Distal Continental Margin Research Articles

Dinoflagellate cysts as proxies of environmental, ocean and climate changes in the Atlantic realm during the quaternary

Over the last four decades, organic-walled dinoflagellate cysts have shown high potential as tracers of past sea-surface conditions during the Quaternary. These microfossils relate to the pelagic productivity of both phototrophic and heterotrophic protist organisms and are recovered in high numbers in almost all marine environment settings from the nearshore and estuarine systems to the distal continental margin. In polar environments, where other conventional proxies are rare or absent, dinoflagellate cysts showed a relatively high diversity of species and a close relationship with sea-ice cover duration, winter and summer temperature, and salinity, enabling quantitative reconstructions of several oceanic variables over time. From the temperate to the tropical latitudes of the Atlantic Ocean, their modern distribution highlights a response to primary productivity and seasonal contrasts in surface temperature. They also have proven that they could be used as tracers of eutrophication in stratified systems and can also highlight human impact on their distribution. In this paper, we present an overview of dinoflagellate cysts as ecological tracers in recent and past sediments of the Atlantic Ocean, from the Arctic to the Antarctic. We provide examples of their use as proxies in paleoclimatic-palaeoceanographic studies at glacial to interglacial time scales, with emphasis on the last ice age to recent (last 25 kyr), the northern North Atlantic and western-eastern tropic North Atlantic. We also discuss their potential as tracers of anthropogenic stress in coastal environments.

Pre-collisional architecture of the European distal margin: Inferences from the high-pressure continental units of central Corsica (France)

Abstract The Lower Units of Alpine Corsica, France, are fragments of continental crust strongly deformed and metamorphosed under high-pressure metamorphic conditions. Three slices of Lower Units are well exposed in the area between the Asco and Tavignano valleys, Central Corsica. Despite their complex structural setting, they provide the opportunity for a reconstruction of the pristine stratigraphic setting of the Lower Units. In our reconstruction, these units consist of a Paleozoic basement topped by Triassic to Early Jurassic sedimentary rocks unconformably covered by Middle to Late Eocene foredeep deposits. However, the three units exposed in the study area display strong differences mainly in the thickness of the Mesozoic sequence. These differences are here interpreted as acquired during the first stage of the rifting process in a setting controlled by normal faults. During the collision-related tectonics and the accretion of the Lower Units to the Alpine orogenic wedge, these normal faults were probably reactivated with a reverse kinematics. The stratigraphic logs of the Lower Units strictly resemble those of the Pre-Piedmont Units from Western Alps. This similarity indicates a common origin of the Lower Units and the Pre-Piedmont Units from the same domain (i.e., the European distal continental margin).

Basin filling and magmatic response to the migration of two-stage rifts: New insights into the late Neoproterozoic tectonics of the northern Tarim Craton

The spatiotemporal evolution of late Neoproterozoic sedimentary basins and their tectono-magmatic context provide essential information regarding the continental rift process of the Tarim Craton during the fragmentation of the Rodinia supercontinent. However, the incomplete exposure of strata and magmatism in the distal continental margin hinders the understanding of different rift phases that trigger continental breakup. The Sawapuqi region is currently located at the northernmost margin of the Tarim Craton and presents an uninterrupted sedimentary sequence and volcanic rocks from the latest Precambrian to the Cambrian period. Herein, we perform an integrated analysis of the Sawapuqi sub-basin and provide critical insights into the Ediacaran stratigraphic and tectono-magmatic evolution that shed light on the final rifting phase. The Sawapuqi sub-basin preserves regressive and transgressive successions, ranging from thick-bedded siltstones and siliceous rocks at the lower unit of the Sugetbrak Formation, through shore gravel-bearing laminated sandstone accumulation (upper unit), and restricted platform stromatolitic carbonates (Qigebrak Formation), to Cambrian mature passive margin sedimentation. An alkaline basaltic layer is interbedded within the coarsening-upward sedimentary packages of the early Ediacaran, recording the final rift-related magmatic pulse in the northwestern Tarim margin. The geochemical signatures of these basalts suggest a relatively shallow-depth derivation from the enriched mantle reservoir in an intraplate setting. The detrital zircon provenance of the coarse-grained siliciclastic to pebbly feldspathic debris sandstone sequence of the upper Sugetbrak Formation indicates the hybrid sediment infills from local metamorphic basements and Neoproterozoic igneous rocks, with progressively increased contribution from the coeval granites in the northern periphery of the Tarim Craton. The stratigraphic evolution from syn-rift clastic rocks to post-rift carbonates and the transition of detrital provenance observed in the Sawapuqi sub-basin suggest the development of a breakup succession at the end of the early Ediacaran period. The margin-localized continental extension and mantle-derived volcanism during the early Ediacaran differed from the distributed extension and bimodal magmatism during the Cryogenian, indicating two-stage extensional modes and subsidences prior to the lithospheric breakup. The rift system of the latter stage, which migrated oceanwards, eventually led to the continental breakup in the northwestern margin of the Tarim Craton. Comparable late Neoproterozoic stratigraphy and magmatic pulses between the Tarim Craton and Yili-Central Tianshan Block (YCTB) support their reconstructions as two conjugate rifted margins. This study indicates that the rift-drift tectonics triggered by this prolonged episodic rifting between the Tarim Craton and the YCTB was closely associated with the final fragmentation of Rodinia.

