment of the Fiji-North Tonga fracture zone, a pure left-lateral strike-slip motion, parallel to the N93°E opening tectonics within the northern Lau basin occurs. In contrast along the N75°E western segment of the Fiji-North Tonga fracture zone, a left-lateral strike-slip motion of 7 cm/y should be accompanied by a N135°E extensional motion of 3.5 cm/y.

Structure of the Kingston orocline in the Appalachian fold-thrust belt, New York

The trend of the post-Taconic Appalachian fold-thrust belt changes at the city of Kingston in the New York recess. To the southwest of Kingston, fold-thrust structures trend northeast-southwest, whereas to the north of Kingston, fold-thrust structures trend due north to north-northeast. This paper discusses the structural geometry and origin of this change in trend. Detailed mapping (1:9,600) suggests that the fold-thrust belt in the vicinity of Kingston can be divided into three structural domains (northern, central, and southern). In the northern domain, megascopic folds are open, significant faults do not intersect the present-day ground surface, Ordovician strata below the Taconic unconformity are not extensively thrust over post-unconformity strata, and shortening is less than 5%. In the southern domain, shortening across the fold-thrust belt is 30%, numerous faults intersect the ground surface, and Ordovician strata are significantly thrust over postunconformity strata. In the central domain, there are numerous accommodation faults, there are locally two noncoaxial-spaced cleavages, and strike-slip lineations overprint dip-slip lineations on many slip surfaces. To explain the contrasts between domains, we suggest that the bend at Kingston is an intersection orocline created when northeast-southwest-trending structures of the Kittatiny-Shawangunk segment of the Appalachian fold-thrust belt overprinted and reoriented older north-south-trending structures of the Hudson Valley fold-thrust belt. Calcite twin strain-gauge measurements indicate that there was less than 2% penetrative tangential stretching accompanying the development of this orocline.

Sandstone petrography and geochemistry of late proterozoic sediments of the armorican massif (France) — A key to basin development during the cadomian orogeny

Brioverian (Late Proterozoic) sediments of the Armorican Massif in Northwest France are contained within isolated basins whose development is linked to Cadomian orogenesis. These basins were formed between c. 650 and 530 Ma ago, mainly by gravitational flow processes. Petrographical study and geochemical analysis of major and trace elements show that the sandstone formations of North Brittany are characterized by great immaturity (10–20% feldspar; 10–20% lithic fragments) and high concentrations of Na 2O, Ba, Sr, V and Cr. The formations from Western and Central Brittany, as well as the southern part of Normandy, are compositionally more mature (< 15% feldspar + lithic fragments; K 2O/Na 2O > 1). Petrographical and geochemical data for North Brittany formations yield plots which suggest a mixed source region composed of a continental active margin (island arc) and ancient basement material; sedimentation of the Brioverian Group was influenced by a distensive tectonic regime. Towards the south, sedimentary formations are characterized by continental source areas and were laid down in a passive tectonic environment. However, REE distribution patterns for sediments of this southern domain preserve evidence for the contribution of magmatic arc material; such formations could have been deposited in a marginal basin.

A structural and metamorphic traverse across the Albany Mobile Belt, Western Australia

The Albany Mobile Belt is divided into three domains (the Southern, Central and Northern Domains) based on their aeromagnetic response and their distinctive structural and metamorphic characteristics. Amphibolite-facies rocks of the Northern Domain are reworked Archaean Yilgarn Block gneisses and dolerite dykes with a progressive, southwards increase in the intensity of deformation and metamorphism. The Northern Domain-Central Domain contact is marked by an oblique thrust zone with a component of dextral transcurrent movement developed during the second deformation event (D 2). The Central Domain consists of two-pyroxene granulites, together with quartz-magnetite gneiss, which has undergone penetrative, ductile, non-coaxial deformation with a dominant dextral transcurrent component during the first deformation event (D 1), and has subsequently been folded around kilometric-and metric-scale folds overturned to the NNW during D 2. The Central Domain grades transitionally southwards into the Southern Domain, where retrograde amphibolite facies rocks dominate. The Albany-Fraser Province is a Proterozoic mobile belt, formed during two prolonged tectonothermal events, both of which consist of a combination of dextral transcurrence and northwards thrusting.

