Continent-ocean Transition Research Articles

Tie points for Gondwana reconstructions from a structural interpretation of the Mozambique Basin, East Africa and the Riiser-Larsen Sea, Antarctica

Abstract. Movements within early East Gondwana dispersal are poorly constrained, and there is debate about conjugate geologic structures and the timing and directions of the rifting and earliest seafloor spreading phases. We present a combined structural interpretation of multichannel reflection seismic profiles from offshore of northern Mozambique (East Africa) and the conjugate Riiser-Larsen Sea (Antarctica). We find similar structural styles at the margins of both basins. At certain positions at the foot of the continental slope close to the continent–ocean transition, the basement is intensely deformed and fractured, a structural style very untypical for rifted continental margins. Sediments overlying the fractured basement are deformed and reveal toplap and onlap geometries, indicating a post-breakup deformation phase. We propose this unique deformation zone as a tie point for Gondwana reconstructions. Accordingly, we interpret the western flank of Gunnerus Ridge, Antarctica as a transform margin similar to the Davie Ridge offshore of Madagascar, implying that they are conjugate features. As the continental slope deformation is post-rift, we propose a two-phase opening scenario. A first phase of rifting and early seafloor spreading, likely in NW–SE direction, was subsequently replaced by a N–S-directed transform deformation phase overprinting the continent–ocean transition. From previously identified magnetic chrons and the sediment stratigraphy, this change in the spreading directions from NW–SE to N–S is suggested to have occurred by the late Middle Jurassic. We suggest that the second phase of deformation corresponds to the strike-slip movement of Madagascar and Antarctica and discuss implications for Gondwana breakup.

Mantle heterogeneity, plume-lithosphere interaction at rift controlled ocean-continent transition zone: Evidence from trace-PGE geochemistry of Vempalle flows, Cuddapah Basin, India

This study reports major, trace, rare earth and platinum group element compositions of lava flows from the Vempalle Formation of Cuddapah Basin through an integrated petrological and geochemical approach to address mantle conditions, magma generation processes and tectonic regimes involved in their formation. Six flows have been identified on the basis of morphological features and systematic three-tier arrangement of vesicular-entablature-colonnade zones. Petrographically, the studied flows are porphyritic basalts with plagioclase and clinopyroxene representing dominant phenocrystal phases. Major and trace element characteristics reflect moderate magmatic differentiation and fractional crystallization of tholeiitic magmas. Chondrite-normalized REE patterns corroborate pronounced LREE/HREE fractionation with LREE enrichment over MREE and HREE. Primitive mantle normalized trace element abundances are marked by LILE-LREE enrichment with relative HFSE depletion collectively conforming to intraplate magmatism with contributions from sub-continental lithospheric mantle (SCLM) and extensive melt-crust interaction. PGE compositions of Vempalle lavas attest to early sulphur-saturated nature of magmas with pronounced sulphide fractionation, while PPGE enrichment over IPGE and higher Pd/Ir ratios accord to the role of a metasomatized lithospheric mantle in the genesis of the lava flows. HFSE-REE-PGE systematics invoke heterogeneous mantle sources comprising depleted asthenospheric MORB type components combined with plume type melts. HFSE-REE variations account for polybaric melting at variable depths ranging from garnet to spinel lherzolite compositional domains of mantle. Intraplate tectonic setting for the Vempalle flows with P-MORB affinity is further substantiated by (i) their origin from a rising mantle plume trapping depleted asthenospheric MORB mantle during ascent, (ii) interaction between plume-derived melts and SCLM, (iii) their rift-controlled intrabasinal emplacement through Archean–Proterozoic cratonic blocks in a subduction-unrelated ocean-continent transition zone (OCTZ). The present study is significant in light of the evolution of Cuddapah basin in the global tectonic framework in terms of its association with Antarctica, plume incubation, lithospheric melting and thinning, asthenospheric infiltration collectively affecting the rifted margin of eastern Dharwar Craton and serving as precursors to supercontinent disintegration.

