Oceanic Basement Research Articles

Monitoring subseafloor temperature and fluid pressure in sealed ODP/IODP boreholes to constrain in-situ hydrological state and processes in igneous oceanic crust

We review the contributions to our understanding of the hydrogeology of the oceanic crust as gained by monitoring subseafloor temperatures and pressures in CORK (“Circulation Obviation Retrofit Kit”) sealed hole observatories that penetrate through marine sediments into igneous basement. CORKs were installed during 1991-2012 in 17 holes drilled by the Ocean Drilling Program (ODP) and Integrated Ocean Drilling Programs (IODP) in igneous oceanic crust (aged 0.4 to 24 Ma) in the thickly sedimented eastern Pacific Ocean and in the thinly sedimented north Atlantic Ocean. Results indicate that the igneous crust in all these settings is highly transmissive and supports vigorous lateral fluid flow and/or convection beneath the sediment cover driven by very small horizontal pressure gradients. Inter-CORK experiments suggest that the horizontal permeability of the young oceanic basement at these sites may be anisotropic, with higher values in the direction parallel to the axis of spreading and tectonic fabric and lower values in a ridge-normal cross-strike direction. Tracer experiments applied to ridge-parallel flow suggest that only a small fraction of the crust is well connected. CORKs also record the response of subseafloor formation fluid pressures to seafloor tidal loading, and, once the tidal responses are filtered out, response to tectonic strain events. Interpretations based on these responses and original two-dimensional modelling of ridge-normal fluid circulation in the crust suggest a scale dependence of permeability of igneous oceanic crust (higher values at larger scales up to tens of km), although recent three-dimensional models suggest a more limited scale effect. CORKs have also provided important geochemical and microbiological results from the igneous oceanic crust, although they are not reviewed in this paper.

Eurasia Basin Composite Tectono-Sedimentary Element

The Eurasia Basin formed by continental break-up and separation of the Lomonosov Ridge from the northern Barents–Kara Shelf at about 56–54 Ma. It was a restricted oceanic basin from its formation up until the Miocene, when extensive water circulation between the North Atlantic and Arctic oceans commenced through the Fram Strait gateway. The opening of the oceanic gateway had a large impact on palaeoceanography and palaeoclimate, which evolved from an Eocene greenhouse to a Neogene icehouse associated with northern hemisphere glaciations. The Eocene–present sedimentary fill of the Eurasia Basin was mainly sourced from surrounding shelf areas in the Barents, Kara and Laptev seas, which experienced widespread Cenozoic uplift and erosion during several phases. The stratigraphy of the Eurasia Basin is poorly constrained due to a lack of boreholes penetrating the basin fill. We therefore have to rely on seismic stratigraphy and tentative ages assigned to sequences and their boundaries terminating on top of oceanic basement of known age. This contribution covers two tectono-sedimentary elements (TSEs): the Eurasia Basin Oceanic TSE and the Eurasia Basin Prograded Margin TSE, which are closely connected and difficult to draw a distinct boundary between.

Seamount and Ridge Subduction at the Java Margin, Indonesia: Effects on Structural Geology and Seismogenesis

AbstractThe Java ‐ Lesser Sunda margin, which features multi‐scale subducting oceanic basement relief, is classified as neutral (Lombok and Sumbawa) to erosional (Central Java to Bali) in comparison to its accretionary counterpart offshore Sumatra. However, a comprehensive analysis of how plate boundary and upper plate structure across the neutral to erosional transition are modulated by the subduction of oceanic basement relief is lacking to date. To shed light on the tectonic parameters that push the margin into the neutral or erosional domain, we combine multi‐channel reflection seismic images derived through a grid‐based P‐wave velocity inversion, and multibeam bathymetric maps. The data document how different scales of subducting topography modify seafloor morphology, upper plate structure, and décollement position. Large‐scale subducting features cause a landward shift of the deformation front, shortening of the accretionary wedge, and seafloor steepening at the relief's trailing edge. Small‐scale subducting ridges primarily impact the frontal prism resulting in over‐steepening at the trench and localized slope failure. Ahead of subducting relief, deformation of the accretionary wedge encompasses enhanced compression and a reduction in seafloor slope but appears independent of the size of the relief. Ridge and seamount subduction induce frontal erosion and basal erosion offshore Lombok and Bali, respectively. Our P‐wave velocity models indicate that the rigidity of the upper plate's base along the eastern Sunda margin is significantly lower than the worldwide trend. We conclude that this favors the genesis of tsunami earthquakes that have occurred on the Java margin.