Hydrothermal activity in the Trinity ophiolite: Water/Rock and chemical mass balances between an oceanic crust and seawater

This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and dunites crop out at Mt. Arpone and Mt. Musinè. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that <1–5% fractional melting of spinel-facies DMM produced the injected melts. Plagioclase peridotites are hybrid rocks resulting from pre-existing spinel peridotites and variable enrichment of plagioclase and micro-gabbroic material by percolating melts. The impregnating melts attained silica-saturation, as testified by widespread orthopyroxene replacement of olivine, during open system migration in the lithosphere. At Mt. Musinè, coarse granular spinel harzburgite and dunite bodies replace the plagioclase peridotites. Most of these replacive, refractory peridotites have interstitial magmatic clinopyroxene with trace element compositions in equilibrium with MORB, while some Cpx have REE-depleted patterns suggesting transient geochemical features of the migrating MORB-type melts, acquired by interaction with the ambient plagioclase peridotite. These replacive spinel harzburgite and dunite bodies are interpreted as channels exploited for focused and reactive migration of silica-undersaturated melts with aggregate MORB compositions. Such melts were unrelated to the silica-saturated melts that refertilized the pre-existing plagioclase peridotites. Finally, MORB melt migration occurred along open fractures, now recorded as gabbroic dikes.Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm.

Large‐scale response to atmospheric forcing of shelf currents and coastal sea level off the west coast of North America: May‐July 1981 and 1982

The response of alongshore currents V over the continental shelf and of coastal sea level ζ along the Pacific coast of North America to atmospheric forcing on large alongshore scales (100–2000 km) during May‐July 1981 (Coastal Ocean Dynamics Experiment (CODE) 1) and May‐July 1982 (CODE 2) has been studied using sea level and wind measurements from coastal stations and buoys between 33°N and 48°N and using current measurements from five locations between 35°N and 48°N. Characteristics of the large scale response of V and ζ to fluctuations in the alongshore component of the wind stress τ varied with latitude and with year in relation to variations in τ. In 1981, large‐scale ζ fluctuations were coherent within, but not between, separate domains north and south of 42°N. In 1982, large‐scale coherent fluctuations were found only in a northern domain that extended much farther south to near 37°N. Accordingly, in 1981, alongshore current V fluctuations from 39°N (CODE) were coherent with V from 35°N, while in 1982, V fluctuations from 39°N were coherent with those north to 48°N. In 1981 the northern domain fluctuations were forced by τ between 42°N and 45°N with southern domain forcing at 35°–36°N. In 1982, northern domain fluctuations were forced by τ over a region of much larger along‐shore extent (37°–48°N). High values of space‐lagged coherences between V and τ(y) (y is the alongshore coordinate) indicate that V at 39°N was strongly wind driven. Maximum squared coherences of V versus τ(y) are found with τ to the south by 100–300 km for low frequencies (ω ≤ 0.23 cpd in 1981, ω ≤ 0.14 cpd in 1982) and with τ from near‐local stations at higher ω, indicating the importance of remote forcing at low frequencies and local forcing at high frequencies. The response of V to τ(y) at 35°N and 43°N was more complicated, but evidence for strong wind forcing is found in several frequency bands for V at 43°N during 1981 and 1982 and at 35°N during 1981. At 48°N, V was forced in 1982 for ω ≤ 0.18 cpd by τ from locations 300–1400 km to the south.

Las unidades de la Zona Bética en la región de Aguilas-Mazarrón (prov. de Murcia)

Three geographic domains are defined in the region: Northern, Central and Southern, separated by vertical fractures trending originally ENE-WSW. Different sets of tectonic units outcrop in each of them. In the Northern Domain over the materials of the Mulhacen Nappe, three alpujarrides units have been distinguished; they are composed of a metapelitic sequence and a carbonate formation which are judged to be Pennotriassic and Triassic, respectively, in age. relative autochthon of the units appearing in the Central Domain is constituted by a tick serie of black schists and quartzites defined here as a new unit of the Veleta Group which has been named The Lomo de Bas This unit is overridden by an alpujarride unit of similar characteristics to the alpujarrides units appearing in the Northern Domain and another unit belonging to the Malaguide Complex. At the contact between these latter and the Lomo de Bas three are remnants of a sedimentary formation similar to the Conglomerate Marble Formation, and of an unit different to hose observed overriding he Veleta Group, which named Minarros Unit. From bottom to top, the Southern Domain displays two alpujarride units composed of a metapelitio sequence of micaschists, phyllites and quartzites with green schist facies metamorphisrn and carbonate formation attributed to the Triassic, Over these appears an alpujarrides unit fonned of micaschists and quartzites, presumibly of Paleozoic age, exhibiting medium-grade metamorphism. Above all this there is a malaguides unit formed of a Triassic and Jurassic series essentially composed of carbonates. Recent wrench faults trending NE-SW and NNE-s5W, are responsible for the present arc-shapped disposition of the structures.