The assessment of the impact of natural and anthropogenic factors on the current stressstrain state of rock massif of burst-hazardous ore deposits in the East of Russia

This paper does a case study of endogenous deposits located in the zone of influence of convergent (converging, colliding) boundaries of tectonic plates in the zone of the continent-ocean transition, where there is increased present seismic and geodynamic activity, covers the issues of the ratio of natural and anthropogenic components of the stress-strain state (SSS) in rock massifs. It has been shown that, despite the confinement of the studied deposits to a single rock structure, as well as the proximity of their common neotectonic position, the SSS of the rock massifs containing them varies significantly. Based on a comprehensive analysis of geological and geophysical data with the use of space data about the Earth (GRACE mission data for calculation of stress fields and radar images of the Earth's surface), the kinematics of the process of modern deformation of the upper part of the Earth's crust within the studied area is specified. It is established that the peculiarity of the region is the pronounced contribution of the natural component to the general present SSS of rock massifs of impactthreatening deposits. On a real example, the possibility of a predictive assessment of SSS has been shown, which is important at the stages of designing and conducting underground mining operations in severe mining-geological and burst -hazardous conditions.

Lower Crustal Strength Controls on Melting and Serpentinization at Magma‐Poor Margins: Potential Implications for the South Atlantic

AbstractRifted continental margins may present a predominantly magmatic continent‐ocean transition (COT), or one characterized by large exposures of serpentinized mantle. In this study we use numerical modeling to show the importance of the lower crustal strength in controlling the amount and onset of melting and serpentinization during rifting. We propose that the relative timing between both events controls the nature of the COT. Numerical experiments for half‐extension velocities <=10 mm/yr suggest there is a genetic link between margin tectonic style and COT nature that strongly depends on the lower crustal strength. Our results imply that very slow extension velocities (< 5 mm/yr) and a strong lower crust lead to margins characterized by large oceanward dipping faults, strong syn‐rift subsidence and abrupt crustal tapering beneath the continental shelf. These margins can be either narrow symmetric or asymmetric and present a COT with exhumed serpentinized mantle underlain by some magmatic products. In contrast, a weak lower crust promotes margins with a gentle crustal tapering, small faults dipping both ocean‐ and landward and small syn‐rift subsidence. Their COT is predominantly magmatic at any ultra‐slow extension velocity and perhaps underlain by some serpentinized mantle. These margins can also be either symmetric or asymmetric. Our models predict that magmatic underplating mostly underlies the wide margin at weak asymmetric conjugates, whereas the wide margin is mainly underlain by serpentinized mantle at strong asymmetric margins. Based on this conceptual template, we propose different natures for the COTs in the South Atlantic.

Structural characteristics of the ocean-continent transition along the rifted continental margin, offshore central Labrador

Studies of the structural deformation and the nature of the ocean-continent transition zone at the rifted continental margin, offshore central Labrador, and its conjugate off West Greenland have been conducted over several decades and helped to define our understanding of the controlling factors in the development of magma-poor margins globally. New two-dimensional seismic reflection data are now available across the Labrador margin which can help to modify and refine this understanding. In this paper, we describe some of these new seismic data and the corresponding potential field data in relation to earlier studies, including an existing deep seismic crustal velocity profile. These data cross the three major structural and compositional zones below the sedimentary wedge defined in earlier deep seismic experiments as: 1) extended zone of continental crust, including the hyper-extended region, partly underlain by serpentinized mantle; 2) exhumed and serpentinized continental mantle zone; and 3) zone of oceanic crust. We are able to extend these zones laterally over 200 km along the margin. The rifted continental crust is hyper-extended over tens of kilometers landward of the breakup location, where many varieties of normal faults and possible low-angle detachment surfaces are present, some of which are near or at the base of the crust. Further seaward, mantle was serpentinized and likely exhumed, and possesses a seismic character very different from the continental crust. Basement in this zone may locally form prominent ridges. Within the oceanic zone we define a further sub-division into a 'proto-oceanic' domain with thin, variable oceanic crust of about 70–65 Ma in age (magnetic chrons 27 to 31) and a steady-state seafloor spreading region further seaward. Thus, seafloor spreading starts at about chron 31 time. Potential field data and models are broadly consistent with these zones. A strongly reflective, post-rift volcanic event is present across much of the region, especially over the exhumed mantle zone and parts of the oceanic crust. These results are discussed and compared to earlier studies of the West Greenland conjugate margin and to other well-studied magma-poor margins. Given that this rift occurred in thick, cold cratonic lithosphere, the results are remarkably similar to those off Iberia and Newfoundland, suggesting that a long rift history and/or mantle metasomatism may have weakened the lithosphere prior to the main rifting event.