Structural evolution of the southern Ecuadorian forearc in the Santa Elena Peninsula region

The southern Ecuadorian forearc system is related to the subduction of the oceanic Farallon/Nazca Plate beneath the continental South American Plate since the Late Cretaceous, and currently evolves with the dynamic of a tectonic block called North Andean Sliver. To explore the structural architecture and processes controlling the Upper Cretaceous-Cenozoic growth of the forearc, we built a ∼143 km-long onshore-offshore crustal-scale cross-section in the Santa Elena Peninsula region using seismic reflection profiles and well and field data. The structure of the Santa Elena Peninsula forearc system is controlled by imbrication of Upper Cretaceous-Palaeocene oceanic basement and Cenozoic sedimentary units, and underplating of distal Cenozoic sequences stacked at the trench zone. This led to the progressive construction of an accretionary wedge through time. The forearc substratum is mainly formed by the Upper Cretaceous-Palaeocene basement developed during the docking of oceanic terranes. It is later deformed by NW-trending landward-dipping, normal to strike-slip faults during the Middle Eocene, and renewed compression by inversion of inherited faults from the Oligocene onwards. Recent deformation consists in N-trending oceanward-dipping normal faults in the frontal slope domain and fault-controlled uplift of marine terraces along the coastal area. Therefore, the Upper Cretaceous to present-day structural evolution of the Santa Elena Peninsula forearc is controlled by the long-lasting subduction dynamics and structural inheritance of the upper plate.

Geology of the Eastern Anatolian Plateau (Turkey): a synthesis

The Eastern Anatolian Plateau (EAP), approximately 2000 m above sea level, is located between the Eastern Pontides to the north, the Arabian Platform to the south, and the Iranian Plateau to the east. It is characterized by approximately 6 km-thick Maastrichtian to Quaternary volcano-sedimentary cover which unconformably overlies continental and oceanic basement units. Overall, the outcrops of the pre-Maastrichtian basement are rare and include both continental and oceanic units. This led to drastically different interpretations of the nature of the pre-Maastrichtian basement as (i) the oceanic accretionary complex or (ii) continental crust and overlying ophiolitic mélange. This synthesis deals with the relationships between continental and oceanic units in light of the recent geological, geophysical, and geochemical studies. Geophysical studies consistently indicate the presence of a spatially thickened continental crust with a lateral variation ranging from 38 to 52 km. Seismological models estimate lithospheric thicknesses to be in the range of 70-80 km, suggesting the presence of a rather thinned lithosphere. The pre-Maastrichtian continental units include late Cretaceous high-T/low-P metamorphic rocks, which are intruded by late Cretaceous basic to acidic intrusions at the base. Protoliths of the high-T/low-P metamorphic rocks can be closely correlated with those of the Anatolide-Tauride Block, probably representing the metamorphosed equivalents of the Anatolide-Tauride Block. The continental crustal nature is also testified by the presence of metasyenite to -granite with igneous crystallization ages of 430-440 Ma. The Late Cretaceous ophiolitic mélanges with locally intact tracks of ophiolite and overlying forearc deposits tectonically sit over the Late Cretaceous high-T/low-P metamorphic rocks. These ophiolitic mélanges probably form part of the North Anatolian ophiolitic belt, related to the İzmir-Ankara-Erzincan suture. Maastrichtian to Quaternary volcano-sedimentary rocks overlie both the continental crustal and tectonically overlying oceanic units, representing probably collisional and postcollisional basin fills. Available geological, geochemical, and geophysical data suggest a pre- Maastrichtian basement that comprises a continental crustal domain and an overlying ophiolitic mélange beneath the Masstrichtian to Quaternary cover.