Permian to Triassic protolith ages of type locality eclogites in the Eastern Alps: Implications for the opening of the Meliata back-arc basin

Abstract The Austroalpine domain contains the type locality of eclogites, but their protolith age is unknown except for a Permian metagabbro. Therefore, we studied the non-gabbroic eclogites from the Saualpe-Koralpe Complex (SKC) representing meta-basalts within a coherent continental rift fragment subducted during the Cretaceous. Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating revealed protolith ages of 283 ± 5 Ma, 255 ± 3 Ma, 251 ± 3 Ma, and 241 ± 3 Ma. Magmatic zircons exhibit 176Hf/177Hf ratios of 0.282935–0.283090 and juvenile εHf(t) values of +10 to +17.4. Typical normal mid-oceanic ridge basalt (N-MORB) geochemistry is established. The SKC host metasedimentary rocks are interpreted as Variscan continental crust close to the margin of the oceanic Meliata basin and were affected by Permian metamorphism. Metamorphic zircon of one SKC eclogite yields an age of 87–93 Ma. These results combined with previous data are used to present a new model for the tectonic evolution of the distal Austroalpine unit associated with the Meliata Ocean in a Wilson cycle: The SKC represents a distal continental margin during Permian to Middle Triassic rifting, which was intruded by few gabbro bodies and numerous basaltic sills and mostly Permian pegmatites. In contrast, structurally separated thick Triassic sedimentary cover successions of the Austroalpine domain lack any magmatism, excluding SKC as Triassic basement of the sedimentary cover successions. The present eclogite-bearing piece of continental crust adjacent to the Meliata oceanic lithosphere was subducted to mantle depth during Late Cretaceous and then exhumed.

Protolith and metamorphic age of the Sieggraben Eclogites: Implications for the Permian to Cretaceous Wilson cycle in the Austroalpine unit

The Austroalpine eclogite-bearing Sieggraben Complex, exposed at the eastern margin of the Eastern Alps, is considered to represent a subducted and then exhumed fragment of the continental rift, which led to the formation of the Meliata oceanic basin. Combined zircon U-Pb dating, whole rock geochemistry and Hf isotope analysis of the Sieggraben Complex revealed that the eclogite sample shows a protolith age of 242.3 ± 2.6 Ma (Middle Triassic) and displays tholeiitic basalt/N-MORB geochemical characteristics. Associated ultramafic rocks as part of oceanic or subcontinental mantle lithosphere suggest a depleted mantle source and a deep subduction environment. Two zircon grains of eclogites with low Th/U ratios yield ages of 113 ± 2 Ma and 86 ± 4 Ma and represent the approximate age of eclogite metamorphism during Cretaceous. A trondhjemite dike cutting through eclogite gives a crystallization age of 82.19 ± 0.4 Ma and formed by partial melting during decompression. The host metasedimentary rocks are interpreted as old continental crust close to the margin of the Meliata basin were affected by Permian migmatitic metamorphism. The results of this study combined with previous results from the Sieggraben Complex are used to present an updated model for the tectonic evolution of the distal Austroalpine unit associated with the oceanic Meliata relics The Austroalpine Sieggraben Complex represents a location on the distal continental margin during Permian rifting. This piece of continental crust of an inferred paleotectonic position adjacent to the Meliata oceanic lithosphere subducted during Early Cretaceous times to mantle depth. The subducted continental crust was then exhumed incorporating even ultramafic mantle rocks. During exhumation and decompression of mafic rocks, partial melting took place forming the trondhjemite dike in Late Cretaceous times. • A Middle Triassic ptotolith age was found for eclogites of the Sieggraben Complex. • A Cretaceous-aged trondhjemite dike report formed by decompressional melting after eclogite metamorphism. • Paragneiss hosts a Panafrican detrital zircon memory and was affected by Permian migmatization. • A new tectonic model for the distal Austroalpine unit relates with the Meliata Ocean in a Wilson cycle.

