The continent-ocean transition near Hatton Bank was studied using a dense grid of single-ship and two-ship multichannel seismic (mcs) profiles. Extensive oceanward dipping reflectors in a sequence of igneous rocks are developed in the upper crust across the entire margin. At the landward (shallowest) end the dipping reflectors overlie continental crust, while at the seaward end they are formed above oceanic crust. Beneath the central and lower part of the margin is a mid-crustal layer approximately 5 km thick that could be either stretched and thinned continental crust or maybe newly formed igneous crust generated at the same time as the dipping reflector sequence. Beneath this mid-crustal layer and above a well defined seismic Moho which rises from 27 km (continental end) to 15 km (oceanic end) across the margin, the present lower crust comprises a 10-15 km thick lens of material with a seismic velocity of 7.3 to 7.4 km/s. We interpret the present lower crustal lens as underplated igneous rocks left after extraction of the extruded basaltic lavas. A considerable quantity of new material has been added to the crust under the rifted margin. The present Moho is a new boundary formed during creation of the margin and cannot, therefore, be used to determine the amount of thinning.
Abstract An expanded sedimentary section provides an opportunity to elucidate conditions in the nascent Chicxulub crater during the hours to millennia after the Cretaceous‐Paleogene (K‐Pg) boundary impact. The sediments were deposited by tsunami followed by seiche waves as energy in the crater declined, culminating in a thin hemipelagic marlstone unit that contains atmospheric fallout. Seiche deposits are predominantly composed of calcite formed by decarbonation of the target limestone during impact followed by carbonation in the water column. Temperatures recorded by clumped isotopes of these carbonates are in excess of 70°C, with heat likely derived from the central impact melt pool. Yet, despite the turbidity and heat, waters within the nascent crater basin soon became a viable habitat for a remarkably diverse cross section of the food chain. The earliest seiche layers deposited with days or weeks of the impact contain earliest Danian nannoplankton and dinocyst survivors. The hemipelagic marlstone representing the subsequent years to a few millennia contains a nearly monogeneric calcareous dinoflagellate resting cyst assemblage suggesting deteriorating environmental conditions, with one interpretation involving low light levels in the impact aftermath. At the same horizon, microbial fossils indicate a thriving bacterial community and unique phosphatic fossils including appendages of pelagic crustaceans, coprolites and bacteria‐tunneled fish bone, suggesting that this rapid recovery of the base of the food chain may have supported the survival of larger, higher trophic‐level organisms. The extraordinarily diverse fossil assemblage indicates that the crater was a unique habitat in the immediate impact aftermath, possibly as a result of heat and nutrients supplied by hydrothermal activity.
ABSTRACT We have previously applied three‐dimensional acoustic, anisotropic, full‐waveform inversion to a shallow‐water, wide‐angle, ocean‐bottom‐cable dataset to obtain a high‐resolution velocity model. This velocity model produced an improved match between synthetic and field data, better flattening of common‐image gathers, a closer fit to well logs, and an improvement in the pre‐stack depth‐migrated image. Nevertheless, close examination reveals that there is a systematic mismatch between the observed and predicted data from this full‐waveform inversion model, with the predicted data being consistently delayed in time. We demonstrate that this mismatch cannot be produced by systematic errors in the starting model, by errors in the assumed source wavelet, by incomplete convergence, or by the use of an insufficiently fine finite‐difference mesh. Throughout these tests, the mismatch is remarkably robust with the significant exception that we do not see an analogous mismatch when inverting synthetic acoustic data. We suspect therefore that the mismatch arises because of inadequacies in the physics that are used during inversion. For ocean‐bottom‐cable data in shallow water at low frequency, apparent observed arrival times, in wide‐angle turning‐ray data, result from the characteristics of the detailed interference pattern between primary refractions, surface ghosts, and a large suite of wide‐angle multiple reflected and/or multiple refracted arrivals. In these circumstances, the dynamics of individual arrivals can strongly influence the apparent arrival times of the resultant compound waveforms. In acoustic full‐waveform inversion, we do not normally know the density of the seabed, and we do not properly account for finite shear velocity, finite attenuation, and fine‐scale anisotropy variation, all of which can influence the relative amplitudes of different interfering arrivals, which in their turn influence the apparent kinematics. Here, we demonstrate that the introduction of a non‐physical offset‐variable water density during acoustic full‐waveform inversion of this ocean‐bottom‐cable field dataset can compensate efficiently and heuristically for these inaccuracies. This approach improves the travel‐time match and consequently increases both the accuracy and resolution of the final velocity model that is obtained using purely acoustic full‐waveform inversion at minimal additional cost.