Dating seafloor spreading of the southwest sub-basin in the South China Sea

Marine magnetic lineations play important role in interpreting the age and spreading processes of the oceanic crust. However, it is difficult to identify magnetic lineations from sea surface observations in a deep and narrow low-latitude slow-spreading inactive ocean basin for many reasons, such as the highly filtered low-amplitude magnetic anomalies, highly oblique magnetization, contamination from narrow stripes, etc., which may hamper accurate lineation discrimination and identification. The spreading history of the Southwestern Sub-basin of the South China Sea (SW-SCS), which is located in such a low-latitude tectonic setting, has been debated for a long time. Near-seafloor deep-tow magnetic measurements provide high-resolution reversal records, making accurate lineation discrimination feasible. Corrections, including diurnal variation correction, regional field correction, and de-skewing, are explored. The results, calibrated with drill sites from the International Ocean Discovery Program (IODP) Expedition 349, show that the spreading ages of the SW-SCS range from ∼ 21 to 15 Ma (from C6A to C5B) and the full spreading rate ranges from 26 to 46 mm/yr, with an average rate of 37 mm/yr. Spreading was initially symmetric but later became asymmetric, with a rate differential reaching 10%. Combined with two seismic profiles, it is found that the roughness of the oceanic basement to the south of the extinct ridge is mainly influenced by faulted blocks, whereas to the north it is mainly influenced by magmatism. The RMS seafloor roughness is estimated to be 211 m in the south and 191 m in the north respectively. This indicates that SW-SCS seafloor spreading tends to be a transition type between axial high and axial low at the intermediate-slow spreading rate. There is a large valley in the spreading center, but the roughness far away from the center is low. the RMS basement roughness is obviously lower than that of the typical slow spreading oceanic basin. we conjecture that the larger than normal syn-spreading magma may be the reason for the low roughness. A long history of being subducted might be the deep-seated reason for all these behaviors where, and the mantle is rich in water and carbon.

Tectonic evolution of the Triassic Songpan‐Ganzi basin as constrained by a synthesis of multi‐proxy provenance data

AbstractThe triangular Songpan‐Ganzi flysch terrane exposes a Triassic turbidite sequence with an average thickness of ca. 8 km. The sediments may have been accumulated in a remnant Paleo‐Tethyan ocean bounded by the converging North China, South China, and the Qiangtang terrane from three sides, or a back‐arc basin with an oceanic basement created during the Triassic closure of the Paleo‐Tethyan ocean. To differentiate the two competing models, we systematically reviewed the available provenance data that include U–Pb detrital zircon ages at the basin scale, paleocurrent directions, sandstone petrography, and heavy‐mineral assemblages from the Triassic Songpan‐Ganzi basin samples. We use the Kolmogorov–Smirnov tests to differentiate competing hypotheses for detrital‐zircon provenance interpretations and DZmix modelling to quantify relative contributions of detrital zircon from all potential source areas for the Triassic Songpan‐Ganzi deposits. The most important result of this work is that the Songpan‐Ganzi basin had a stable and locally derived source system: the western, central and eastern sub‐basins were mainly sourced from the north whereas the easternmost and southeastern sub‐basins were mainly sourced from westernmost South China (i.e., the Longmen Shan area) and the Qiangtang terrane. The stability of the source areas around the Songpan‐Ganzi basin throughout the Triassic is most compatible with the remnant ocean model that predicts a long‐lived marine basin with a pre‐Triassic oceanic/continental basement trapped between converging continental blocks during the Triassic.

Lower oceanic crust formed by in situ melt crystallization revealed by seismic layering

Oceanic crust forms at mid-ocean spreading centres through a combination of magmatic and tectonic processes, with the magmatic processes creating two distinct layers: the upper and the lower crust. While the upper crust is known to form from lava flows and basaltic dykes based on geophysical and drilling results, the formation of the gabbroic lower crust is still debated. Here we perform a full waveform inversion of wide-angle seismic data from relatively young (7–12-Myr-old) crust formed at the slow-spreading Mid-Atlantic Ridge. The seismic velocity model reveals alternating, 400–500 m thick, high- and low-velocity layers with ±200 m s−1 velocity variations, below ~2 km from the oceanic basement. The uppermost low-velocity layer is consistent with hydrothermal alteration, defining the base of extensive hydrothermal circulation near the ridge axis. The underlying layering supports that the lower crust is formed through the intrusion of melt as sills at different depths, which cool and crystallize in situ. The layering extends up to 5–15 km distance along the seismic profile, covering 300,000–800,000 years, suggesting that this form of lower crustal accretion is a stable process.