Triple-stage India-Asia collision involving arc-continent collision and subsequent two-stage continent-continent collision

The enigmatic geodynamic processes involved in the India-Asia collision shape our understanding of uplift and deformation of the Tibetan Plateau and the subsequent changes in the land-sea distribution pattern, monsoon-arid environments and biotic evolution in Central-Eastern Asia. However, the current geodynamic models dealing with the India-Asia collision history exhibit significant discrepancies in how, when and where exactly the collision of India with Asia occurred. Recently, we proposed a new hypothesis that favored the opening and closure of a North India Sea during the late Cretaceous to Paleogene. Here we present new additional paleomagnetic data from the red siliceous shales and cherts of the Sangdanlin Formation, which were deposited on the most distal continental margin of the Tethyan Himalaya terrane, i.e. the southern margin of the Neo-Tethys Ocean, during the middle Paleocene. The combination of our new and previously reported paleomagnetic data shows that the Tethyan Himalaya was at a paleolatitude of 14.1° ± 1.9°N during the interval 62.5–59.2 Ma and refines the maximum size of the North India Sea to about 2200 ± 500 km. Combining our geodynamic model with recently published paleomagnetic data from the Kohistan-Ladakh arc, we conclude that the India-Asia collision was a triple-stage process, which involved an arc-continent collision followed by a two-stage continent-continent collision. The arc-continent collision occurred at ca. 64 Ma between the Tibetan Himalaya (Tethyan Himalaya plus Greater Himalaya) and the Trans-Tethyan subduction zone at a paleolatitude of ~8°N. The initial continent-continent collision took place at ca. 61 Ma between the Tibetan Himalaya (plus the accreted Trans-Tethyan subduction zone) and Lhasa terranes at a paleolatitude of ~14°N. The final continent-continent collision occurred during 53–47 Ma between India and the Tibetan Himalaya, diachronously suturing the North India Sea from west to east. The triple-stage collision scenario is in agreement with the history of India-Asia convergence rates, and is reflected by the presence of three slab remnants below Tibet and India as documented by seismic tomographic imaging. Our updated collision scenario reconciles multidisciplinary evidence for the India-Asia collision and provides better constraints on the resulting paleoclimatic and paleoenvironmental changes in Central-Eastern Asia. • The Tethyan Himalaya was paleomagnetically constrained to a paleolatitude of 14.1° ± 1.9°N at ca. 61 Ma. • The arc-continent collision between the Tibetan Himalaya and the Trans-Tethyan subduction zone was estimated to be ca. 64 Ma. • The India-Asia collision was a triple-stage process.

Palaeoenvironmental context and significance of ferruginous tubular bioforms and other authigenic mineral formations in source-to-sink sedimentary systems

Surficial sediments on the seafloor from passive continental margins can provide insight into recent Late Quaternary sedimentary dynamics acting over offshore sedimentary systems. This work focuses on the study of some particular ferruginous tubular structures resembling bioforms (FTB) located in the distal Galician Continental Margin (NW Iberian Margin) at water depths between ~ 1550 and ~ 2200 m. The characterisation of these structures made it possible to study in depth their formation environment and subsequent sedimentary evolution during the Late Pleistocene and Holocene. The FTB consist of goethite with a framboidal texture. They were interpreted as formed by an initial pyrite precipitation in reducing microenvironments conditioned by the activity of sediment-dwelling organisms during the early diagenesis. This is followed by the oxidation of pyrite by a combination of hydrothermal fluids and erosional processes, which triggers the formation of the framboidal oxyhydroxides. The data allowed obtaining a comprehensive understanding of the environmental context and the significance of these ferruginous tubules, as there are no previous studies in the scientific literature that describe these structures in a source-to-sink sedimentary system.

Ediacaran ophiolite relics in the SE Brazilian coast: Field, geochemical and geochronological evidence from metabasites and paragneisses

