ALBERT

Description: Rifted margins are commonly characterised by an extension discrepancy: the amount of extension measurable from the observed faulting is far less than that required to explain the crustal thinning. It is shown here that polyphase faulting may provide a simple explanation for this paradox, but can be very hard to recognise on seismic sections. However at the west Galicia rifted margin (the Galicia Interior Basin between the Galicia Bank and the mainland, and the deep Galicia margin to the west of the Galicia Bank), a combination of high quality depth images, seismic velocity information and stratigraphic control through ODP drilling and submersible sampling, provide complementary evidence for polyphase faulting. Berriasian–Hauterivian rifting in the Galicia Interior Basin occurred along two sets of faults: the first unroofed deep crustal rocks, evidenced by high seismic velocities close to top basement; the second cut and dismembered these early faults. Further rifting (up until the Aptian) then focussed west of the Galicia Bank, where two further phases of faulting can be inferred from the diachronous nature of seismostratigraphic units tilted within fault blocks. Removal of the latest phase of faulting aligns discontinuous reflections within the fault blocks into an anastomosing network of earlier faults; restoration along these brings the crust back to its early Hauterivian state, similar to the present structure of the Interior Basin.

Description: Highlights • Magmatism, detachment faulting and changing symmetry of crustal accretion at a ridge segment end. • PmPs from an OCC argue for constant magma production during detachment faulting. • Refraction seismic modelling and PmP events reveal very thin (4 km) oceanic crust at a segment end. A wide-angle seismic section across the Mid-Atlantic Ridge just south of the Ascension transform system reveals laterally varying crustal thickness, and to the east a strongly distorted Moho that appears to result from slip along a large-offset normal fault, termed an oceanic detachment fault. Gravity modelling supports the inferred crustal structure. We investigate the interplay between magmatism, detachment faulting and the changing asymmetry of crustal accretion, and consider several possible scenarios. The one that appears most likely is remarkably simple: an episode of detachment faulting which accommodates all plate divergence and results in the westward migration of the ridge axis, is interspersed with dominantly magmatic and moderately asymmetric (most on the western side) spreading which moves the spreading axis back towards the east. Following the runaway weakening of a normal fault and its development into an oceanic detachment fault, magma both intrudes the footwall to the fault, producing a layer of gabbro (subsequently partially exhumed).

Description: “Non-volcanic” rifted margins exhibit very little evidence of synrift magmatism, even where the continental crust has been thinned to such an extent that the mantle has been exhumed across a transitional zone (up to ∼100 km wide), called the continent–ocean transition (COT). Using dynamical models of rifting, we explore how extension velocity, mantle composition and potential temperature influence the nature and extent of the COT and compare our results to observations at the West Iberia margin (WIM) and the ancient margins of the Liguria-Piemonte Ocean (LP) now exposed in the Alps. We find a first order relationship between extension velocity and the amagmatic exposure of mantle at the COT. For very slow half extension velocities, (〈 6 mm/yr), mantle exhumation begins before melting. At these velocities, by the time melting starts at the rift centre, the area of exhumed mantle has moved sideways creating a COT, the width of which increases with decreasing velocities. However, at 10 mm/yr, a velocity probably appropriate for the exhumation of mantle at the WIM and LP, melting starts prior to mantle exhumation. In this case, our models show that by the time mantle exhumation starts, a ∼4.5 km column of melt has been produced, much more than the ∼2 km maximum mean melt thickness inferred at the COT of these margins. Even considering that 25% of the produced melt may be trapped in the mantle, as in slow-spreading mid-ocean ridges, still more melt is produced in the models than inferred from observations. Thus, extension velocity alone cannot explain the practical absence of synrift magmatism at the COT of the WIM and LP. We find that the formation of a wide, amagmatic COT requires that either the mantle was depleted in basaltic constituents by 〉 10% prior to rifting or that its potential temperature was ∼50 °C lower than normal (≤ 1250 °C).

Description: High-resolution seismic experiments, employing arrays of closely spaced, four-component ocean-bottom seismic recorders, were conducted at a site off western Svalbard and a site on the northern margin of the Storegga slide, off Norway to investigate how well seismic data can be used to determine the concentration of methane hydrate beneath the seabed. Data from P-waves and from S-waves generated by P–S conversion on reflection were inverted for P- and S-wave velocity (Vp and Vs), using 3D travel-time tomography, 2D ray-tracing inversion and 1D waveform inversion. At the NW Svalbard site, positive Vp anomalies above a sea-bottom-simulating reflector (BSR) indicate the presence of gas hydrate. A zone containing free gas up to 150-m thick, lying immediately beneath the BSR, is indicated by a large reduction in Vp without significant reduction in Vs. At the Storegga site, the lateral and vertical variation in Vp and Vs and the variation in amplitude and polarity of reflectors indicate a heterogeneous distribution of hydrate that is related to a stratigraphically mediated distribution of free gas beneath the BSR. Derivation of hydrate content from Vp and Vs was evaluated, using different models for how hydrate affects the seismic properties of the sediment host and different approaches for estimating the background-velocity of the sediment host. The error in the average Vp of an interval of 20-m thickness is about 2.5%, at 95% confidence, and yields a resolution of hydrate concentration of about 3%, if hydrate forms a connected framework, or about 7%, if it is both pore-filling and framework-forming. At NW Svalbard, in a zone about 90-m thick above the BSR, a Biot-theory-based method predicts hydrate concentrations of up to 11% of pore space, and an effective-medium-based method predicts concentrations of up to 6%, if hydrate forms a connected framework, or 12%, if hydrate is both pore-filling and framework-forming. At Storegga, hydrate concentrations of up to 10% or 20% were predicted, depending on the hydrate model, in a zone about 120-m thick above a BSR. With seismic techniques alone, we can only estimate with any confidence the average hydrate content of broad intervals containing more than one layer, not only because of the uncertainty in the layer-by-layer variation in lithology, but also because of the negative correlation in the errors of estimation of velocity between adjacent layers. In this investigation, an interval of about 20-m thickness (equivalent to between 2 and 5 layers in the model used for waveform inversion) was the smallest within which one could sensibly estimate the hydrate content. If lithological layering much thinner than 20-m thickness controls hydrate content, then hydrate concentrations within layers could significantly exceed or fall below the average values derived from seismic data.