Arc-crustal compression and its effects on the underlying mantle geometry as elucidated from the potential field signatures of the buckled Cretaceous Cebu lithosphere, Philippines

The occurrence of long wavelength Moho undulations in continental and oceanic lithospheres is attributed to lithospheric buckling. This has been reported for Cretaceous or older lithospheres under compressional regimes. However, there is very limited data on lithospheric buckling of oceanic island arcs. The region where the Cretaceous Cebu arc is under compression due to the impingement of the Palawan micro-Continental Block (PCB) to the Philippine Mobile Belt (PMB). The configuration of the inferred buckled Cretaceous Cebu arc lithosphere is derived from geologic and combined onshore and airborne gravity and magnetics datasets. Subsurface modeling using gravity data revealed the presence of mantle upwelling beneath Central Cebu. This could be considered as one of the crests of long-wavelength Moho undulations commonly attributed to a buckled lithosphere. The oceanic basement was also modelled to determine its configuration. Euler solutions derived from magnetic data showed that the contact between the arc crustal units of the Cansi Volcanics and Pandan Formation is affected by short wavelength folding with wavelengths ranging from 5 to 15 km. The younger arc crustal units are affected by tighter folding with wavelengths ranging from 1 to 5 km. The occurrence of a buckled arc lithosphere is being considered for the first time in this area. The consistent NE-SW folding axes inferred for the folding events strongly suggest that these are consequences of the Middle Miocene collision involving the Philippine Mobile Belt and the Palawan micro-Continental Block.

Authigenic Clays Versus Carbonate Formation as Products of Marine Silicate Weathering in the Input Sequence to the Sumatra Subduction Zone

AbstractWe use geochemical and petrographic data from anoxic sequences of the Nicobar Fan to document extensive marine silicate weathering (MSiW) in the input sediment of the Sumatra subduction zone and the conditions that result in authigenic minerals originating from this reaction: precipitation of authigenic carbonate—which sequesters carbon—and formation of authigenic clay—which releases CO2. Increase in 87Sr/86Sr in pore fluids from International Ocean Discovery Program Expedition 362 (Site U1480 to 0.71376 and Site U1481 to 0.71296) reveals a radiogenic strontium contribution from alteration of the Himalayan continental sediment that dominates the Nicobar Fan. Peaks in the dissolved strontium isotope data coincide with zones of methane presence, consistent with MSiW reactions driven by CO2 generation during methanogenesis. Later‐stage fan sequences from 24 to 400 mbsf (meters below seafloor) contain only minor carbonate with 87Sr/86Sr ratios that deviate only slightly from the co‐eval seawater values (0.70920–0.70930); geochemical data in this zone point to a contribution of authigenic clay formation. In contrast, microscopy and elemental mapping of the carbonate‐cemented zones in the earliest fan deposits (>780 mbsf) show replacement of feldspars and dense minerals by carbonate, which ranges in volume from a few percent of the grain to near total grain obliteration. This deeper authigenic carbonate is significantly enriched in radiogenic 87Sr (0.71136–0.71328). Thus, MSiW leads to distinct products, likely in response to a weathering‐derived supply of silica in the younger setting versus calcium enrichment via diffusion from oceanic basement in the older sequence.

Tectonic control on sedimentary dynamics in intraplate oceanic settings: A geomorphological image of the eastern Canary Basin and insights on its middle-upper Miocene to quaternary volcano-tectonic-sedimentary evolution