Metabasites and paragneisses from the high-grade Ediacaran-Cambrian Cabo Frio Tectonic Domain, SE Brazilian coast, are investigated with field, geochronological (U-Pb combined with Lu-Hf), isotopic (Sm-Nd) and geochemical analyses. These units, tectonically interleaved with a Paleoproterozoic basement, record the pre-collisional setting within the eastern Ribeira Orogen. The four analyzed metabasite bodies indicate tholeiitic basalts/gabbros with OIB affinity as protoliths tending to high-Mg terms, also corroborated by their higher LILE, LREE and HFSE contents, in comparison to the E-MORB and N-MORB basalts. The Nb and Ta (Nb/Yb – alkaline proxy) and Ti/Yb (plume-melting proxy) resemble present-day OIBs values. One metabasite provided mainly metamorphic zircon crystals (n = 20) with a U-Pb age of 530 Ma. Only three crystals show oscillatory zoning, revealing ages of 1890 Ma, 676 Ma and 580 Ma. The Paleoproterozoic zircon, interpreted as a xenocryst, and the Epsilon Nd values (+3 and −6) for whole rock indicate crustal contamination. U-Pb data from detrital zircon of two paragneiss layers, structurally below and above the metabasite, constrain maximum depositional ages of 626 Ma and 549 Ma, respectively. The older has a major 750-620 Ma zircon source, similar to the age of Cryogenian magmatic units in the African terranes. The sediments of this layer are here interpreted as deposited in a distal continental passive margin, coeval with the OIB magmatism (metabasites) crystallized near or at the Ocean-Continent Transitional zone. The upper paragneiss layer shows a major juvenile Ediacaran detrital zircon source between 620 and 550 Ma, with mainly positive Epsilon Hf values (0 to +13). It also presents disrupted blocks of the metabasites, forming a sedimentary breccia. This latter evidence, in addition to its medium P-high T metamorphic paragenesis, supports the hypothesis of an accretionary prism system. The metabasites are here interpreted as ophiolite relics from an Ediacaran Ocean that predated the Gondwana amalgamation. The subsequent ca. 550-530 Ma continental collision preserved this tectonic sliver between the Paleoproterozoic continental crust, below, and the Ediacaran forearc sedimentary sequence, above (Palmital Succession). The formed Cambrian suture was later reactivated as one of the Cretaceous rift branches that evolved to the South Atlantic.

Evolution of salt structures of the Pyrenean rift (Chaînons Béarnais, France): From hyper-extension to tectonic inversion

The Chaînons Béarnais is a salt-detached fold belt in the northern Pyrenees that formerly occupied the axis of the Cretaceous Pyrenean rift. Geological map revision and cross-section construction from surface geology and industrial well and seismic reflection data emphasize the role of salt diapirism in the folding of the belt during the Cretaceous extension and the subsequent Pyrenean orogeny. Pre-rift Triassic evaporites played a fundamental role during rifting, allowing the sedimentary basin lying above to detach and slide down the hyper-extended margins onto a central exhumed mantle tract. Since the Early Cretaceous (and locally probably since the Jurassic) a system of low-amplitude salt walls evolved in shallow marine environments punctuated by episodic emersion. During the main stage of crustal extension, in late Aptian to early Cenomanian times, carbonate shelves rapidly drowned giving rise to deeper marine sedimentation. This was a period of major rise of salt walls, progressively detached from their substratum. These salt walls enclosed minibasins that accumulated thick flysch deposits arranged in growth stratal patterns. Depocenter migration and foundering of previous diapiric highs controlled further flysch deposition during the Late Cretaceous, while moderate extension probably persisted until the onset of the Pyrenean compression. During the late Santonian to Paleogene Pyrenean orogeny, the sedimentary lid of the Chaînons Béarnais basin climbed back along the Triassic detachment onto the colliding continental margins, leading to salt wall squeezing and further rising. Based on the Cretaceous timing and style of growth folding, we suggest that salt wall squeezing was not solely related to the Pyrenean compression, but shortening affected the diapiric ridges during the syn-rift sliding by gravity and crowding in the basin center, as the rifted margins were pulled apart from beneath. This makes the Chaînons Béarnais belt a unique field analog for contractional salt-tectonic systems in distal continental margins.

Cretaceous and Eocene tectono-thermal events determined in the Inner Western Carpathians orogenic front Infratatricum

