Recognizing detachment-mode seafloor spreading in the deep geological past.

Large-offset oceanic detachment faults are a characteristic of slow- and ultraslow-spreading ridges, leading to the formation of oceanic core complexes (OCCs) that expose upper mantle and lower crustal rocks on the seafloor. The lithospheric extension accommodated by these structures is now recognized as a fundamentally distinct “detachment-mode” of seafloor spreading compared to classical magmatic accretion. Here we demonstrate a paleomagnetic methodology that allows unequivocal recognition of detachment-mode seafloor spreading in ancient ophiolites and apply this to a potential Jurassic detachment fault system in the Mirdita ophiolite (Albania). We show that footwall and hanging wall blocks either side of an inferred detachment have significantly different magnetizations that can only be explained by relative rotation during seafloor spreading. The style of rotation is shown to be identical to rolling hinge footwall rotation documented recently in OCCs in the Atlantic, confirming that detachment-mode spreading operated at least as far back as the Jurassic

Louisville Seamount Trail: implications for geodynamic mantle flow models and the geochemical evolution of primary hotspots

The Louisville Seamount Trail is a 4300 km long volcanic chain that has been built in the past 80 m.y. as the Pacific plate moved over a persistent mantle melting anomaly or hotspot. Because of its linear morphology and its long-lived age-progressive volcanism, Louisville is the South Pacific counterpart of the much better studied Hawaiian-Emperor Seamount Trail. Together, Louisville and Hawaii are textbook examples of two primary hotspots that have been keystones in deciphering the motion of the Pacific plate relative to a set of "fixed" deep-mantle plumes. However, drilling during Ocean Drilling Program (ODP) Leg 197 in the Emperor Seamounts documented a large ~15° southward motion of the Hawaiian hotspot prior to 50 Ma. Is it possible that the Hawaiian and Louisville hotspots moved in concert and thus constitute a moving reference frame for modeling plate motion in the Pacific? Alternatively, could they have moved independently, as predicted by mantle flow models that reproduce the observed latitudinal motion for Hawaii but that predict a largely longitudinal shift for the Louisville hotspot? These two end-member geodynamic models were tested during Integrated Ocean Drilling Program (IODP) Expedition 330 to the Louisville Seamount Trail. In addition, existing data from dredged lavas suggest that the mantle plume source of the Louisville hotspot has been remarkably homogeneous for as long as 80 m.y. These lavas are predominantly alkali basalts and likely represent a mostly alkalic shield-building stage, which is in sharp contrast to the massive tholeiitic shield-building stage of Hawaiian volcanoes. Geochemical and isotopic data for the recovered lavas during Expedition 330 will provide insights into the magmatic evolution and melting processes of individual Louisville volcanoes, their progression from shield-building to postshield and (maybe) posterosional stages, the temperature and depth of partial melting of their mantle plume source, and the enigmatic long-lived and apparent geochemical homogeneity of the Louisville mantle source. Collectively, this will enable us to characterize the Louisville Seamount Trail as a product of one of the few global primary hotspots, to better constrain its plume-lithosphere interactions, and to further test the hypothesis that the Ontong Java Plateau formed from the plume head of the Louisville mantle plume around 120 Ma. During Expedition 330 we replicated the drilling strategy of Leg 197, the first expedition to provide compelling evidence for the motion of the Hawaiian mantle plume between 80 and 50 Ma. For that reason we targeted Louisville seamounts that have ages similar to Detroit, Suiko, Nintoku, and Koko Seamounts in the Emperor Seamount Trail. In total, five seamounts were drilled in the Louisville Seamount Trail: Canopus, Rigil, Burton, Achernar, and Hadar Guyots (old to young). By analyzing a large number of time-independent in situ lava flows (and other volcanic eruptive products) from these seamounts using modern paleomagnetic, 40Ar/39Ar geochronological, and geochemical techniques, we will be able to directly compare the paleolatitude estimates and geochemical signatures between the two longest-lived hotspot systems in the Pacific Ocean. We drilled into the summits of the five Louisville guyots and reached volcanic basement at four of these drilling targets. In two cases we targeted larger seamount structures and drilled near the flanks of these ancient volcanoes, and in the other three cases we selected smaller edifices that we drilled closer to their centers. Drilling and logging plans for each of these sites were similar, with coring reaching 522.0 meters below seafloor (mbsf) for Site U1374 and 232.9, 65.7, 11.5, 182.8, and 53.3 mbsf for Sites U1372, U1373, U1375, U1376, and U1377, respectively. Some Expedition 330 drill sites were capped with only a thin layer of pelagic ooze between 6.6 and 13.5 m thick, and, if present, these were cored by using a low-rotation gravity-push technique with the rotary core barrel to maximize recovery. However, at Sites U1373 and U1376 no pelagic ooze was present, and the holes needed to be started directly into cobble-rich hardgrounds. In all cases, the bulk of the seamount sediment cover comprised sequences of volcanic sandstones and various kinds of basalt breccia or basalt conglomerate, which often were interspersed with basaltic lava flows, the spatter/tephra products of submarine eruptions, or other volcanic products, including auto-brecciated flows or peperites. Also several intervals of carbonate were cored, with the special occurrence of a ~15 m thick algal limestone reef at Site U1376 on Burton Guyot. In addition, some condensed pelagic limestone units were recovered on three of the other seamounts, but these did not exceed 30 cm in thickness. Despite their limited presence in the drilled sediment, these limestones provide valuable insights for the paleoclimate record at high ~50° southern latitudes since Mesozoic times. Several Louisville sites progressed from subaerial conditions in the top of volcanic basement into submarine eruptive environments, or drilling of the igneous basement immediately started in submarine volcanic sequences, as was the case for Sites U1376 and U1377 on Burton and Hadar Guyots. At three sites we cored >100 m into the igneous basement: 187.3 m at Site U1372, 505.3 m at Site U1374, and 140.9 m at Site U1376. At the other sites we did not core into basement (Site U1375) or we cored only 38.2 m (Site U1377) because of unstable hole conditions. Even so, drilling during Expedition 330 resulted in a large number of in situ lava flows, pillow basalts, or other types of volcanic products such as auto-brecciated lava flows, intrusive sheets or dikes, and peperites. In particular, the three holes on Canopus and Rigil Guyots (the two oldest seamounts drilled in the Louisville Seamount Trail), resulted in adequate numbers of in situ lava flows to average out paleosecular variation, with probable eruption ages estimated at ~78 and 73 Ma, respectively. Remarkably, at all drill sites large quantities of hyaloclastites, volcanic sandstones, and basaltic breccias were also recovered, which in many cases show consistent paleomagnetic inclinations compared to the lava flows bracketing these units. For Site U1374 on Rigil Guyot we also observed a magnetic polarity reversal in the cored sequence. Overall, this is very promising for determining a reliable paleolatitude record for the Louisville Seamounts following detailed postcruise examinations. The deeper penetrations of several hundred meters required bit changes and reentries using free-fall funnels. Basement penetration rates were 1.8–2.5 m/h depending on drill depth. In total, 1114 m of sediment and igneous basement at five seamounts was drilled, and 806 m was recovered (average recovery = 72.4%). At Site U1374 on Rigil Guyot, a total of 522 m was drilled, with a record-breaking 87.8% recovery. Most outstandingly, nearly all Expedition 330 core material is characterized by low degrees of alteration, providing us with a large quantity of samples of mostly well-preserved basalt, containing, for example, pristine olivine crystals with melt inclusions, fresh volcanic glass, unaltered plagioclase, carbonate, zeolite and celadonite alteration minerals, various micro- and macrofossils, and, in one case, mantle xenoliths and xenocrysts. The large quantity and excellent quality of the recovered sample material allow us to address all the scientific objectives of this expedition and beyond