This paper integrates sedimentary, tectonic and volcanic geological processes inside a model of volcano-tectonic activity in oceanic intraplate domains related to rifted continental margins. The study case, the eastern Canary Basin (NE Atlantic), is one of the few places in the world where giant MDTs and Quaternary volcanic and hydrothermal edifices take place in intraplate domains. In this paper, we analyse how two structural systems (WNW-ESE and NNE-SSW) matching with the oceanic fabric control the location of volcanic systems, seafloor tectonic reliefs and subsequently the distribution of main sedimentary systems. Linear turbidite channels, debris flow lobes and the lateral continuity of structural and volcanic reliefs follow a WNW-ESE trend matching the tracks of the oceanic fracture zones. Furthermore, escarpments, anticline axes and volcanic ridges follow a NNE-SSW trend matching normal faults delimiting blocks of oceanic basement. The morpho-structural analysis of all the above geomorphological features shows evidence of a volcanic and tectonic activity from the middle–upper Miocene to the Lower–Middle Pleistocene spread over the whole of the eastern Canary Basin that reached the western Canary Islands. This reactivation changes the paradigm in the seamount province of Canary Islands reported inactive since Cretaceous. A tecto-sedimentary model is proposed for this period of time that can be applied in other intraplate domains of the world. A tectonic uplift in the study area with a thermal anomaly triggered volcanic and hydrothermal activity and the subsequent flank collapse and emplacement of mass transport deposits on the Western Canary Slope. Furthermore, this uplift reactivated the normal basement faults, both trending WNW-ESE and NNE-SSW, generating folds and faults that control the location of turbidite channels, escarpments, mass transport deposits and volcanic edifices.

Alteration of abyssal peridotites is a major sink in the W geochemical cycle

Arc lavas and the continental crust exhibit a selective enrichment of W relative to similarly incompatible elements, indicating that subduction zone environments are a tectonic setting where W is mobilized from the subducting slab. Here we present evidence that ultramafic portions of altered oceanic lithosphere are a major sink for W and evaluate how its W budget needs to be included in constraining the global geochemical cycle of W. We report high precision W, HFSE and U-Th data obtained by isotope dilution for altered basement formed at super-slow spreading rates along the Mid-Atlantic Ridge drilled during ODP Leg 209 (between 14°N and 16°N). The oceanic basement consists of abyssal peridotites and associated gabbroic rocks exhumed in a magma starved setting. Hydrothermal alteration styles covered in the sample set are serpentinization, talc alteration, and low-T seawater alteration. The drilled rock portions are among the most depleted in HFSE that have ever been studied.Tungsten concentrations range from 4 ppb in the least altered harzburgites to 500 ppb in strongly serpentinized dunites. Locally, W is significantly enriched relative to U, Th and Ta by factors of up to 100 compared to canonical mantle values, much higher than in mafic portions of altered oceanic crust such as hole 1256D (up to 10). The samples from ODP Leg 209 show strong W enrichment during progressive serpentinization (holes 1268A, 1270 and 1271) and during late-stage oxidative seawater alteration (hole 1270D and 1272A). In contrast, silica metasomatism associated with talc alteration (hole 1268A) is associated with selective W depletion relative to Th and Ta. Trace element modelling indicates that hydrothermal enrichment of W by redistribution is a more likely source than seawater-derived W. Collectively, our data show that altered ultramafic rocks likely constitute an important geochemical reservoir in the global geochemical cycle of W contributing to the enrichment of W found in arc lavas and to the recycling of W into the Earth’s mantle.

Sub-arc mantle enrichment in the Sunda rear-arc inferred from HFSE systematics in high-K lavas from Java

Many terrestrial silicate reservoirs display a characteristic depletion in Nb, which has been explained in some studies by the presence of reservoirs on Earth with superchondritic Nb/Ta. As one classical example, K-rich lavas from the Sunda rear-arc, Indonesia, have been invoked to tap such a high-Nb/Ta reservoir. To elucidate the petrogenetic processes active beneath the Java rear-arc and the causes for the superchondritic Nb/Ta in some of these lavas, we studied samples from the somewhat enigmatic Javanese rear-arc volcano Muria, which allow conclusions regarding the across-arc variations in volcanic output, source mineralogy and subduction components. We additionally report some data for an along-arc sequence of lavas from the Indonesian part of the Sunda arc, extending from Krakatoa in the west to the islands of Bali and Lombok in the east. We present major and trace element concentrations, Sr–Nd–Hf–Pb isotope compositions, and high-field-strength element (HFSE: Nb, Ta, Zr, Hf, W) concentrations obtained via isotope dilution and MC-ICP-MS analyses. The geochemical data are complemented by melting models covering different source compositions with slab melts formed at variable P–T conditions. The radiogenic isotope compositions of the frontal arc lavas in combination with their trace element systematics confirm previously established regional variations of subduction components along the arc. Melting models show a clear contribution of a sediment-derived component to the HFSE budget of the frontal arc lavas, particularly affecting Zr–Hf and W. In contrast, the K-rich rear-arc lavas tap more hybrid and enriched mantle sources. The HFSE budget of the rear-arc lavas is in particular characterized by superchondritic Nb/Ta (up to 25) that are attributed to deep melting involving overprint by slab melts formed from an enriched garnet–rutile-bearing eclogitic residue. Sub-arc slab melting was potentially triggered along a slab tear beneath the Sunda arc, which is the result of the forced subduction of an oceanic basement relief ~ 8 Myr ago as confirmed by geophysical studies. The purported age of the slab tear coincides with a paucity in arc volcanism, widespread thrusting of the Javanese basement crust as well as the short-lived nature of the K-rich rear-arc volcanism at that time.