Tectono-sedimentary evolution of the eastern Gulf of Aden conjugate passive margins: Narrowness and asymmetry in oblique rifting context

Here, we focus on the yet unexplored eastern Gulf of Aden, on Socotra Island (Yemen), Southeastern Oman and offshore conjugate passive margins between the Socotra-Hadbeen (SHFZ) and the eastern Gulf of Aden fracture zones. Our interpretation leads to onshore-offshore stratigraphic correlation between the passive margins. We present a new map reflecting the boundaries between the crustal domains (proximal, necking, hyper-extended, exhumed mantle, proto-oceanic and oceanic domains) and structures using bathymetry, magnetic surveys and seismic reflection data. The most striking result is that the magma-poor conjugate margins exhibit asymmetrical architecture since the thinning phase (Upper Rupelian–Burdigalian). Their necking domains are sharp (~40–10km wide) and their hyper-extended domains are narrow and asymmetric (~10–40km wide on the Socotra margin and ~50–80km wide on the Omani margin). We suggest that this asymmetry is related to the migration of the rift center producing significant lower crustal flow and sequential faulting in the hyper-extended domain. Throughout the Oligo-Miocene rifting, far-field forces dominate and the deformation is accommodated along EW to N110°E northward-dipping low angle normal faults. Convection in the mantle near the SHFZ may be responsible of change in fault dip polarity in the Omani hyper-extended domain. We show the existence of a northward-dipping detachment fault formed at the beginning of the exhumation phase (Burdigalien). It separates the northern upper plate (Oman) from southern lower plate (Socotra Island) and may have generated rift-induced decompression melting and volcanism affecting the upper plate. We highlight multiple generations of detachment faults exhuming serpentinized subcontinental mantle in the ocean-continent transition. Associated to significant decompression melting, final detachment fault may have triggered the formation of a proto-oceanic crust at 17.6Ma and induced late volcanism up to ~10Ma. Finally, the setting up of a steady-state oceanic spreading center occurs at ~17Ma.

Detrital zircon age patterns from turbidites of the Balagne and Piedmont nappes of Alpine Corsica (France): Evidence for an European margin source

At the front of metamorphic Cenozoic Alpine nappe of Schistes Lustrés, Western Alpine Corsica (France) exposes non- to very low grade metamorphic nappes, such as the Piedmont nappes, Upper nappes, and the Balagne nappe. The provenance of the Balagne nappe remains still opened: an origin close to the Corsican continental margin; or an origin far East from the Corsican margin toward the “Apenninic” oceanic domain. This would constrain that the Balagne ophiolite be derived from the opposite OCT (Ocean - Continent transition), close to a microcontinent located to the East of the Mesozoic Corsican margin. A systematic U-Pb dating of 586 detrital zircon grains collected from the turbidites in the Balagne and Piedmont nappes has been performed to constrain the source of sediments. The zircon grains yield U-Pb age spectra ranging from Neoarchean to Late Paleozoic with age peaks at 2600Ma, 2080Ma, 1830Ma, 910Ma, 600Ma, 560Ma, 450Ma, 330Ma and 280Ma with different continental model ages (TDM2) from 3.5Ga to 1.0Ga. The variety of composition of the Corsican batholith, unique in its present Mediterranean environment, and in spite of Alpine transcurrent movements, provide a key to analyze the detrital zircon age distribution patterns of sedimentary rocks. These new results i) confirm the lack of any Cretaceous zircon that validates absence of a magmatic arc of this age, at least in the surroundings of the turbiditic formations from the Balagne and the Piedmont nappes; ii) fully support an European provenance of detritus of the Balagne nappe, iii) put forward evidence that no ophiolitic zircon was contained neither in the Cretaceous nor in the Eocene turbidites samples, and iv) question both the deposition of the Piedmont Narbinco flysch within the ocean continent transition and its possible relationships with the Late Cretaceous Pyrenean basins.

