Showing papers on "Continental margin published in 1993"

An indentation model for the North and South China collision and the development of the Tan‐Lu and Honam Fault Systems, eastern Asia

Abstract: Passive continental margins are geometrically irregular as a consequence of either triple-junction evolution or the development of transfer zones in detachment fault systems, whereas active continental margins are smoothly arc-shaped due to subduction of plates on the Earth's spherical surface. We propose that this basic difference in boundary geometry has played an important role in the latest Paleozoic-early Mesozoic collision of North and South China. In particular, we suggest that prior to collision, the active southern margin of the North China Block (NCB) was contiguous across the Qilian Shan, Qinling, Dabie Shan, Shandong peninsula of east central China to the Imjingang area of central Korea. The passive northern margin of the South China Block (SCB), in contrast, had a more irregular shape, such that its northeastern segment in northern Jiangsu and eastern Anhui provinces of China extended some 500 km farther north than its western counterparts in northern Sichuan, southern Shaanxi, and northern Hubei provinces. Collision of the NCB and the SCB began by indentation of the northeastern SCB into the eastern NCB in the late Early Permian and lasted until the Late Triassic-Early Jurassic. The indentation produced the left-slip Tan-Lu fault in northeastern China and the right-slip Honam shear zone in southeastern Korea and caused the northward displacement of the Shandong and the Imjingang metamorphic belts. This model predicts that collision along the Dabie and Qinling metamorphic belt occurred significantly later than along the Shandong belt, which is consistent with radiometric and depositional constraints on the time of collision. The proposed model accounts for the abrupt termination of the Tan-Lu fault at its south end and the drastic decrease in slip along the Tan-Lu fault north of the Shandong metamorphic belt. The model also predicts the distribution and ages of metamorphism along the suture and the observed local but intense Triassic deformation (=Indosinian orogeny) in northeastern China and northern Korea, which was previously an enigmatic feature in this region.

Cratons, mobile belts, alkaline rocks and continental lithospheric mantle : the Pan-African testimony

Abstract: Several late-collision and intraplate features are not entirely integrated in the classical plate tectonic model. The Pan-African orogeny (730–550 Ma) in Saharan Africa provides some insight into the contrasting behaviour of cratons and mobile belts. Simple geophysical considerations and geological observations indicate that rigidity and persistence of cratons are linked to the presence of a thick mechanical boundary layer, the upper brittle part of the continental lithospheric mantle, well attached to an ancient weakly radioactive crust. The surrounding Pan-African mobile belts, characterized by a much thinner mechanical boundary layer and more radioactive crust, were the locus of A-type granitoids, volcanism, tectonic reactivation and basin development during the Phanerozoic. During oceanic closures leading to the assembly of Gondwana, lithosphere behaviour was controlled by its mechanical boundary layer, the crust being much less rigid. We suggest that the 5000 km wide Pan-African domain of Saharan Africa, a collage of juvenile and old reactivated basement terranes, has suffered regional continental lithospheric mantle delamination during the early stages of this orogeny, as has been postulated for the more recent Himalayan orogeny in Tibet. Delamination of the continental lithospheric mantle and juxtaposition of crust against hot asthenosphere can explain many features of the late Pan-African (around 600 Ma): reactivation of old terrains, abundant late-tectonic high-K calc-alkaline granitoids, high temperature-low pressure metamorphism, important displacements along mega-shear zones and mantle-derived post-tectonic granitoids linked to a rapid change in mantle source. Recycling into the asthenosphere of large amounts of continental lithospheric mantle delaminated during the Pan-African, can provide one of the reservoirs needed to explain the isotopic compositions of ocean island basalts. Lastly, the lithospheric control over the location of the alkaline rocks enjoins us to consider the thermal boundary layer (the lower ductile part of the continental lithospheric mantle) as a major mixing source zone for these rocks.