Newly discovered late Devonian and early Carboniferous ophiolite fragments in the Diyanmiao mélange in southeastern Inner Mongolia: Implications for the late Paleozoic tectonic evolution of the southeastern Central Asian Orogenic Belt

The Late Paleozoic tectonic evolution of the southeastern Central Asian Orogenic Belt (CAOB) has long been controversial, and one of the most contentious issues has been whether there was a collisional orogenic event during the Middle Paleozoic. Here we report the discovery of a Late Devonian (373.5 ± 6.8 Ma) ophiolite fragment in the Diyanmiao mélange of southeastern Inner Mongolia. This fragment, coupled with previously reported Carboniferous ophiolites in the same region, confirm the former presence of a Late Devonian ocean basin that evolved into the Carboniferous basin. The geochemical characteristics of the Devonian and Carboniferous mafic rocks in the ophiolites are consistent with evolutionary trends from mid-ocean ridge (MOR)-like to supra-subduction zone (SSZ) compositions. The presence of a Late Devonian-Carboniferous trench-arc-basin system in the region, which is represented by the Daqing-Diyanmiao and Bayan'aobao-Hongger ophiolite belts, the Baolidao arc, and the Erenhot-Hegenshan Oceanic Basin (EHB), confirms the presence of a late Devonian-Carboniferous orogenic belt in the southeastern CAOB. The western segment of the Baolidao arc was developed on continental basement (the Xilin Gol complex), whereas the eastern segment was formed on oceanic basement. This tectonic configuration is similar to that of the modern Aleutian arc, Kamchatka-Kuril Islands and Japan-Ryukyu arc system in the northern and western Pacific Ocean. During the early Permian the oceanic basin represented by the Erenhot-Hegenshan basin closed, and the Baolidao arc was accreted onto the southern continental margin of the Southern Mongolian continental block.

Geochemical consequences of oxygen diffusion from the oceanic crust into overlying sediments and its significance for biogeochemical cycles based on sediments of the northeast Pacific

Abstract. Exchange of dissolved substances at the sediment–water interface provides an important link between the short-term and long-term geochemical cycles in the ocean. A second, as yet poorly understood sediment–water exchange is supported by low-temperature circulation of seawater through the oceanic basement underneath the sediments. From the basement, upwards diffusing oxygen and other dissolved species modify the sediment, whereas reaction products diffuse from the sediment down into the basement where they are transported by the basement fluid and released to the ocean. Here, we investigate the impact of this “second” route with respect to transport, release and consumption of oxygen, nitrate, manganese, nickel and cobalt on the basis of sediment cores retrieved from the Clarion Clipperton Zone (CCZ) in the equatorial Pacific Ocean. We show that in this abyssal ocean region characterised by low organic carbon burial and sedimentation rates vast areas exist where the downward- and upward-directed diffusive fluxes of oxygen meet so that the sediments are oxic throughout. This is especially the case where sediments are thin or in the proximity of faults. Oxygen diffusing upward from the basaltic crust into the sediment contributes to the degradation of sedimentary organic matter. Where the sediments are entirely oxic, nitrate produced in the upper sediment by nitrification is lost both by upward diffusion into the bottom water and by downward diffusion into the fluids circulating within the basement. Where the oxygen profiles do not meet, they are separated by a suboxic sediment interval characterised by Mn2+ in the porewater. Where porewater Mn2+ in the suboxic zones remains low, nitrate consumption is low and the sediment continues to deliver nitrate to the ocean bottom waters and basement fluid. We observe that at elevated porewater manganese concentrations, nitrate consumption exceeds production and nitrate diffuses from the basement fluid into the sediment. Within the suboxic zone, not only manganese but also cobalt and nickel are released into the porewater by reduction of Mn oxides, diffusing towards the oxic–suboxic fronts above and below where they precipitate, effectively removing these metals from the suboxic zone and concentrating them at the two oxic–suboxic redox boundaries. We show that not only do diffusive fluxes in the top part of deep-sea sediments modify the geochemical composition over time but also diffusive fluxes of dissolved constituents from the basement into the bottom layers of the sediment. Hence, the palaeoceanographic interpretation of sedimentary layers should carefully consider such deep secondary modifications in order to prevent the misinterpretation of primary signatures.