Understanding the tectonic history offshore Southern Gabon with high resolution seismic, gravity and magnetics

Having produced oil since 1957, Gabon has several proven petroleum systems, both onshore and offshore. Over the last decade interest has moved towards the deep offshore area where a number of early exploration wells have been successful in finding hydrocarbons, but also highlighted the need to answer questions about trap formation, source maturation and timing of charge in this new area. Following this exploration trend, and aiming to answer these key questions, a recent 3D broadband seismic survey was acquired in conjunction with shipborne gravity and magnetic data in the southern deep offshore area of Gabon. Understanding the tectonic evolution of the basin and its impact on the petroleum systems, requires the integration of seismic with the concurrently acquired gravity and magnetic data. Potential fields data provide deep crustal information where it may be challenging for seismic to resolve deep geological features, and vice versa for the shallower sedimentary section. One of the key objectives of the potential fields study was to model crustal types and thicknesses for input into basin modelling. Upper and lower crust ratios, as well as the crustal characteristics and processes in the continental-oceanic transition zones, are important for understanding the temperature regimes undergone by potential source and reservoir rocks. Knowing the distribution of lower- or higher-density crust, and zones of higher susceptibility, provides insight into the characteristics and timing of formation/deformation of the crust along the margin.

Geophysical evidence for the crustal variation and distribution of magmatism along the central coast of Mozambique

For our understanding of the timing and geometry of the initial Gondwana break-up, still a consistent image of the crustal composition of the conjugated margins of central Mozambique and Antarctica and the location of their continent-ocean boundaries is missing. In this regard, a main objective is the explanation for the source of the different magnetic signature of the conjugate margins. Based on a revised investigation of wide-angle seismic data along two profiles across the Mozambican margin by means of an amplitude modelling, this study presents the crustal composition across and along the continental margin of central Mozambique. Supported by 2D magnetic modelling, the results are compared to the conjugate margin in Antarctica and allow new conclusions about their joined tectonic evolution. An observed crustal diversity between the north-eastern and south-western parts of the central Mozambican margin, testifies to the complex break-up history of this area. Conspicuous is the equal spatial extent of the HVLCB along the margin of 190–215km. The onset of oceanic crust at the central Mozambican margin is refined to chron M38n.2n (164.1Ma). Magnetic modelling supports the presence of reversed polarized SDRs in the continent-ocean transition that were mainly emplaced between 168.5 and 166.8Ma (M42–M40). Inferred SDRs in the Riiser-Larsen Sea might be emplaced sometime between 166.8 and 164.1Ma (M39–M38), but got overprinted by normal polarized intrusions of a late stage of rift volcanism, causing the opposite magnetic signature of the conjugate margins. The distribution of the magmatic material along the central coast of Mozambique clearly indicates the eastern extension of the north-eastern branch of the Karoo triple rift along the entire margin. The main magmatic phase affecting this area lasted for at least 12Myr between 169 and 157Ma, followed by the cease of the magmatism, perhaps due to the relative southwards motion of the magmatic centre.

Structural evolution and basin architecture of the Traill Ø region, NE Greenland: A record of polyphase rifting of the East Greenland continental margin

Fault block basins exposed along NE Greenland provide insights into the tectonic evolution of East Greenland and the Norwegian-Greenland Sea. We present a new geological map and cross sections of the Traill O region, NE Greenland, which formed the western margin of the Voring Basin prior to Ceno­zoic seafloor spreading. Observations support a polyphase rift evolution with three rift phases during Devonian–Triassic, Jurassic–Cretaceous, and Cenozoic time. The greatest amounts of faulting and block rotation occurred during Cenozoic rifting, which we correlate with development of the continent-ocean transition after ca. 56 Ma and the Jan Mayen microcontinent after ca. 36 Ma. A newly devised macrofaunal-based stratigraphic framework for the Cretaceous sandy mudstone succession provides insights into Jurassic–Cretaceous rifting. We identify a reduction in sedimentation rates during the Late Cretaceous; this corresponds to a transition from structurally confined to unconfined sedimentation that coincides with increased clastic sedimentation to the Voring and More Basins derived from East Greenland. With each rift phase we record an increase in the number of active faults and a decrease in the spacing between them. We attribute this to fault block rotation that leads to an excess build-up of stress that can only be released by the creation of new steep faults. In addition, we observe a stepwise migration of deformation toward the rift axis that we attribute to preexisting lithospheric heterogeneity that was modified during subsequent rift and post-rift phases. Such observations are not readily conformable to classic rift evolution models and highlight the importance of post-rift lithospheric processes that occur during polyphase rift evolution.