Large igneous province on the US Atlantic margin and implications for magmatism during continental breakup

Abstract: RIFTED continental margins commonly include sections of igneous rock more than twice as thick as normal oceanic crust. Explanations for this voluminous magmatic accretion during rifting include plume models1–3, which require a deep-seated thermal or chemical anomaly in upwelling mantle, and non-plume models4–7, which call on broad, shallow thermal anomalies and/or rapid upwelling of mantle through the melting zone. New seismic models from two transects across the continent-ocean transition on the US Atlantic margin8–10 confirm the presence of a 20–25-km-thick igneous section. Here we argue that the similarity of the crustal structure on these and two previous transects, spanning 1,000 km of the margin, and the association of thick igneous crust with the East Coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3.2 × 106 km3. The distribution of volcanic and plutonic rocks, details of the seismic structure, and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non-plume processes created the thick igneous crust.

A 300 KYR Record of Upwelling Off Oman during the Late Quaternary: Evidence of the Asian Southwest Monsoon

Abstract: In the northwest Arabian Sea upwelling occurs each summer, driven by the strong SW monsoon winds. Upwelling results in high biological productivity and a distinctive assemblage of plankton species in the surface waters off Oman that are preserved in the sediments along the Oman continental margin, creating a geologic record of monsoon-driven upwelling. Sediments recovered from the Oman continental margin during Ocean Drilling Program leg 117 provide an opportunity to examine how upwelling has varied during the late Quaternary, spanning a longer interval than piston cores recovered prior to the ODP cruise. Variations in foraminifer shell accumulation and in the relative abundance of Globigerina bulloides indicate dominant cycles of variation at 1/100 kyr, the dominant frequency of glacial-interglacial variations, and at 1/23 kyr, the frequency of precessionally driven cycles in seasonal insolation. The strongest monsoon winds (indicated by increased upwelling) occurred during interglacial times when perihelion was aligned with the summer solstice, an orbital change that increased the insolation received during summer in the northern hemisphere. During glacial times upwelling was reduced, and although the precessional cycles were still present their amplitude was smaller. At both frequencies the upwelling cycles are in phase with minimum ice volume, evidence that glacial-interglacial climate changes also include changes to the climate system that influence the low-latitude monsoon. We attribute the decrease in the monsoon winds observed during glacial times to changes in bare land albedo over Asia and/or to changes in the areal extent and seasonal cycle in Asian snow cover that decrease the summer land-sea temperature contrast. Other mechanisms may also be involved. These new upwelling time series differ substantially from previous results, however the previous work relied on cores located farther offshore where upwelling is less intense and other physical mechanisms become important. Our results support the observations derived from atmospheric general circulation models of the atmosphere that indicate that both glacial boundary conditions, and the strength of summer insolation are important variables contributing to cycles in the monsoon winds during the late Quaternary.

History and modes of Mesozoic accretion in Southeastern Russia

Abstract: The history of Mesozoic accretion and growth of the Asia eastern margin, occupied by Southeastern Russia, includes five main events; two main tectonic regimes were responsible for the growth of the continent. In the Triassic-Jurassic, Early Cretaceous and Late Cretaceous-Paleogene, the subduction of the oceanic lithosphere resulted in the formation of wide accretionary wedges of the Mongol-Okhotsk, Khingan-Okhotsk and Eastern Sikhote-Alin active continental margins, respectively. These stages of the comparatively slow growth of the continent were broken by stages of rapid growth and drastic changes in the shape of the continent, since at these stages large terranes of various tectonic nature collided with active continental margins. At the end of the Early-Middle Jurassic, the Bureya terranes collided with the Mongol-Okhotsk active margin, and at the beginning of the Late Cretaceous there was collision of the Central and Southern Sikhote-Alin terranes with the Khingan-Okhotsk active margin. Collision-related structural styles in all cases are indicative of oblique collision and great strike-slip motions along the main sutures. The peculiarities of the terrane's geological structure show that prior to collision with the Mongol-Okhotsk and Khingan-Okhotsk active margins, they had already accreted to Asia and then migrated along its margins along the strike-slip faults. The Bureya terranes were squeezed out of the compression zone between Siberia and North China. This compression zone originated after the Paleozoic oceans which divided these cratons had closed. The Khanka terranes and Mesozoic accretionary wedge terranes of the Sikhote-Alin shifted along the strike-slip faults subparallel to the Asia Pacific margin. Strike-slip motions resulted in duplication of the primary tectonic zonation.