History of Deep-sea Ocean Basement Drilling Programs and Contributions to the Earth Sciences

History of Deep-sea Ocean Basement Drilling Programs and Contributions to the Earth Sciences

Structure of an accreted intra-oceanic arc: potential-field model of a crustal cross-section through the Macquarie Arc, Lachlan Orogen, southeastern Australia

Construction of a 3D geological model for the eastern Lachlan Orogen provided a new reference model for a combined magnetic and gravity model of the Macquarie Arc, an intra-oceanic arc accreted to Gondwana during the Late Ordovician to early Silurian Benambran Orogeny. Geologically constrained, iterative 2.5 D forward modelling represented the entire thickness of the crust over a west-to-east profile extending for 230 km, approximately coinciding with seismic reflection profiles 99AGSL1 and 99AGSL2. The output model, referred to here as the Forbes model, confirms west-dipping thrust stacks of the Junee-Narromine and Molong volcanic belts of the Macquarie Arc, in packages that also comprise underlying arc basement, overlying Ordovician turbidites, and overlying Silurian–Devonian volcanic and sedimentary units. Ordovician rocks in the Parkes Fault Zone are distributed in wedges within a flower-fault structure. These structural elements are broadly similar to an earlier interpretation of the seismic profiles and corresponding potential-field models but differ in the nature of the substrate to the Macquarie Arc. The arc substrate is best fitted with physical properties broadly that of an intermediate calc-alkaline rock, rather than mid-ocean ridge basalt (MORB) as previously inferred. Highly magnetic metamafic rocks of the Jindalee Group appear to occupy relatively narrow fault-bounded slivers and may correspond to mafic volcanic rocks in the forearc rather than fragments of MORB basement. The thick intermediate-composition crustal profile required by the Forbes model cannot distinguish between an Izu-Bonin-like thickened arc or a compound of an Ordovician arc developed on a pre-existing Cambrian arc, but the simple MORB substrate previously assumed for the Macquarie Arc can be excluded, as can obduction of the Macquarie Arc as a nappe over the Wagga Belt. KEY POINTS A 3D model for the Lachlan Orogen provides geological constraints on joint magnetic and gravity model across the accreted intra-oceanic Macquarie Arc. The substrate of the arc is best fitted with magnetic susceptibility and density typical of an intermediate composition, in contrast with previous interpretations that the substrate preserves an oceanic crust basement. Western and central belts of the Macquarie Arc are stacked by west-dipping thrusts, inconsistent with emplacement as an allochthonous nappe.

Subducting oceanic basement roughness impacts on upper-plate tectonic structure and a backstop splay fault zone activated in the southern Kodiak aftershock region of the Mw 9.2, 1964 megathrust rupture, Alaska