This paper re-assesses the tectono-thermal evolution of the frontal Infratatricum part of the Inner Western Carpathians orogen based on new geological-petrological and zircon fission track (ZFT) data and published 40Ar–39Ar and K–Ar data. The study area is in the Považský Inovec Mts. in Western Slovakia, where the Infratatric Inovec Nappe comprises a micaschist-gneiss basement and Upper Carboniferous–Lower Cretaceous cover with the Jurassic–Lower Cretaceous Humienec Succession; this latter is reconstructed from olistoliths in the Horne Belice Succession Upper Cretaceous flysch. The Infratatric Inovec Nappe posterior part is thrust over its frontal Humienec tectonic Slice with infolded Upper Cretaceous sediments. This nappe exhibits very low-temperature tectono-thermal overprinting, and this is documented by electron probe microanalysis of metamorphic phases and P–T estimates. The late Early Cretaceous age of this D1 stage event at approximately 115–95 Ma is recorded by 114 ±2 and 106 ±4 Ma phengitic white mica 40Ar–39Ar ages from a Lower Cretaceous cherty slate, and by 101 ±3 Ma 40Ar–39Ar age and 102±4 Ma ZFT age from Permian meta-sandstone. The Inovec Nappe was thus derived from an Albian–Early Cenomanian accretionary wedge. The hemipelagic Upper Turonian to Lower Santonian red marls and upward syn-orogenic Upper Santonian to Maastrichtian flysch were deposited on the frontal attenuated part of the Inovec Nappe-type basement in the inferred Pieninic–Vahic (~South-Penninic) zone. This flysch contains olistolithic to clastogeneous metamorphosed material, including the Lower Cretaceous slates, supplied from the posterior part of the Inovec Nappe. The 40Ar–39Ar age of 86 ±2 Ma from this nappes’ Permian meta-sandstone is consistent with D2 thrusting at approximately 95–85 Ma and Horne Belice foreland flysch basin supply in the D3 stage at ~85–65 Ma. The Inovec Nappe frontal Humienec tectonic Slice with infolded Upper Cretaceous sediments suggests formation of a Paleocene–Eocene accretionary wedge at approximately 65–40 Ma in the D4 stage. This event is constrained by 40Ar–39Ar age of 48 ±2 Ma from the Tatricum hanging wall blastomylonites and whole-rock K–Ar age of 46 ±3 Ma from a Permian basalt olistolith in the footwall Infratatricum Horne Belice flysch. The Infratatricum finally became part of an Eocene accretionary wedge north of the Hradok–Zlatniky thrust-fault, and reheating and exhumation cooling is documented by 57–37 Ma ZFT ages. Although the Infratatricum exhibits Late Cretaceous and Eocene tectono-thermal imprints, it is a remnant of the Early Cretaceous structure at the Inner Western Carpathian front. This imposes the Infratatricum as a distal continental margin of the Penninicum. Normal 0 21 false false false PL X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:Standardowy; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin-top:0cm; mso-para-margin-right:0cm; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:12.0pt; font-family:Arial,sans-serif; mso-bidi-font-family:CG Times 12pt; mso-fareast-language:EN-US;}

Middle and Late Jurassic radiolarians from the Neotethys suture in the Eastern Alps

AbstractCherty limestones, marls and radiolarites are widespread in the Hallstatt Mélange of the Northern Calcareous Alps. The mélange was formed during the Neotethyan orogeny in a series of deep-water basins that progressively developed in front of the advancing nappe front. The low thermal overprint of these rocks favors the good preservation of radiolarians (polycystines) that have been used for dating and reconstructing the Jurassic tectonostratigraphy of the area. This paper describes rich Middle–Late Jurassic radiolarian faunas from four localities in the Hallstatt Mélange near Bad Mitterndorf in Austria. Two different successions, both spanning from the Bathonian to the Oxfordian, are dated. In the first succession, the radiolarites are intercalated between or occur as matrix in mass-flow deposits originating from the accretionary wedge. The second succession is nearly 100 m thick but is devoid of mass-flow deposits and documents a continuous radiolarite deposition in greater distance from the nappe stack. Both successions are ascribed to the Sandlingalm Basin, which evolved on a relatively distal continental margin during early stages of the orogeny.The highly diverse and well-preserved radiolarian assemblages have been used for a detailed taxonomic study. Two new families are described: Minocapsidaen. fam. andXitomitridaen. fam.; six new genera are describedDoliocapsan. gen.,Crococapsan. gen.,Parvimitrellan. gen.,Xitomitran. gen.,Campanomitran. gen., andMizukidellan. gen. In addition, one new replacement name,Takemuraella(proTriversus), is introduced, the diagnoses of 6 genera are emended, and two new species are described:Hemicryptocapsa nonagintan. sp. andMizukidella mokaensisn. sp.