Ocean‐continent Transition to Suprasubduction Zone Origin of the Western Yarlung Zangbo Ophiolites in SW Tibet, China: Multi‐stage, Transient Evolution of the Neotethyan Oceanic Lithosphere

The ophiolites that crop out discontinuously along the ∼2000 km Yarlung Zangbo Suture zone (YZSZ) between the Nanga Parbat and Namche Barwa syntaxes in southern Tibet represent the remnants of Neotethyan oceanic lithosphere (Fig. 1a). We have investigated the internal structure and the geochemical makeup of mafic‐ultramafic rock assemblages that are exposed in the westernmost segment of the YZSZ where the suture zone architecture displays two distinct sub‐belts of ophiolitic and mélange units separated by a continental Zhongba terrane (Fig. 1b). These two sub‐belts include the Daba – Xiugugabu in the south (Southern sub‐belt, SSB) and the Dajiweng – Saga in the north (Northern sub‐belt, NSB). We present new structural, geochemical, geochronological data from upper mantle peridotites and mafic dike intrusions occurring in these two sub‐belts and discuss their tectonomagmatic origin.In‐situ analysis of zircon grains obtained from mafic dikes within the Baer, Cuobuzha and Jianabeng massifs in the NSB, and within the Dongbo, Purang, Xiugugabu, Zhaga and Zhongba in the SSB have yielded crystallization ages ranging between130 and 122 Ma. Dike rocks in both sub‐belts show N‐MORB REE patterns and negative Nb, Ta and Ti anomalies, reminiscent of those documented from SSZ ophiolites. Harzburgitic host rocks of the mafic dike intrusions mainly display geochemical compositions of abyssal peridotites (Fig. 2), with the exception of the Dajiweng harzburgites, which show the geochemical signatures of forearc peridotites (Lian et al., 2016). Extrusive rocks that are spatially associated with these peridotite massifs in both sub‐belts also have varying compositional and geochemical features. Tithonian to Valanginian (150 – 135 Ma) basaltic rocks in the Dongbo massif have OIB‐like geochemistry and 138 Ma basaltic lavas in the Purang massif have EMORB‐like geochemistry (Liu et al., 2015). Tuffaceous rocks in the Dajiweng massif are 140 Ma in age and show OIB‐like geochemistry. We interpret these age and geochemical data to reflect a rifted continental margin origin of the extrusive rock units in both sub‐belts.These data and structural observations show that the western Yarluang Zangbo ophiolites represent fragments of an Ocean‐Continent Transition (OCT) peridotites altered by fluids in an initial supersubduction setting. We infer that mafic‐ultramafic rock assemblages exposed in the SSB and NSB initially formed in an ocean – continent transition zone (OCTZ) during the late Jurassic, and that they were subsequently emplaced in the forearc setting of an intraoceanic subduction zone within a Neotethyan seaway during 130 to 122 Ma. The NSB and SSB are hence part of a single, S‐directed nappe sheet derived from a Neotethyan seaway located north of the Zhongba terrane.

Deep seismic structure of the northeastern South China Sea: Origin of a high‐velocity layer in the lower crust

