Timothy J. Reston

Mechanical decoupling and basal duplex formation observed in sandbox experiments with application to the Western Mediterranean Ridge accretionary complex

Sandbox experiments of accretionary wedges were performed incorporating a thin weak layer of micro glass beads. The impact of heterogeneous sedimentary input on wedge mechanics, evolution and mass transfer was investigated. We report the first experimentally documented growth of basal duplexes. These occurred for high basal friction conditions, with restricted output of the lower section. The upper and lower sections were completely decoupled due to the intervening layer of glass beads, with frontal accretion occurring in the upper section simultaneously with basal duplex formation and underplating of subsequent generations of duplexes. IMERSE multichannel seismic reflection data from the Western Mediterranean Ridge (WMR) image Tertiary clastics beneath a thick section of Messinian evaporites. The base of the evaporites is identified as the primary decollement for deformation in the frontal part of the accretionary complex. Constriction of the channel of subducting Tertiary sediments, as well as internal deformation observed as arcward-dipping reflectors argue for basal underplating and/or two different active decollements. We propose an evolution of the WMR in accordance with the sandbox experimental results. A weak mid-level detachment (base of evaporites) combined with a strong basal detachment produce mechanical decoupling and basal accretion of toeward-verging duplexes.

Contemporaneous mass extinctions, continental flood basalts, and ‘impact signals’: are mantle plume-induced lithospheric gas explosions the causal link?

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.

Detachment faulting at ocean core complexes

Intracrustal reflections from a Cretaceous inside corner are interpreted as a detachment fault that dips gently within the basement and can be followed up the flank of a domed high. The fault heave is sufficient for ridge-normal extension to have exhumed deep-crustal and probably mantle rocks, as sampled at some inside corner domed massifs. The morphology of these massifs closely resembles that of the Cretaceous high, implying that the structures are analogous and supporting the notion that modern inside corner massifs are oceanic core complexes formed by unroofing the footwalls of similar detachment faults, partially exposed as corrugated slip surfaces. This is the first demonstration of the overall geometry of an oceanic core complex detachment fault.

Detachment faulting, mantle serpentinization, and serpentinite- mud volcanism beneath the Porcupine Basin, southwest of Ireland

The Porcupine Basin southwest of Ireland provides an opportunity to study the symmetry of rifting at stretching factors approaching continental breakup. Profiles across the basin image a bright reflection that appears to represent a detachment fault, and may in part be a decollement at the top of partially serpentinized mantle. Although overall the basin appears symmetric, the consistent westward structural dip of the detachment implies that, at high stretching factors, extension was asymmetric. Farther south, the Porcupine median high appears in cross section to be a triangular construction overlying tilted fault blocks and onlapped by postrift sediment. Despite no evidence for synrift magmatism, this high has previously been interpreted as a basaltic structure. However, it may represent a serpentinite-mud volcano or diapir: we suggest that such structures produce the serpentinite breccias found within the rifted continent-ocean transition of nonvolcanic margins.

REFLECTIVE OCEANIC CRUST FORMED AT A FAST-SPREADING CENTER IN THE PACIFIC

We present new seismic images of Cretaceous crust formed at a fast-spreading center in the Pacific. The high crustal reflectivity observed in these data contradicts the conventional wisdom that accretionary structures formed at fast-spreading centers are not seismically detectable. Subhorizontal reflections can be traced at 600–800 ms two-way time below the top of basement for tens of kilometres, suggesting the presence of a widespread seismic boundary, possibly a structural discontinuity related to the maximum depth of hydrothermal circulation at the spreading center or the base of the sheeted dikes. Lower-crustal reflections, dipping dominantly toward the paleo–spreading center, may represent mafic-ultramafic banding similar to that observed in the lower crust of reconstructed ophiolite sections.

Serpentinization and magmatism during extension at non-volcanic margins: the effect of initial lithospheric structure

Abstract At several non-volcanic margins serpentinized peridotites occur within a wide continent-ocean transition (COT) and beneath the edge of the thinned continental crust. However, other margins such as the Woodlark Basin appear to have a sharp COT and no reported serpentinites. We investigate the thermal, magmatic and rheological evolution of margins during extension as a function of initial lithospheric structure, rift duration and stretching factor. For cratonic and old orogen models, the entire crust should become brittle at stretching factors c. 3–4. The resultant crust-cutting faults allow water to reach and serpentinize the mantle, leading to the development of serpentinite décollements at the crust-mantle boundary and exhumation of mantle at the COT. Our predictions are consistent with the spatial limit and thickness of serpentinites at the SW Greenland and West Iberia margins, and the Rockall Trough. They also explain the absence of a broad zone of unroofed, serpentinized mantle at the COT of the Woodlark Basin: here the crust was too thick and hot for serpentinites to form before break-up. Larger melt production than in the West Iberia type margins and concentration of the lithospheric strength in the crust leads to synchronous crustal separation and lithospheric failure, yielding a sharp COT.

The structure of Cretaceous oceanic crust of the NW Pacific: Constraints on processes at fast spreading centers

A 725 km long transect along a flow line in the NW Pacific provides new images of the internal structure of Cretaceous oceanic crust formed at fast spreading rates. Reflections from the Moho transition zone document changes in crustal travel time along the profile: variations with a wavelength of 100–150 km may representing fluctuations in the magma supply with a 2–3 m.y. periodicity; to the south the crust may be several kilometers thicker than “normal” in places, possibly resulting from increased magma supply due to the distal effects of the Shatsky hotspot. At about the depth expected for the boundary between seismic layers 2 and 3, sub-horizontal reflections are imaged over much of the profile; the clearest are positive polarity and of an amplitude consistent with a reflection coefficient of ∼0.1, implying a sharp velocity and density discontinuity. Northward (ridgeward) dipping reflections in the lower crust are imaged over much of the profile, particularly toward the north (away from the region possibly affected by the hotspot). These reflections stop at the Moho reflection but die out perhaps 1 s beneath top basement; they probably represent lithological layering as predicted by models of ductile flow accompanying passive upwelling during plate separation, although they could represent secondary shears generated by basal drag during active mantle upwelling beneath the spreading center. The discrete character and, in places, regular spacing (every ∼8 km) of these features hint at a cyclicity in the layering and thus at some episodicity in magmatic processes over a timescale of 0.1–0.2 m.y.