Palynological evidence for sustained deep-marine conditions during the Eocene–Miocene in the southern Gulf of Mexico distal continental margin

The present study represents the first attempt to provide palynological data about the southern Gulf of Mexico distal continental margin during the Eocene−Miocene time interval. In addition to documenting the types of fossil palynomorphs preserved in these economically critical rock sequences, this study also provides important information about the accumulation and preservation of total sedimentary particulate organic matter along with their paleoenvironmental and paleoceanographic implications. This was accomplished through the analysis of cored sediments from the Deep Sea Drilling Project Site 94 north of the Yucatan Peninsula and Site 540 south of the Western Florida Escarpment. The present findings from the deep waters of southern Gulf of Mexico are expected to greatly assist hydrocarbon geoscientists in their exploration and development efforts, reducing investment risk and drilling uncertainty. Two palynofacies assemblages were recognized based on particulate organic matter analysis: marine-dominated palynofacies A and terrestrially influenced palynofacies B. The organic compositions of these two palynofacies along with the recovered dinoflagellate cyst community suggest that while sustained oceanic depositional conditions were prevalent, occasional influxes of terrestrial organic material of variable magnitudes occurred. These short-term influxes were especially pronounced during the late Eocene and earliest Oligocene and appear to have happened in response to the cooling and slight global sea-level drop that occurred at ca. 38−33.5 Ma. The preserved terrestrial palynomorphs in the samples indicate coastal plain and estuarine vegetation sources in general, except during the Eocene-Oligocene transition, when anemophilous pollen specimens mostly derived from farther inland and montane sources were recovered.

Late Norian (Late Triassic) Radiolarian assemblages from chert pebbles of the Pentalofos Formation (Mesohellenic basin, Greece). Palaeogeographic implications

Abstract Well-preserved Late Norian Radiolaria were extracted from two pebbles of radiolarian chert included in Lower Miocene conglomerates of the Pentalofos Formation, in the area of Meteora (Mesohellenic basin, Greece). Both samples yielded a similar assemblage characterized by the presence of species Betraccium deweveri PESSAGNO & BLOME, Bipedis acrostylus BRAGIN, Pylostephanidium ankaraense BRAGIN & TEKIN and several other taxa typical of the Betraccium deweveri zone (uppermost Norian). This is the first evidence of Upper Norian radiolarian cherts from Greece. It is likely that the studied chert pebbles were derived from ophiolitic complexes or distal continental margin series situated to the east of the Mesohellenic basin; they were part of the Upper Norian oceanic realm that was obducted, during the Late Jurassic, over the Pelagonian continental crust. During the Late Miocene they were still present east of Meteora and they have been eroded since.

Crustal accretion in the Manila trench accretionary wedge at the transition from subduction to mountain-building in Taiwan

New marine seismic reflection and coincident wide-angle ocean-bottom seismometer data acquired offshore Taiwan provide high-resolution constraints on the crustal structure of an incipient mountain belt during the earliest stage of arc–continent collision. The new seismic reflection image and travel-time tomography velocity model show evidence for crust of the distal southern Chinese continental margin being subducted eastward beneath the Manila trench and underplated to the accretionary wedge before collision with the southern Chinese continental shelf. The distal margin crust consists of highly extended continental crust interspersed with volcanic bodies and a high-velocity lower crustal layer of likely magmatic intrusions. The distal margin crust is 10–14km thick outboard of the trench, but thins to 6km thick beneath the lower slope of the Manila trench accretionary wedge. Along the lower slope of the accretionary prism, we image westward-verging imbricate thrusts and folded strata up to 10km thick. A sharp decrease in bathymetry marks the transition from lower to upper slope, where we observe a fast (>6.0km/s) seismic velocity anomaly at the base of the wedge that we interpret as structurally underplated crust from the distal continental margin. Our results support a model of arc–continent collision in Taiwan where the accretionary wedge is first thickened by structural underplating of distal margin crust prior to collision with the continental shelf. The crustal rocks exposed throughout the Central Range in Taiwan may be similarly derived from subducted and structurally underplated crust from the highly extended distal continental margin.

Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins

Abstract. Past characterizations of the land–ocean continuum were constructed either from a continental perspective through an analysis of watershed river basin properties (COSCATs: COastal Segmentation and related CATchments) or from an oceanic perspective, through a regionalization of the proximal and distal continental margins (LMEs: large marine ecosystems). Here, we present a global-scale coastal segmentation, composed of three consistent levels, that includes the whole aquatic continuum with its riverine, estuarine and shelf sea components. Our work delineates comprehensive ensembles by harmonizing previous segmentations and typologies in order to retain the most important physical characteristics of both the land and shelf areas. The proposed multi-scale segmentation results in a distribution of global exorheic watersheds, estuaries and continental shelf seas among 45 major zones (MARCATS: MARgins and CATchments Segmentation) and 149 sub-units (COSCATs). Geographic and hydrologic parameters such as the surface area, volume and freshwater residence time are calculated for each coastal unit as well as different hypsometric profiles. Our analysis provides detailed insights into the distributions of coastal and continental shelf areas and how they connect with incoming riverine fluxes. The segmentation is also used to re-evaluate the global estuarine CO2 flux at the air–water interface combining global and regional average emission rates derived from local studies.