AbstractWe present a 2‐D seismic tomographic image of the crustal structure along the OBS2012 profile, which delineates the Moho morphology and magmatic features of the northeastern South China Sea margin. The image was created by forward modeling (RayInvr) and traveltime tomographic inversion (Tomo2D). Overall, the continental crust thins seaward from ~27 km to ~21 km within the continental shelf across the Zhu I Depression and Dongsha Rise, with slight local thickening beneath the Dongsha Rise accompanying the increase in the Moho depth. The Dongsha Rise is also characterized by ~4–7 km thick high‐velocity layer (HVL) (~7.0–7.6 km/s) in the lower crust and exhibits a relatively high velocity (~5.5–6.4 km/s) in the upper crust with a velocity gradient lower than those of the Zhu I Depression and Tainan Basin. Across the continental slope and continent‐ocean transition (COT), which contain the Tainan Basin, the crust sharply thins from 20 km to 10 km seaward and a ~2–3 km thick HVL is imaged in the lower crust. We observed that volcanoes are located only within the COT, but none exist in the continental shelf; the Dongsha Rise exhibits a high magnetic anomaly zone and different geochemical characteristics from the COT. Based on those observations, we conclude that the HVL underlying the COT is probably extension related resulting from the decompression melting in the Cenozoic, whereas the HVL beneath the Dongsha Rise is probably arc related and associated with the subduction of the paleo‐Pacific plate. These findings are inconsistent with those of some previous studies.

Timing of initial seafloor spreading in the Newfoundland-Iberia rift

Broad areas of subcontinental lithospheric mantle are commonly exposed along ocean-continent transition zones in magma-poor rifts and are thought to be exhumed along lithospheric-scale detachment faults during the final stages of rifting. However, the nature of the transition from final rifting to seafloor spreading is controversial. We present the first high-precision U-Pb zircon geochronologic and Hf isotopic data from gabbros that intrude exhumed mantle at Ocean Drilling Program (ODP) Sites 1070 and 1277 in the Newfoundland-Iberia rift (North Atlantic). The sites are conjugate to one another within crust that is commonly considered to have been emplaced during early seafloor spreading. Magnetic data suggest that crustal accretion occurred at both sites during magnetic polarity chrons M3–M0 (130–126 Ma). However, our data indicate that asthenospheric melts were emplaced over brief intervals (≤1 m.y.) prior to or coeval with mantle exhumation at 124 Ma at ODP Site 1070 and 115 Ma at ODP Site 1277. We suggest that this discrepancy is the result of continued mantle exhumation along large, west-dipping detachment faults until lithospheric breakup. The breakup location is likely coincident with the large-amplitude magnetic J anomaly, and our 115 Ma date for magmatism within this anomaly provides the best available age constraint for breakup along the studied transect.

The onset of Walvis Ridge: Plume influence at the continental margin

The opening of the South Atlantic is a classical example for a plume related continental breakup. Flood basalts are present on both conjugate margins as well as aseismic ridges connecting them with the current plume location at Tristan da Cunha. To determine the effect of the proposed plume head on the continental crust, we acquired wide-angle seismic data at the junction of the Walvis Ridge with the African continent and modelled the P-wave velocity structure in a forward approach. The profile extends 430km along the ridge and continues onshore to a length of 720km. Crustal velocities beneath the Walvis Ridge vary between 5.5km/s and 7.0km/s, a typical range for oceanic crust. The crustal thickness of 22km, however, is approximately three times larger than of normal oceanic crust. The continent-ocean transition is characterized by 30km thick crust with strong lateral velocity variations in the upper crust and a high-velocity lower crust (HVLC), where velocities reach up to 7.5km/s. The HVLC is 100 to 130km wider at the Walvis Ridge than it is farther south, and impinges onto the continental crust of the Kaoko fold belt. Such high seismic velocities indicate Mg-rich igneous material intruded into the continental crust during the initial rifting stage. However, the remaining continental crust seems unaffected by intrusions and the root of the 40km-thick crust of the Kaoko belt is not thermally abraded. We conclude that the plume head did not modify the continental crust on a large scale, but caused rather local effects. Thus, it seems unlikely that a plume drove or initiated the breakup process. We further propose that the plume already existed underneath the continent prior to the breakup, and ponded melt erupted at emerging rift structures providing the magma for continental flood basalts.