Siliciclastic sequence stratigraphic patterns in foreland, ramp-type basins

Abstract: Stratal patterns within shelf depositional sequences are dependent on tectonically controlled subsidence rates and their regional patterns. In active tectonic basin settings, regional subsidence patterns can be very different from passive continental margin settings, resulting in substantial modifications of the basic sequence stratigraphic model. In ramp-type foreland basins, on the tectonically active side, subsidence rates decrease seaward in contrast with passive continental margins, where the opposite subsidence pattern exists. Analysis of the interplay between eustasy and subsidence suggests the existence of two tectono-stratigraphic zones, occurring proximally and distally with respect to the basin margin. Zone A is defined as the region within which the rate of subsidence always exceeds the rate of eustatic fall. Consequently, relative sea level rises continuously during a eustatic cycle, albeit at varying rates. Zone B is defined as the region within which the rate of eustatic fall periodically exceeds the rate of subsidence, resulting in an interval of relative sea-level fall during a eustatic cycle. On the tectonically active sides of foreland basins, zone A lies on the landward side of the basin margin, proximal to the orogenic belt, and zone B lies seaward off zone A, away from the orogenic belt where subsidence rates are lower. On passive continental margins, because of the opposite subsidence patterns, zone A lies seaward of zone B, where subsidence rates are greater. The location of the shoreline relative to these zones determines the stacking patterns and stratal discontinuities within a depositional sequence. If the shoreline remains in zone A, then only type 2 sequence boundaries will occur; if sufficient sediment flux is available, allowing the shoreline to prograde into zone B, then type 1 sequence boundaries can occur. In foreland basins, these type 1 sequence boundaries would become type 2 sequence boundaries updip in zone A. The pattern of seaward-decreasing subsidence on the tectonically active sides of foreland basins results in characteristic longitudinal facies and stratal patterns. When the two zones occur, the updip region of the basin proximal to the orogenic belt is characterized by nearly continuous nonmarine deposition, albeit at varying rates. The downdip region is characterized by forced regressions and deposition primarily of lowstand and transgressive systems tracts. A transitional region containing deposits of all three systems tracts commonly occurs between the updip and downdip areas.

Contrasting styles of rifting: Models and examples from the Eastern Canadian Margin

Abstract: We present the results of a dynamical model of lithospheric rifting and rupture which show that a wide range of crustal thinning patterns across rifted passive margins can be produced by varying the steady state geotherm, lithospheric composition (dry versus wet materials), and strain rate. The basic mechanism of continental rupture is assumed to be passive rifting and necking. We use a numerical thermomechanical model of lithosphere extension based on a finite element approach. When plasticity is significant (i.e., at lower temperatures or for “harder” materials) deformation is unstable and thinning takes place abruptly, over a narrow area. Conversely, a progressive thinning across the margin is observed when creep is dominant (i.e., in warm or ductile conditions). Cooling and associated hardening of the thinned area can occur during extension and cause the locus of extension to migrate laterally. In these circumstances, rupture is likely to take place asymmetrically along one edge of the thinned area, producing a narrow margin and a very wide conjugate. The eastern margins of Canada and their conjugates across the North Atlantic provide examples which cover this range of theoretical profiles. The crustal thinning patterns, inferred from deep seismic data, and the duration of rifting compare well with model results. We discuss also the constraints that these geodynamical models provide on such current issues as the seismic reflectivity of the lower crust, or the location of the ocean-continent boundary in wide areas supposedly underlain by 5-km thin continental crust.

Deep and bottom water of the Weddell Sea's western rim

Abstract: Oceanographic observations from the Ice Station Weddell 1 show that the western rim of the Weddell Gyre contributes to Weddell Sea Bottom Water. A thin (< 300 meters), highly oxygenated benthic layer is composed of a low-salinity type of bottom water overlying a high-salinity component. This complex layering disappears near 66°S because of vertical mixing and further inflow from the continental margin. The bottom water flowing out of the western rim is a blend of the two types. Additionally, the data show that a narrow band of warmer Weddell Deep Water hugged the continental margin as it flowed into the western rim, providing the continental margin with the salt required for bottom-water production.