From embryonic tectonics and geosynclines to modern concepts of rifting, spreading, and Alpine orogenesis: a summary of contributions to the 9th Alpine Workshop (Cogne, 2009)

The 9th Alpine Workshop was held in September of 2009 in the village of Cogne (Italy) amid, the spectacular geology and scenery of the Western Alps. This volume of the International Journal of Earth Science comprises a selection of papers read at the workshop, which in keeping with the workshop’s tradition, encompassed a broad range of themes on the processes and evolution of Alpine-type mountain belts. The workshop itself was dedicated to the memory of Rudolph Trumpy (1921–2009), a key figure in Alpine Geology and a frequent participant of past Alpine workshops. The keynote paper by Sengor and Bernoulli reviews Trumpy’s scientific oevre in the context of his time, especially his landmark paleogeographic reconstructions of Alpine Tethys and his early recognition of the role of strike-slip tectonics in the evolution of the Alps. Beyond these contributions, their paper provides insight into how Trumpy inspired colleagues and students alike to look carefully at rocks and to think critically about the geologic record, even as he acknowledged the need for a deductive approach to further our understanding of orogenesis. One of Trumpy’s many insights was that Alpine Tethys resulted from Mesozoic extension (rather than shortening) and that the structure of the adjacent European and Adriatic continental margins did not coincide with the impressive nappe structures that formed later during Alpine subduction and collision. It is therefore fitting that several papers in this volume deal with this early evolution and its effect on Alpine orogenic structure. Mohn, Manatschal, Massini, and Muntener present a revised interpretation of Austroalpine units that represent the distal Adriatic continental margin as preserved at the junction of Eastern and Central/ Western Alps in eastern Switzerland. In a series of E-W oriented palinspastic reconstructions, the authors show how Early Mesozoic rift-related structures of this margin determined the complex sequence of nappe stacking during late Cretaceous east–west shortening. Further complications arose during Tertiary north–south shortening, yet the original pre-orogenic structure of the passive margin is shown to be remarkably well preserved. In the Western Alps, Loprieno, Bousquet, Bucher, Ceriani, Fugenschuh, and Schmid present new structural and stratigraphic evidence for the existence of Valais oceanic lithosphere in Cretaceous time. Internal Valais units (Versoyen ophiolites and underlying magmatic and sedimentary rocks) are shown to have originated at the ocean–continent transition in contrast with external Valais units that did not have an oceanic lithosphere and probably derived from the European distal continental margin. This interpretation will be controversial to some Alpine geologists, as it refutes previous interpretations of the ‘‘Versoyen ophiolites’’ as a Carboniferous-age complex in tectonic (rather than stratigraphic) contact with post-rift sediments of the internal Valais unit. The Early Mesozoic paleogeography of the eastern part of the European margin of Alpine Tethys has been relatively poorly constrained until now. By careful dating of pre-Mesozoic basement rocks in the western part of the Tauern Window, Vesela, Sollner, Finger, and Gerdes correlate the sub-Penninic ‘‘Zentral Gneiss’’ there with the External basement Massifs in the Western Alps of Switzerland and France. Synsedimentary rhyodacitic and rhyolitic horizons (310–304 Ma) in these units along the Alpine chain are interpreted to mark the onset of sedimentation in a post-Variscan extensional basins, just prior to the onset of Early Perman intracontinental rifting. M. R. Handy C. L. Rosenberg (&) Freie Universitat Berlin, Berlin, Germany e-mail: cla@zedat.fu-berlin.de

Mantle exhumation and lithospheric spreading: An historical perspective from investigations in the Oceans and in the Alps-Apennines ophiolites