AbstractIn 1964, the Alaska margin ruptured in a giant Mw 9.2 megathrust earthquake, the second largest during worldwide instrumental recording. The coseismic slip and aftershock region offshore Kodiak Island was surveyed in 1977–1981 to understand the region’s tectonics. We re-processed multichannel seismic (MCS) field data using current standard Kirchhoff depth migration and/or MCS traveltime tomography. Additional surveys in 1994 added P-wave velocity structure from wide-angle seismic lines and multibeam bathymetry. Published regional gravity, backscatter, and earthquake compilations also became available at this time.Beneath the trench, rough oceanic crust is covered by ∼3–5-km-thick sediment. Sediment on the subducting plate modulates the plate interface relief. The imbricate thrust faults of the accreted prism have a complex P-wave velocity structure. Landward, an accelerated increase in P-wave velocities is marked by a backstop splay fault zone (BSFZ) that marks a transition from the prism to the higher rigidity rock beneath the middle and upper slope. Structures associated with this feature may indicate fluid flow. Farther upslope, another fault extends >100 km along strike across the middle slope. Erosion from subducting seamounts leaves embayments in the frontal prism.Plate interface roughness varies along the subduction zone. Beneath the lower and middle slope, 2.5D plate interface images show modest relief, whereas the oceanic basement image is rougher. The 1964 earthquake slip maximum coincides with the leading and/or landward flank of a subducting seamount and the BSFZ. The BSFZ is a potentially active structure and should be considered in tsunami hazard assessments.

Thermal Conductivity Profile in the Nankai Accretionary Prism at IODP NanTroSEIZE Site C0002: Estimations From High‐Pressure Experiments Using Input Site Sediments

AbstractDepth profiles of sediment thermal conductivity are required for understanding the thermal structure in active seismogenic zones. During the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a scientific drilling project of the International Ocean Discovery Program (IODP), a borehole was penetrated to a depth of 3,262.5 m below seafloor (mbsf) at Site C0002. Because core samples obtained from below ~1,100 mbsf in an accretionary prism are limited, a thermal conductivity profile over such depths usually determined by laboratory measurements using core samples is not available. To obtain the thermal conductivity profile at Site C0002, we used core samples collected from sediments that overlay the incoming subducting oceanic basement at Nankai Trough Seismogenic Zone Experiment Site C0012, which can be considered to have the same mineral composition as the accretional prism at Site C0002. The thermal conductivity of the C0012 core samples was measured at high pressure to simulate subduction by reducing the sample porosity. We measured the thermal conductivity of six core samples from 144–518 mbsf at Site C0012 up to a maximum effective pressure of ~50 MPa, corresponding to depths greater than ~4 km below seafloor. We obtained an empirical relation between thermal conductivity λBulk in Wm‐1K‐1 and fractional porosity ϕ for the Nankai Trough accretionary prism as λBulk = exp(−1.09ϕ + 0.977). Based on porosity data measured using core/cuttings samples and data derived from P wave velocity logs, we estimate two consistent and complete thermal conductivity profiles down to ~3 km below seafloor in the Nankai Trough accretionary prism. These profiles are consistent with the existing thermal conductivity data measured using limited core samples.

Potential role of strike-slip faults in opening up the South China Sea.

Radiometric dates of key rock units indicate that a remnant Late Mesozoic ocean of the Huatung Basin is still preserved today east of the South China Sea (SCS). We integrate regional geology with a Cretaceous oceanic basement in the vicinity of the Huatung Basin to reconstruct the Huatung Plate east of the Eurasian continent. Results of geophysical investigations, four expeditions of deep-sea drilling and a renaissance of regional geology allow us to propose a hypothesis that the mechanism responsible for the SCS opening was raised from strike-slip fault on the east. The hypothesis suggests that the SCS opening could highly relate to the strike-slip faults inherited from Late Mesozoic structures onshore–offshore the SE Cathaysia Block to develop rhombic-shaped extensional basins en echelon on the thinned Eurasian continental crust in the Early Cenozoic. It was followed by sinistral strike-slip movements along the boundary between the Eurasian Plate and the Huatung Plate driven by oblique subduction of the Huatung Plate to the northwest coupled with slab-pull force by southward subduction of the Proto-SCS to open up the triangle-shaped oceanic East Sub-basin in the Early Oligocene (33/34 Ma). The spreading ridge then propagated southwestward in the step-over segment between the Zhongnan-Lile and the Red River strike-slip fault systems to open the triangle-shaped oceanic Southwest Sub-basin by 23 Ma. The plate boundary fault was subsequently converted into the Manila Trench when the Eocene Sierra Madre arc of the Huatung Plate had moved from the south to its present latitude by the Middle Miocene.