Preserved organic matter in a fossil Ocean Continent Transition in the Alps: the example of Totalp, SE Switzerland

Evidence from ultraslow spreading mid-ocean ridges and both fossil and present-day Ocean–Continent Transitions (OCT) demonstrates that mantle serpentinization resulting from the interaction of mantle rock and water during tectonic exhumation is widespread. Observations at white smokers in modern ocean settings suggest that methane produced by serpentinization can support methanotrophic bio-systems, which use methane as the only source of carbon. An important question is whether such bio-systems are more generally pervasive in their association with serpentinized mantle in the subsurface. In this study, we examined whether there is evidence for such a methanotrophic system in exhumed serpentinized mantle at a magma-poor rifted continental margin, by probing for characteristic biological markers in these and associated sedimentary rocks in the Totalp unit of SE Switzerland. This unit represents a remnant of the former OCT of the southern Alpine Tethyan margin and was chosen because of its mild Alpine tectonic and low-grade metamorphic overprint during Alpine orogeny, hence giving potential for the preservation of indigenous organic matter (OM). Totalp samples are characterized by low organic carbon contents of 11–647 ppm. The majority of the samples contain hydrocarbons in the form of n-alkanes in the range C17–C36. Some sediments contain isoprenoids, for example pristane and phytane and a suite of steranes that are consistent with a marine origin for the OM preserved in the rocks. Traces of marine planktonic and bacterial OM are preserved in the serpentinized mantle and overlying sediments of this ancient Tethyan OCT, but there is no evidence that the OM has been generated from methanotrophic bio-systems.

An aeromagnetic survey system based on an unmanned autonomous helicopter: Development, experiment, and analysis

ABSTRACTChina has very long coastline, but special surface conditions make it very difficult to conduct ground magnetic and marine geomagnetic surveys over beach–shallow sea transition areas such as mariculture areas, harbours, and wharfs; as a result, marine survey data cannot be linked to continental survey data, giving rise to missing data. To address this issue, an aeromagnetic survey technology based on an unmanned autonomous helicopter is proposed. Not only is an unmanned autonomous helicopter capable of aeromagnetic surveying the beach–shallow sea transition area inaccessible to vehicles and vessels but its high efficiency and low cost also enable it to be deployed on a vessel, thus making it suitable for aeromagnetic surveying oceanic islands, too. The setup of the aeromagnetic survey system based on an unmanned aerial vehicle (UAV) is introduced, and its trial flight information and processed and analysed magnetic survey data obtained in trial flights are discussed. The measurement accuracy of UAV aeromagnetic data is shown to be comparable to that of marine geomagnetic survey data, and the system effectively compensates for missing magnetic data from inshore and continent–ocean transition zones.

Isostasy as a tool to validate interpretations of regional geophysical datasets – application to the mid-Norwegian continental margin

Abstract Isostasy is a well understood concept, yet rarely applied to its full capacity in regional interpretations of crustal structures. In this study, we utilize a recent density model for the entire NE Atlantic, based on refraction seismic data and gravity inversion, to calculate isostatically balanced bathymetry along the mid-Norwegian margin. Since gravity and isostatically balanced elevation are independent observables but both depend on the underlying density model, consistencies and discrepancies point towards model deficits, erroneously interpreted or poorly understood areas. Four areas of large isostatic residuals are identified. Along the outer Vøring Margin, a mass deficit points to more extensive high-density bodies or a shallower Moho than currently mapped. Farther seaward, along the Vøring Marginal High, a mass excess indicates inaccurate mapping of the continent–ocean boundary and surrounding structures. A number of eclogitic bodies along the proximal mid-Norwegian margin have been described in recent publications and their presence is now also confirmed by isostatic calculations. Major elevation and gravity residuals along the transition between the Vøring and Møre margins signify that the structure of this region is poorly understood and modifications to the mapped continent–ocean transition may be required.

Vestiges of a continental margin ophiolite type in the Novo Oriente region, Borborema Province, NE Brazil