Thin crust at the western Iberia Ocean‐Continent transition and ophiolites

Abstract: Western Iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments. The age of breakup decreases from south to north. Seismic refraction and reflection profiles, and magnetic and gravity data from each segment, show a consistent pattern of geophysical observations across the ocean-continent transition (OCT) zone, which is a few tens of kilometers wide. We emphasize here the discovery of thin (2–4 km) oceanic crust underlain by 7.6 km s−1 material within the OCT. The available evidence favors the suggestion that the 7.6 km s−1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup. This thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities.

Continental magmatic underplating

Abstract: Three arguments based on geological evidence are put forward to support the importance of magmatic underplating processes during continental flood basalt vulcanism. (1) Petrological evidence of gabbro fractionation in erupted basaltic sequences allows estimates to be made of the minimum total mass of concealed cumulate material, which is retained in deep crustal magma chambers, possibly along the Moho, and is comparable in amount to the erupted material. (2) In the Karoo province (southern Africa) large volumes of rhyolite along the S.E. continental margin were generated from basaltic precursors, either as partial melts of alreadyemplaced solid basic material or as crystal fractionation products of large volumes of basic magma. In either case very substantial volumes of concealed basic rocks are at least locally implied. (3) Studies of geomorphology suggest that the area of the Karoo province experienced at least 1 km of permanent uplift associated with the vulcanism. This appears to be the consequence of the emplacement of an underplated gabbroic layer ca . 5km thick.

Late Mesozoic–Cenozoic evolution of the southwestern Barents Sea

Abstract: Three geological provinces are recognized, separated by major fault zones: the oceanic Lofoten Basin and the Vestbakken volcanic province in the west; the southwestern Barents Sea basin province; and the eastern region which has largely acted as a stable platform since Late Paleozoic times. Since Middle Jurassic times, two structural stages are recognized in the southwestern Barents Sea: Late Mesozoic rifting and basin formation; and Early Tertiary rifting and opening of the Norwegian–Greenland Sea. This evolution reflects the main plate tectonic episodes in the North Atlantic–Arctic break-up of Pangea. Middle–Late Jurassic and Early Cretaceous structuration were characterized by regional extension accompanied by strike-slip adjustments along old structural lineaments, which developed as the Bjornoya, Tromso and Harstad basins. Late Cretaceous development was more complex, with extension west of the Senja Ridge and the Veslemoy High, and halokinesis in the Tromso Basin. Tertiary structuration was related to the two-stage opening of the Norwegian–Greenland Sea and the formation of the predominantly sheared western Barents Sea continental margin. Tectonic activity shifted towards the west in successive phases. The southwestern Barents Sea basin province developed within the De Geer Zone in a region of rift-shear interaction. Initially, oblique extension linked the Arctic and North Atlantic rift systems (Middle Jurassic–Early Cretaceous). Later, a continental megashear developed (Late Cretaceous–Paleocene), and finally a sheared-rifted margin formed during the opening of the Norwegian–Greenland Sea (Eocene–Recent).

The southern West Greenland continental margin: rifting history, basin development, and petroleum potential