New and revised 14C dates for Hawaiian surface lava flows: paleomagnetic and geomagnetic implications

Radiocarbon dates have been obtained for 30 charcoal samples corresponding to 27 surface lava flows from the Mauna Loa and Kilauea volcanoes on the Island of Hawaii. The submitted charcoal was a mixture of fresh and archived material. Preparation and analysis was undertaken at the NERC Radiocarbon Laboratory in Glasgow, Scotland, and the associated SUERC Accelerator Mass Spectrometry facility. The resulting dates range from 390 years B.P. to 12,910 years B.P. with corresponding error bars an order of magnitude smaller than previously obtained using the gas-counting method. The new and revised 14C data set can aid hazard and risk assessment on the island. The data presented here also have implications for geomagnetic modelling, which at present is limited by large dating errors. <br/

Data report: spatial and temporal evolution of slow spread oceanic crust - graphics sections of core recovered from IODP Hole U1309D, Atlantis Massif, 30N, MAR (including Pb/U zircon geochronology and magnetic remanence data)

This contribution presents a graphical downhole log for core recovered from Integrated Ocean Drilling Program (IODP) Hole U1309D made during postcruise study at the Bremen Core Repository. More than 3500 observations of the rock composition and structural relations are presented, including igneous composition, texture and grain size variations, layering and intrusive contact orientation, and younger/older intrusive relations, together with observations of fault/shear zone thickness and orientation and alteration mineral assemblages. In addition, remanence components for 523 new specimens are shown along with shipboard magnetic susceptibility and 18 Pb/U zircon ages from evolved Fe-Ti oxide gabbro and felsic dikes from Hole U1309D. This compilation is a resource for researchers interested in the composition, construction, and deformation of slow-spreading gabbroic crust, in particular the scale/thickness of intrusive bodies and the order of intrusion

Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples

Oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large-slip detachment faults. The common occurrence of these structures in slow and ultra-slow spread oceanic crust suggests that they accommodate a significant component of plate divergence. However, the subsurface geometry of detachment faults in oceanic core complexes remains unclear. Competing models involve either: (a) displacement on planar, low-angle faults with little tectonic rotation; or (b) progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading (the “rolling-hinge” model). We address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1.4 km long footwall section of gabbroic rocks recovered by Integrated Ocean Drilling Program (IODP) sampling at Atlantis Massif oceanic core complex on the Mid-Atlantic Ridge (MAR). These rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. For the first time we have independently reoriented drill-core samples of lower crustal gabbros, that were initially azimuthally unconstrained, to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall. This allows reorientation of the palaeomagnetic data, placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data. Analysis of the reoriented high temperature reversed component of magnetization indicates a 46° ± 6° anticlockwise rotation of the footwall around a MAR-parallel horizontal axis trending 011° ± 6°. Reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the Jaramillo chron (0.99 Ma). The data provide unequivocal confirmation of the key prediction of flexural, rolling-hinge models for oceanic core complexes, whereby oceanic detachment faults initiate at higher dips and rotate to their present day low-angle geometries as displacement increases.<br/

Microwave palaeointensities from Holocene age Hawaiian lavas: investigation of magnetic properties and comparison with thermal palaeointensities

Sixteen -dated Hawaiian surface lava flows spanning 0–4.5 ka have been investigated using the microwave perpendicular applied field palaeointensity technique and classical thermal Thellier–Thellier palaeointensity technique in parallel. The overall microwave experimental success rate is 63% compared to 59% in the thermal experiments. Nineteen percent of the microwave results are deemed to be first class compared to 52% of the thermal results. High palaeointensities and a large amount of within-flow variation are seen in the microwave results with reliable palaeointensity estimates ranging from 25.10 to 82.94 T. These variations do not appear to be systematically related to variations in rock magnetic properties such as mineralogy, grain size, texture or oxidation state. The flow-means from the microwave and thermal studies agree for 9 of the 13 flows that can be directly compared and the average difference between the two techniques is not significantly different from zero. Neither experimental technique produces mean palaeointensities systematically lower or more precise than the other, suggesting that the differences seen are a reflection of natural within-flow variation rather than experimental technique.<br/

Evaluation of prognostic risk models for postoperative pulmonary complications in adult patients undergoing major abdominal surgery: a systematic review and international external validation cohort study

Background Stratifying risk of postoperative pulmonary complications after major abdominal surgery allows clinicians to modify risk through targeted interventions and enhanced monitoring. In this study, we aimed to identify and validate prognostic models against a new consensus definition of postoperative pulmonary complications. Methods We did a systematic review and international external validation cohort study. The systematic review was done in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. We searched MEDLINE and Embase on March 1, 2020, for articles published in English that reported on risk prediction models for postoperative pulmonary complications following abdominal surgery. External validation of existing models was done within a prospective international cohort study of adult patients (≥18 years) undergoing major abdominal surgery. Data were collected between Jan 1, 2019, and April 30, 2019, in the UK, Ireland, and Australia. Discriminative ability and prognostic accuracy summary statistics were compared between models for the 30-day postoperative pulmonary complication rate as defined by the Standardised Endpoints in Perioperative Medicine Core Outcome Measures in Perioperative and Anaesthetic Care (StEP-COMPAC). Model performance was compared using the area under the receiver operating characteristic curve (AUROCC). Findings In total, we identified 2903 records from our literature search; of which, 2514 (86·6%) unique records were screened, 121 (4·8%) of 2514 full texts were assessed for eligibility, and 29 unique prognostic models were identified. Nine (31·0%) of 29 models had score development reported only, 19 (65·5%) had undergone internal validation, and only four (13·8%) had been externally validated. Data to validate six eligible models were collected in the international external validation cohort study. Data from 11 591 patients were available, with an overall postoperative pulmonary complication rate of 7·8% (n=903). None of the six models showed good discrimination (defined as AUROCC ≥0·70) for identifying postoperative pulmonary complications, with the Assess Respiratory Risk in Surgical Patients in Catalonia score showing the best discrimination (AUROCC 0·700 [95% CI 0·683–0·717]). Interpretation In the pre-COVID-19 pandemic data, variability in the risk of pulmonary complications (StEP-COMPAC definition) following major abdominal surgery was poorly described by existing prognostication tools. To improve surgical safety during the COVID-19 pandemic recovery and beyond, novel risk stratification tools are required. Funding British Journal of Surgery Society