The Novo Oriente Group is a restricted well-preserved metasedimentary sequence, composed of two tectonic-stratigraphic sequences in the southwestern portion of the Ceará Central Domain, NE Brazil. The Bonsucesso Formation comprises mainly quartzite and metamafic rocks and the Caraúbas Formation is dominantly metapelitic, with chemical sedimentary contribution, metamafic and metaultramafic rocks. New integrated field, geochemical data and Sm-Nd isotopes of the metaultramafic and metamafic rocks of the two formations have been investigated in order to determine their tectonic setting. The metaultramafic rocks are dominantly composed of deformed and undeformed serpentinites, chloritites, actinolitites, talc-chlorite schists, serpentine-talc schists, talc-rich siliceous rocks and subordinated listwänites. Geochemical data indicate that the serpentinites correspond to rocks resulting from the alteration of dunites depleted in HREE, similar to the pattern presented by subduction-zone serpentinites generated from exhumed sub-continental peridotites and hydrated during ocean-continent transition (OCT) rifting. The metamafic rocks, represented by metagabbros, hornblende metagabbros and metabasalts, consist of basic rocks of basaltic and tholeiitic affinity with signatures between E- and N-MORB and variable contamination by crustal components similar to the rocks formed from the interaction between mantle plumes and heavily thinned continental crust. Isotopic analysis indicates crustal assimilation with negative ɛNd and Paleoproterozoic TDM ages. The data suggest that metaultramafic and metamafic rocks correspond, respectively, to continental sub-lithospheric mantle exhumed in an area of ocean-continent transition (OCT), and mafic magmatism associated with the development of a magma-poor passive margin generated by the break-up of the Rodinia Supercontinent, which was later dismembered by the Brasiliano/Pan-African Orogeny collisional phase and preserved as a Continental Margin Ophiolite type.

A bottom-driven mechanism for distributed faulting in the Gulf of California rift

Observations of active faulting in the continent-ocean transition of the Northern Gulf of California show multiple oblique-slip faults distributed in a 200×70km2 area developed some time after a westward relocation of the plate boundary at ~2Ma. In contrast, north and south of this broad pull-apart structure, major transform faults accommodate Pacific-North America plate motion. Here we propose that the mechanism for distributed brittle deformation results from the boundary conditions present in the Northern Gulf, where basal shear is distributed between the Cerro Prieto strike-slip fault (southernmost fault of the San Andreas fault system) and the Ballenas Transform Fault. We hypothesize that in oblique-extensional settings whether deformation is partitioned in a few dip-slip and strike-slip faults, or in numerous oblique-slip faults may depend on (1) bottom-driven, distributed extension and shear deformation of the lower crust or upper mantle, and (2) the rift obliquity. To test this idea, we explore the effects of bottom-driven shear on the deformation of a brittle elastic-plastic layer with the help of pseudo-three dimensional numerical models that include side forces. Strain localization results when the basal shear abruptly increases in a step-function manner while oblique-slip on numerous faults dominates when basal shear is distributed. We further explore how the style of faulting varies with obliquity and demonstrate that the style of delocalized faulting observed in the Northern Gulf of California is reproduced in models with an obliquity of 0.7 and distributed basal shear boundary conditions, consistent with the interpreted obliquity and boundary conditions of the study area.

Thinned crustal structure and tectonic boundary of the Nansha Block, southern South China Sea

The southern South China Sea margin consists of the thinned crustal Nansha Block and a compressional collision zone. The Nansha Block’s deep structure and tectonic evolution contains critical information about the South China Sea’s rifting. Multiple geophysical data sets, including regional magnetic, gravity and reflection seismic data, reveal the deep structure and rifting processes. Curie point depth (CPD), estimated from magnetic anomalies using a windowed wavenumber-domain algorithm, enables us to image thermal structures. To derive a 3D Moho topography and crustal thickness model, we apply Oldenburg algorithm to the gravity anomaly, which was extracted from the observed free air gravity anomaly data after removing the gravity effect of density variations of sediments, and temperature and pressure variations of the lithospheric mantle. We found that the Moho depth (20 km) is shallower than the CPD (24 km) in the Northwest Borneo Trough, possibly caused by thinned crust, low heat flow and a low vertical geothermal gradient. The Nansha Block’s northern boundary is a narrow continent–ocean transition zone constrained by magnetic anomalies, reflection seismic data, gravity anomalies and an interpretation of Moho depth (about 13 km). The block extends southward beneath a gravity-driven deformed sediment wedge caused by uplift on land after a collision, with a contribution from deep crustal flow. Its southwestern boundary is close to the Lupar Line defined by a significant negative reduction to the pole (RTP) of magnetic anomaly and short-length-scale variation in crustal thickness, increasing from 18 to 26 km.