Abstract: The development of the continental margin of West Greenland is closely related to the processes that led to the opening of the Labrador Sea. The opening of the Labrador Sea began in the Early Paleocene (anomaly 27N), and not in the Late Cretaceous as previously supposed. Modelling of magnetic data and new interpretation of seismic data indicate that a large area previously regarded as underlain by oceanic crust is in fact underlain by block-faulted continental crust overlain by syn- and post-rift sedimentary sequences. The ocean–continent transition is now placed 100–150 km southwest of the foot of the continental slope instead of at the foot of this slope. Rifting in the Labrador Sea area began, however, in the Early Cretaceous. The earliest sediments are the syn-rift lower and upper members of the Bjarni Formation on the Labrador shelf and their likely equivalents, the pre- to syn-rift Kitsissut and Appat sequences on the Greenland margin. The age of these units is Barremian (or older) to Albian. The units are overlain by widespread mudstone-dominated units, the Markland Formation of the Labrador shelf and the Kangeq Sequence on the Greenland margin. The former is Cenomanian–Danian in age. By analogy the base of the Kangeq Sequence is probably Cenomanian (or Turonian), while the top is known from well ties to be at the Cretaceous–Tertiary boundary. Rifting was subdued during deposition of these mudstone units. Rifting was renewed in the Early Paleocene, and mudstones, siltstones and very fine sandstones were deposited. With the initiation of sea-floor spreading there was considerable igneous activity at the ocean–continent transition, as well as in the onshore area where picrites followed by plagioclase-porphyritic basalts were erupted. After the end of the Paleocene there was little rifting in the region, but compressional structures were formed locally as a response to transpression related to strike-slip movements that transferred plate motion from the Labrador Sea to Baffin Bay. A marked Early Oligocene unconformity separates the syn-drift Paleocene–Eocene succession from the post-drift middle Oligocene–Quaternary sediments. Sediments deposited since the Paleocene are dominated by sands. The main hydrocarbon play types offshore West Greenland are related to tilted fault blocks. Source rocks are anticipated near the base of the Kangeq Sequence, which is also the seal, and reservoirs are sandstones in the Appat and Kitsissut sequences. These two sequences were not reached by any of the exploration wells drilled in the 1970s.

Heavy-Mineral Assemblages of Continental Margins as Indicators of Plate-Tectonic Environments

Abstract: Heavy-mineral analyses of fifty Quaternary sediments from the North Sea, Red Sea, East China Sea, South China Sea, and Vancouver Island area (western seaboard of Canada) supplemented by over 1000 published analyses of sediments from many other sites in the world define accessory clastic mineral assemblages indicative of the principal plate-tectonic settings (excluding transform plate boundaries) associated with continental margins. Assemblages of all continental margins studied differ significantly from those of the intraoceanic, island-arc, and deep marginal-sea assemblages by possessing relatively high contents of zircon, tourmaline, garnet, epidote, amphibole (as well as other less common minerals), derived chiefly from metamorphic and sialic intrusive rocks. This suite is accompan ed by olivine, iddingsite, and brown (titanium-rich) clinopyroxene in regions containing rifting-type volcaniclastic sediments (i.e., near divergent plate boundaries), and with orthopyroxene, green clinopyroxene, and green-brown hornblende in arc-type volcaniclastic deposits (areas near convergent plate boundaries). On passive continental margins, both volcaniclastic suites are absent or present in negligible amounts.

The transition from subduction to continental collision: crustal structure in the North Canterbury region, New Zealand

Abstract: SUMMARY The North Canterbury region marks the transition from Pacific plate subduction to continental collision in the South Island of New Zealand. Details of the seismicity, structure and tectonics of this region have been revealed by an 11-week microearthquake survey using 24 portable digital seismographs. Arrival time data from a well-recorded subset of microearthquakes have been combined with those from three explosions at the corners of the microearthquake network in a simultaneous inversion for both hypocentres and velocity structure. The velocity structure is consistent with the crust in North Canterbury being an extension of the converging Chatham Rise. The crust is about 27 km thick, and consists of an 11 km thick seismic upper crust and 7 km thick seismic lower crust, with the middle part of the crust being relatively aseismic. Seismic velocities are consistent with the upper and middle crust being composed of greywacke and schist respectively, while several lines of evidence suggest that the lower crust is the lower part of the old oceanic crust on which the overlying rocks were originally deposited. The distribution of relocated earthquakes deeper than 15 km indicates that the seismic lower crust changes dip markedly near 43"s. To the south-west it is subhorizontal, while to the north-east it dips north-west at about lo". Fault-plane solutions for these earthquakes also change near 43"s. For events to the south, P-axes trend approximately normal to the plate boundary (reflecting continental collision), while for events to the north, T-axes are aligned down the dip of the subducted plate (reflecting slab pull). While lithospheric subduction is continuous across the transition, it is not clear whether the lower crust near 43"s is flexed or