Oxygen-isotope geochemistry of Archean granitoid gneisses and related rocks in the English River Subprovince, northwestern Ontario

Whole-rock δ 18O values for Archean granitoid gneisses from the southern domain of the English River Subprovince, northwestern Ontario, increase from an average of +6.5 for 3000 Ma old tonalites in the eastern Lac Seul area to an average of +7.9 for 2750–2800 Ma old tonalites, trondhjemites and granodiorites that occur in the Kenora and eastern Lac Seul areas. Still younger granitoid plutons and dykes (2660-2560 Ma) that intrude the older gneisses also have average δ 18O = +7.9. These values lie towards the low end of the range known for normal granitoid rocks. Oxygen-isotope data for minerals from the granitoid rocks indicate minor isotopic disequilibrium, but no evidence exists for large-scale depletion in 18O subsequent to granitoid formation. Rocks rich in 18O are abundant only in the northern domain of the English River Subprovince, which is dominated by paragneiss and metasedimentary migmatite (δ 18O +9−12). Within the Kenora area of the southern domain, however, a remnant supracrustal sequence characterized by a thin metachert/iron formation unit (δ 18O = +8−10) has been recognized. The low whole-rock oxygen-isotope values indicate that pre-existing 18O-rich supracrustal rocks were not involved in the genesis of the 3000 Ma old gneisses and have made only limited, if any, contribution to the magmas that formed the younger units. This conclusion is supported by the low initial Sr-isotope ratios of these rocks (0.7009-0.7027). The oxygen- and strontium-isotope data require that most of the granitoid rocks in the southern domain of the English River Subprovince comprise juvenile or near-juvenile additions to the continental crust.

Strain distribution over the east Mediterranean ridge: A synthesis incorporating new Sea-Beam data

We present the results of several multi-narrow-beam (Sea-Beam) traverses of the Mediterranean ridge on board R.V. “Jean Charcot”. These results can be considered to be complementary, in resolution and coverage, to the previously published long-range side-scan sonar (Gloria) results. A characteristic of the Mediterranean ridge is the presence over about half of its surface of an extensive fold system affecting the Messinian and Plio-Quaternary cover, which is dissected by conjugate strike-slip faults. The resolution of Sea-Beam was used to map the main characteristics of folding and faulting. This analysis, together with previously available data, enables us to define the strain pattern over the Mediterranean ridge. We thus now have an overall view of the strain pattern from a marginal basin (Aegea) through the Hellenic trench and adjacent sea-floor to the opposite continent. The bottom of the Hellenic trench is a boundary between an extensional domain to the north and a compressional one to the south. The southern domain is strongly affected by the presence of the Lybian promontary. It is related to the formation of an accretionary ridge due to the inability of the upper 3–4 km of sediments to subduct. This ridge is similar to the accretionary prism but it is much wider with respect to its height, probably because of the existence of a decollement over a level of overpressure. The Hellenic trench then is in the situation of a fore-arc basin. Its origin is due to the fact that the Aegean continental slope is not vertical but rather has a small 4° dip. As a result, a sedimentary wedge, symmetric but shorter than the main accretionary wedge develops over the buried portion of the slope. It is characterized by landward thrusts. Such an origin probably applies to fore-arc basins in general.

A Hittite Cylinder Seal in the Yale Babylonian Collection

Recently Professor Harald Ingholt generously donated to the Yale Babylonian Collection a Hittite cylinder seal from the empire period, which has received the inventory number YBC 16575. The piece, which is of copper and measures 24·5 × 11 mm., is in excellent condition and shows little sign of wear – see Plate XX and Figure 1. According to Professor Ingholt, he purchased the seal in the 1930s in the bazaar of Beirut, a city which lies outside of the area controlled by the Hittite empire at the greatest extent of its expansion, but which at the time of the acquisition of the seal was linked by daily train service to Adana in Cilicia, the rail line running through the heart of what had once been the southern domains of the Hittites.