Description: Seismic velocities obtained from ocean-bottom hydrophone, expanding spread profile and multi-channel seismic data were used to compile a velocity model for the Mediterranean Ridge along a 220-km-long transect extending from the Sirte Abyssal Plain to the Cleft region near the Hellenic Trough. A 200–300-m-thin layer of Plio–Quaternary sediments with velocities of 1800–2200 m s−1 covers the whole Ridge. The Messinian evaporites (4000–4500 m s−1) occur in the southwest as a tectonically thickened layer and in a basin just northeast of the crest of the Ridge. In the intervening region however, the evaporites appear absent and the seismic velocities are generally lower. Arched reflectors, imaged in the depth-migrated section, suggest that the sediments beneath the Ridge crest belong to a Pre-Messinian accretionary wedge. Beneath the Messinian evaporites a northeastward-thinning layer of probable Tertiary sediments shows laterally increasing velocities from 3300 m s−1 to 4600 m s−1. Assuming that the layer thinning is caused by compaction due to increased overburden alone, we have calculated a porosity reduction from 15% to 4% and an associated fluid expulsion of 10 km3 km−1 along the trench axis. This corresponds to c. 60% of the initial fluid volume of an undeformed sediment column from the abyssal plain. The almost impermeable evaporitic cap over these sediments leads to high fluid pressures at the base of the evaporites, likely to make this horizon the basal décollement of the modern accretionary system. A 2.5-km-thick unit of probable Mesozoic carbonates with velocities of 4500–4600 m s−1 is inferred at c. 8 km depth. The top of the oceanic crust occurs at a depth of about 10 km. The results from this study have widespread implications for the understanding of the regional geological history.

Description: Contemporaneous occurrences of the geologic signals of ‘large impacts’, craton-associated continental flood basalts, and mass extinctions have occurred far too often during the past 400 Myr to be plausibly attributed to random coincidence. While there is only a 1 in 8 chance that even one synchronous large impact within the interval of a continental flood basalt and mass extinction event should have happened during this period, there is now geologic evidence of four such ‘coincidences’, implying causal links between them. The ∼66 Ma (K–T) evidence suggests that impacts do not trigger flood basalts, since the Deccan flood basalt had started erupting well before the Chicxulub impact event. If extraterrestrial impacts do not trigger continental flood basalt volcanism, then we are really only left with two possible resolutions to the dilemma posed by these mega-coincidences: either the reported ‘impact signals’ at the times of great mass extinctions are spurious or misleading, or – somehow – a terrestrial process linked to continental rifting and the eruption of cratonic flood basalts is sometimes able to generate the shocked quartz, microspherules, and other geologic traces commonly attributed to large extraterrestrial impacts, while also triggering a mass extinction event. Here we explore a promising mechanistic link: a large explosive carbon-rich gas release event from cratonic lithosphere, triggered by mantle plume incubation beneath cratonic lithosphere, and typically associated with the onset phase of continental rifting. Sudden CO2/CO and SO2 release into the atmosphere would provide the primary killing mechanism of the induced extinction event. Such explosive deep-lithospheric blasts could create shock waves, cavitation, and mass jet formation within the venting region that could both create and transport a sufficiently large mass of shocked crust and mantle into globally dispersive super-stratospheric trajectories. We suggest these be called ‘Verneshot’ events.

Description: The Mediterranean Ridge is a unique accretionary complex, consisting of five key elements: the frontal slope, the upper slope, the crest of the Ridge, the Cleft area, and the Inner Plateau. The IMERSE data show that these correspond roughly to the locus of frontal accretion, of underplating, of a pre-Messinian wedge, of complex faulting and possible strike-slip tectonics, and of a backstop of Hellenic nappes covered by a Messinian forearc basin. The frontal portion of the complex is a post-Messinian accretionary wedge (composed of Messinian evaporites and overlying tightly folded Plio–Quaternary sediments), underlain by pre-Messinian sequences attached to the African Plate. The basal detachment at the front of the wedge occurs at the base of the evaporites. Moving further to the northeast (the upper slope), the basal detachment cuts to deeper levels leading to the development of duplex structures where pre-Messinian units are subcreted beneath the Messinian evaporites. Just behind the subcreted units, the evaporites thin and may be absent on the crest of the Ridge. This region we interpret as the site of a pre-Messinian accretionary wedge: we suggest that following the deposition of thick evaporites in the Messinian, the pre-Messinian accretionary tectonics continued as subsurface accretion (subcretion) beneath the evaporites. Although the crest of the Ridge is largely devoid of evaporites, local deep evaporite basins observed here formed as local closed basins on top of the pre-Messinian wedge. We infer that the Messinian sealevel was at about the level of the Ridge crest, that is 3000 m below present. Allowing for isostatic adjustment to the removal of the water load, this would imply a sealevel drop of at least 2000 m. The Cleft basins mark the northeast limit of the accretionary complex. Thick evaporite deposits to the northeast (beneath the Inner Plateau) may have been deposited in a Messinian forearc basin. The evaporites of the Inner Plateau are underlain by a thin pre-Messinian sequence and by crystalline basement of the Hellenic nappes. This basement forms the backstop to the accretionary complex.