Geological transect across the Northwestern Himalaya in eastern Ladakh and Lahul (a model for the continental collision of India and Asia)

Abstract: The detailed geological mapping and structural study of a complete transect across the northwestern Himalaya allow to describe the tectonic evolution of the north Indian continental margin during the Tethys ocean opening and the Himalayan Orogeny. The Late Paleozoic Tethys rifting is associated with several tectonomagmatic events. In Upper Lahul and SE Zanskar, this extensional phase is recorded by Lower Carboniferous synsedimentary transtensional faults, a Lower Permian stratigraphic unconformity, a Lower Permian granitic intrusion and middle Permian basaltic extrusions (Panjal Traps). In eastern Ladakh, a Permian listric normal fault is also related to this phase. The scarcity of synsedimentary faults and the gradual increase of the Permian syn-rift sediment thickness towards the NE suggest a flexural type margin. The collision of India and Asia is characterized by a succession of contrasting orogenic phases. South of the Suture Zone, the initiation of the SW vergent Nyimaling-Tsarap Nappe corresponds to an early phase of continental underthrusting. To the S, in Lahul, an opposite underthrusting within the Indian plate is recorded by the NE vergent Tandi Syncline. This structure is associated with the newly defined Shikar Beh Nappe, now partly eroded, which is responsible for the high grade (amphibolite facies) regional metamorphism of South Lahul. The main thrusting of the Nyimaling-Tsarap Nappe followed the formation of the Shikar Beh Nappe. The Nyimaling-Tsarap Nappe developed by ductile shear of the upper part of the subducted Indian continental margin and is responsible for the progressive regional metamorphism of SE Zanskar, reaching amphibolite facies below the frontal part of the nappe, near Sarchu. In Upper Lahul, the frontal parts of the Nyimaling-Tsarap and Shikar Beh nappes are separated by a zone of low grade metamorphic rocks (pumpellyite-actinolite facies to lower greenschist facies). At high structural level, the Nyimaling-Tsarap Nappe is characterized by imbricate structures, which grade into a large ductile shear zone with depth. The related crustal shortening is about 87 km. The root zone and the frontal part of this nappe have been subsequently affected by two zones of dextral transpression and underthrusting: the Nyimaling Shear Zone and the Sarchu Shear Zone. These shear zones are interpreted as consequences of the counterclockwise rotation of the continental underthrusting direction of India relative to Asia, which occurred some 45 and 36 Ma ago, according to plate tectonic models. Later, a phase of NE vergent `'backfolding'' developed on these two zones of dextral transpression, creating isoclinal folds in SE Zanskar and more open folds in the Nyimaling Dome and in the Indus Molasse sediments. During a late stage of the Himalayan Orogeny, the frontal part of the Nyimaling-Tsarap Nappe underwent an extension of about 15 km. This phase is represented by two types of structures, responsible for the tectonic unroofing of the amphibolite facies rocks of the Sarchu area: the Sarchu high angle Normal Fault, cutting a first set of low angle normal faults, which have been created by reactivation of older thrust planes related to the Nyimaling-Tsarap Nappe.

The ocean-continent boundary in the Gulf of Lion from analysis of expanding spread profiles and gravity modelling