Abstract– The 1.4–1.6 km thick Onaping Formation consists of a complex series of breccias and “melt bodies” lying above the Sudbury Igneous Complex (SIC) at the Sudbury impact structure. Based on the presence of shocked lithic clasts and various “glassy” phases, the Onaping has been described as a “suevitic” breccia, with an origin, at least in part, as fallback material. Recent mapping and a redefined stratigraphy have emphasized similarities and differences in its various vitric phases, both as clast types and discrete intrusive bodies. The nature of the Onaping and that of other “suevitic” breccias overlying impact melt sheets is reviewed. The relative thickness, internal stratigraphic and lithological character, and the relative chronology of depositional units indicate multiple processes were involved over some time in the formation of the Onaping. The Sudbury structure formed in a foreland basin and water played an essential role in the evolution of the Onaping, as indicated by a major hydrothermal system generated during its formation. Taken together, observations and interpretations of the Onaping suggest a working hypothesis for the origin of the Onaping that includes not only impact but also the interaction of sea water with the impact melt, resulting in repeated explosive interactions involving proto‐SIC materials and mixing with pre‐existing lithologies. This is complicated by additional brecciation events due to the intrusion of proto‐SIC materials into the evolving and thickening Onaping. Fragmentation mechanisms changed as the system evolved and involved vesiculation in the formation of the upper two‐thirds of the Onaping.
<p>We present velocity images across a bottom-simulating reflector (BSR) recovered using 3D high-resolution full-waveform inversion (FWI) and discuss its use as a tool for understanding the nature of the BSR.</p><p>FWI is a seismic imaging technique which generates highly resolved physical property models of the subsurface. FWI uses the full recorded waveform for inversion which leads to a superior resolution compared to other imaging methods, but also makes it computationally more expensive. Relative to 2D inversions, 3D FWI leads to image improvements due to an increase in azimuthal coverage and ability to map out-of-plane arrivals to the correct location, which is particularly important in a complex geological setting. Therefore, next to the advantage of a fully resolved 3D structure, the model will also be more accurate.</p><p>Caused by gas hydrate in an upper layer and/or free gas in a lower layer, a BSR indicates the base of the gas hydrate stability zone. This significant change of the physical properties in the upper few hundred meters of the marine sediment produces a distinct reflection, i.e. the BSR, that can be seen in the seismic image.</p><p>We are imaging and investigating a BSR at Puke Ridge, a thrust ridge on the accretionary wedge of the northern Hikurangi subduction margin, offshore the North Island of New Zealand. We are using seismic multichannel streamer data, belonging to the NZ3D dataset collected in 2018, to invert for the P-wave velocity. The resolved velocity model displays the geometry and the structure of a BSR characterised by a velocity increase followed by a sudden decrease and provides us with accurate velocities which we can use for rock physics modelling and interpretation.</p>
Abstract Geophysical data indicate that the 65.5 million years ago Chicxulub impact structure is a multi‐ring basin, with three sets of semicontinuous, arcuate ring faults and a topographic peak ring (PR). Slump blocks define a terrace zone, which steps down from the inner rim into the annular trough. Fault blocks underlie the PR, which exhibits variable relief, due to target asymmetries. The central structural uplift is >10 km, and the Moho is displaced by 1–2 km. The working hypothesis for the formation of Chicxulub is: a 50 km radius transient cavity, lined with melt and impact breccia, formed within 10 s of the impact, and within minutes, weakened rebounding crust rose kilometers above the surface, the transient crater rim underwent localized deformation and collapsed into large slump blocks, resulting in a inner rim at 70–85 km radius, and outer ring faults at 70–130 km radius. The overheightened structural uplift collapsed outward, buried the inner slump blocks, and formed the PR. Most of the impact melt was ultimately emplaced as a coherent <3 km thick melt sheet within the central basin that shallows within the inner regions of the PR. Smaller pockets of melt flowed into the annular trough. Subsequently, slope collapse, ejecta, ground surge, and tsunami waves infilled the annular trough and annular basin with sediments up to 3 km and 900 m thick, respectively. Testing this working hypothesis requires direct observation of the impactites, within and adjacent to the PR and central basin.
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