This historical perspective provides an overall view of the main steps that led to the acceptance of the concept of mantle exhumation accompanying lithosphere stretching and extreme crustal thinning. We first remember that the presence of exposures of mantle rocks along oceanic spreading ridges was early reported in the 60's due to the results of sediment cores and dredge hauls collected along the Mid-Atlantic Ridge. In the meantime, detailed analysis of the relationships between oceanic sediments of Late Jurassic age and their primary basement in the Apennine ophiolites led many authors to point to the importance of exhumation of mantle rocks on ancient seafloors. By contrast to the thick sections of classical ophiolites (e.g. Oman), the Alpine-Corsican-Appennine ophiolites are characterized by a relatively small amount of mafic rocks (gabbros and basalts), by the absence of any sheeted dyke complex and by the frequent occurrence of oceanic sediments stratigraphically overlying mantle-derived peridotites and associated gabbroic intrusions. They display some of the characteristics of slow-spreading ridge systems, but, due to their highly tectonized character, they have been interpreted successively either as remnants of oceanic fracture zones of «normal» ocean or as remnants of very-poorly organized, «abnormal» oceanic basement. This review shows how the concept of mantle exhumation has been elaborated more or less in the same period both by marine geoscientists and by geologists conducting investigations in ophiolitic units of the Alps-Apennines mountain belt. Dredging and diving results from the Gorringe Bank, the Iberia margin, the Thyrrenian Sea and the Central Atlantic in the 1980's and 1990's provided additional proofs that the mantle is currently exhumed in various oceanic environments, including distal continental margins, back-arc basins and slow-spreading ridges of wide oceans. It is shown finally how renewed cross-information from mountain belts and the oceans, multiplying the examples of sedimentary reworking of mantle material, help better constrain the mechanisms of mantle exhumation. This mechanism progressively appeared as a fundamental step during the processes of extension of both continental and oceanic lithospheres in numerous geological situations worldwide.

The Rhenohercynian passive margin of SW England: Development, inversion and extensional reactivation

The SW England Rhenohercynian passive margin initiated with rift-related non-marine sedimentation and bimodal magmatism (Late Lockhovian). Continued lithospheric extension resulted in the exhumation of mantle peridotites and limited seafloor spreading (Emsian-Eifelian). Variscan convergence commenced during the Late Eifelian and was coeval with rifting further north. Collision was marked by the Early Carboniferous emergence of deep marine sedimentary/volcanic rocks from the distal continental margin, oceanic lithosphere, pre-rift basement and upper plate gneisses (correlated with the Mid-German Crystalline High of the Saxothuringian Zone). Progressive inversion of the passive margin was strongly influenced by rift basin geometry. Convergence ceased in the Late Carboniferous and was replaced by an extensional regime that reactivated basin controlling/thrust faults and reorientated earlier fabrics (Start-Perranporth Zone). The resultant exhumation of the lower plate was accompanied by emplacement of the Early Permian SW England granites and was contemporaneous with upper plate sedimentary basin formation above the reactivated Rhenohercynian suture. The Rhenohercynian passive margin probably developed in a marginal basin north of the Rheic Ocean or, possibly, a successor basin following its closure. The Lizard ophiolite is unlikely to represent Rheic Ocean floor or associated forearc (SSZ) crust. The Rheic and Rhenohercynian sutures may be coincident or the Rheic suture may be located further south in the Léon Domain.

Provenance of the Bosnian Flysch

Sandwiched between the Adriatic Carbonate Platform and the Dinaride Ophiolite Zone, the Bosnian Flysch forms a c 3000 m thick, intensely folded stack of Upper Jurassic to Cretaceous mixed carbonate and siliciclastic sediments in the Dinarides. New petrographic, heavy mineral, zircon U/Pb and fission-track data as well as biostratigraphic evidence allow us to reconstruct the palaeogeology of the source areas of the Bosnian Fysch basin in late Mesozoic times. Middle Jurassic intraoceanic subduction of the Neotethys was shortly followed by exhumation of the overriding oceanic plate. Trench sedimentation was controlled by a dual sediment supply from the sub-ophiolitic high-grade metamorphic soles and from the distal continental margin of the Adriatic plate. Following obduction onto Adria, from the Jurassic-Cretaceous transition onwards a vast clastic wedge (Vranduk Formation) was developed in front of the leading edge, fed by continental basement units of Adria that experienced Early Cretaceous synsedimentary cooling, by the overlying ophiolitic thrust sheets and by redeposited elements of coeval Urgonian facies reefs grown on the thrust wedge complex. Following mid-Cretaceous deformation and thermal overprint of the Vranduk Formation, the depozone migrated further towards SW and received increasing amounts of redeposited carbonate detritus released from the Adriatic Carbonate Platform margin (Ugar Formation). Subordinate siliciclastic source components indicate changing source rocks on the upper plate, with ophiolites becoming subordinate. The zone of the continental basement previously affected by the Late Jurassic-Early Cretaceous thermal imprint has been removed; instead, the basement mostly supplied detritus with a wide range of pre-Jurassic cooling ages. However, a c. 80 Ma, largely synsedimentary cooling event is also recorded by the Ugar Formation, that contrasts the predominantly Early Cretaceous cooling of the Adriatic basement and suggests at least locally, a fast exhumation.

Miocene collision-related conglomerates near Dazhuqu and Xigaze, Yarlung Tsangpo suture zone, Tibet

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 112