Abstract: SUMMARY Two-ship multichannel seismic profiles, deep penetration (ECORS) and conventional seismic lines (LIGO surveys) are used to study the crustal structure of the Gulf of Lion (Western Mediterranean). 11 full ESPs (Expanded Spread Profiles) with total shot-receiver ranges up to 60 km were shot in 1981 perpendicular to the margin of the Gulf of Lion and in 1988 a deep MCS seismic profile (ECORS-CROP program) was performed parallel to the ESPs. These ESPs were analysed by matching traveltime and amplitude variations in both the x-t and τ-p domains. The resulting P-wave velocity/depth model has the following features, (a) beneath the continental slope of the Provencal margin a rapid rise of the Moho from 20 to 14 km and the existence of an anomalous 7.2-7.4 km s-1 velocity layer, (b) from the base of the slope to the extensive salt-domes domain a 5-6 km thin crust which does not appear typically oceanic in nature, (c) quite typical oceanic crust up to the Sardinian margin. Gravity modelling is consistent with the seismic results. The OCB (ocean-continent boundary) could be placed north of that postulated by previous authors, where the data indicate a remarkably narrow transition between continental and ‘oceanic’crust, or south where a typical oceanic crust, which correlates well with the domain of the salt domes and of large magnetic anomalies, has been determined. A very prominent reflector is clearly seen, at the base of the continental slope, on the seismic reflection profiles and corresponds to the top of an 7.2-7.4 km s-1 velocity layer. The high-velocity layer is 2-3 km thick where the crust is thinnest and has a limited lateral extent seawards. This anomalous crustal structure could be the result of extremely thinned and possibly broken up, continental crust underplated and intruded by partial melt, or could represent serpentinized peridotite material. Important questions about the evolution of the Gulf of Lion cannot be addressed using these new results alone without addition of other constraints. Nevertheless a two-stage mechanism of drifting and rifting of this part of the Western Mediterranean Sea is proposed.

Emplacement and deformation history of the western margin of the Idaho batholith near McCall, Idaho: Influence of a major terrane boundary

Abstract: Cretaceous plutons of the western margin of the Idaho batholith were emplaced along and to the west of the major terrane boundary separating middle Proterozoic and Paleozoic continental rocks from mostly Mesozoic accreted oceanic-arc terranes of the Blue Mountain Province. This boundary is marked by a change in the lithology of pendants and inclusions within the batholith. Plutons form two newly named complexes of igneous and metamorphosed igneous rocks. The Hazard Creek Complex, emplaced west of the boundary between the oceanic arc and the continental margin, consists primarily of a series of variably deformed and metamorphosed quartz diorite to trondhjemite plutons. The Little Goose Creek Complex, which intruded the boundary between the oceanic arc and the continental margin, is primarily porphyritic granodiorite to granite orthogneiss. A preliminary U-Pb age of 111 Ma for this porphyritic orthogneiss is a minimum age for the formation of the oceanic-arc-continent boundary. The plutonic rocks were deformed both during and after emplacement in response to east-west compressive stresses. Cretaceous deformation was localized along the boundary between the accreted terranes and the continental margin and is interpreted to have occurred after the formation of this boundary. The major deformation of the Hazard Creek Complex occurred during its emplacement. The dominant fabric in the Little Goose Creek Complex is due to subsolidus ductile deformation. The localization of two deformation events along the pre-existing boundary between the accreted terranes and the continental margin suggests that a terrane boundary may form a long-lasting, crustal flaw.

A Late Palaeozoic‐Early Mesozoic marginal basin along the actives sourthern continental margin of Eurasia: Evidence from the central Pontides (Turkey) and adjacent regions

Abstract: Remnants of two ‘Palaeotethyan’ oceanic basins are exposed in the Central Pontides of northern Turkey, separated by a continental sliver and an oceanic arc. The southern basin corresponds to the main Tethys (‘Palaeotethys’), which partially closed in Early Mesozoic time following northward subduction under the southern, active continental margin of Eurasia. The northern basin (Kure Complex) opened above the ‘Palaeotethyan’ subduction zone as a marginal basin, following rifting of a continental fragment (Istanbul fragment) from Eurasia. Marginal basin opening apparently dates from the Late Palaeozoic in the east (Kure basin) and from the Triassic in the west (Kocaeli basin). Basin closure was achieved by southward subduction-accretion, in pre-Late Jurassic times, leaving ‘Neotethys’ open to the south. Counterparts of the Kure Complex are found in the adjacent Crimea (Taurian Series), Istranca (Zabernevo Complex), Dobrogea (Nalbant flysch) and Caucasus (pre-Late Jurassic Southern Slope Basin) regions. Basin opening was accompanied by oceanic crust genesis, at least in the Pontides and Caucasus. Closure before Mid-Jurassic time was achieved by subduction-accretion processes, whereby oceanic crust and deep-sea sediments (including sulphides) were detached and structurally assembled, while oceanic basement was subducted. Marginal basin opening and closure is seen as one in a series of events along a long-lived, active south Eurasian continental margin.