Seismic evidence for a sharp lithospheric base persisting to the lowermost mantle beneath the Caribbean

Broad-band data from South American earthquakes recorded by Californian seismic networks are analysed using a newly developed seismic wave migration method—the slowness backazimuth weighted migration (SBWM). Using the SBWM, out-of-plane seismic P -wave reflections have been observed. The reflection locations extend throughout the Earth's lower mantle, down to the core–mantle boundary (CMB) and coincide with the edges of tomographically mapped high seismic velocities. Modelling using synthetic seismograms suggests that a narrow (10–15 km) low- or high-velocity lamella with about 2 per cent velocity contrast can reproduce the observed reflected waveforms, but other explanations may exist. Considering the reflection locations and synthetic modelling, the observed out-of-plane energy is well explained by underside reflections off a sharp reflector at the base of the subducted lithosphere. We also detect weaker reflections corresponding to the tomographically mapped top of the slab, which may arise from the boundary between the Nazca plate and the overlying former basaltic oceanic crust. The joint interpretation of the waveform modelling and geodynamic considerations indicate mass flux of the former oceanic lithosphere and basaltic crust across the 660 km discontinuity, linking processes and structure at the top and bottom of the Earth's mantle, supporting the idea of whole mantle convection.     

Active tectonics of an apparently aseismic region: distributed active strike-slip faulting in the Hangay Mountains of central Mongolia

We identify and describe a series of east–west left-lateral strike-slip faults (named the Songino-Margats, the Hag Nuur, the Uliastay and the South Hangay fault systems) in the Hangay mountains of central Mongolia: an area that has little in the way of recorded seismicity and which is often considered as a rigid block within the India–Eurasia collision zone. The strike-slip faults of central Mongolia constitute a previously unrecognized hazard in this part of Mongolia. Each of the strike-slip faults show indications of late Quaternary activity in the form of aligned sequences of sag-ponds and pressure-ridges developed in alluvial deposits. Total bed-rock displacements of ∼3 km are measured on both the Songino-Margats and South Hangay fault systems. Bed-rock displacements of 11 km are observed across the Hag Nuur fault. Cumulative offset across the Uliastay fault systems are unknown but are unlikely to be large. We have no quantitative constraint on the age of faulting in the Hangay. The ≤20 km of cumulative slip on the Hangay faults might, at least in part, be inherited from earlier tectonic movements. Our observations show that, despite the almost complete absence of instrumentally recorded seismicity in the Hangay, this part of Mongolia is cut through by numerous distributed strike-slip faults that accommodate regional left-lateral shear between Siberia and China. Central Mongolia is thus an important component of the India–Eurasia collision that would be overlooked in models of the active tectonics based on the distribution of seismicity. We suggest that active faults such as those identified in the Hangay of Mongolia might exist in other, apparently aseismic, regions within continental collision zones.     

Deep geoelectric structure over the Lower Brahmaputra valley and Shillong Plateau, NE India using magnetotellurics

Magnetotelluric studies over the Shillong plateau and lower Brahmaputra sediments have delineated the Dauki fault as a NE–SW striking thrust zone with a dip angle of about 30°, along which the low resistivity layer of Bengal sediments and the underlying oceanic crust subduct to the northwest. At present, about 50 km length of these sequences has subducted beneath the Shillong plateau and is traced up to depth of about 40 km. Another thrust zone, sub parallel to the Dauki thrust is observed in the lower Brahmaputra valley, corresponding to the Brahmaputra fault. This is interpreted to be an intracratonic thrust within the Indian plate. These results suggest that a large fraction of the seismicity over the Shillong plateau is associated with the NE–SW striking Dauki thrust, contrary to the earlier belief that this fault zone is relatively aseismic. The present studies also suggest that the Shillong plateau and the adjoining sedimentary layers act as a supracrustal block, not directly participating in the subduction process. However in response to the compressive tectonic forces generated by the Himalayan and Indo-Burman subduction processes the Shillong plateau, together with the Brahmaputra sediments overlying the Indian crust drift eastwards relative to the Bengal sediments along the surface expression of the Dauki fault leading to a dextral strike slip movement. We thus propose that the NE Indian crust responds to the compressive forces differently at different depths, governed by the rheological considerations. At deeper levels the crustal readjustments take place through the subduction along the Dauki and Brahmaputra thrusts where as, at the shallow levels the relative deformability of the supracrustal blocks have a strong influence on the tectonics, leading to the strike slip mechanism along the surface expression of the Dauki fault.     

On the use of satellite altimetry to infer the earthquake rupture characteristics: application to the 2004 Sumatra event

Standard data and methods, such as the inversion of seismic and GPS data, have been used extensively to infer the details of the 2004 December 26 earthquake. The unprecedented large size of this event gave the opportunity to modern altimeters to provide the first clear records of a tsunami in deep ocean, therefore allowing us to study the rupture history from an independent perspective. We invert the Jason-1 and Topex–Poseidon altimetry records, considering the new constraints available on the geometry of the fault plane, and taking them into account in a 3-D rupture model. The data are corrected for the non-negligible effect of satellite motion during measurements. Our results show that the rupture propagated over the 1500 km of subduction zone initially identified by the aftershock distribution, with a magnitude of   M _w= 9.1  . Our solution compares well with the latitudinal distribution of slip inferred from other data sets, with a maximum of energy release north of Sumatra, and two other slip patches near the Nicobar and Andaman islands. Based on waveform comparison, we assert that the shallow portion of the megathrust offshore Banda Aceh had slip amplitudes of more than 20 m. Also, we find that significant amounts of slip (about 10 m) concentrated below the Andaman islands and did not propagate on the shallow portion of the interface. Although synthetic tests tend to show less resolution in the northern part of the rupture, this solution is compatible with the near-field data (GPS, coral heads and imagery), and would allow one to explain the apparent paradox between the large local displacements and the moderate tsunami observed locally. Finally, we demonstrate the rapidly dominating effect of propagation and slip distribution over the rupture velocity, and how it precludes the direct estimate of this latter parameter.     

Crustal thickness estimation beneath the southern central Andes at 30°S and 36°S from S wave receiver function analysis

We have used the S wave receiver function (SRF) technique to investigate the crustal thickness beneath two seismic profiles from the CHARGE project in the southern central Andes. A previous study employing the P wave receiver function method has observed the Moho interface beneath much of the profiles. They found, however, that the amplitude of the P to S conversion was diminished in the western part of the profiles and have attributed it to a reduction of the impedance contrast at the Moho due to lower crustal ecologitization. With SRF, we have successfully detected S to P converted waves from the Moho as well as possible conversions from other lithospheric boundaries. The continental South American crust reaches its maximum thickness of ∼70 km (along 30°S between 70°W and 68.5°W) beneath the Principal Cordillera and the Famatina system and becomes thinner towards the Sierras Pampeanas with a thickness of ∼40 km. Negative phases, possibly related to the base of the continental and oceanic lithosphere, can be recognized in the summation traces at different depths. By comparing our results with data obtained from previous investigations, we are able to further constrain the thickness of the crust and lithosphere beneath the central Andes.     

Impact of regional mantle flow on subducting plate geometry and interplate stress: insights from physical modelling

Physical models of subduction investigate the impact of regional mantle flow on the structure of the subducted slab and deformation of the downgoing and overriding plates. The initial mantle flow direction beneath the overriding plate can be horizontal or vertical, depending on its location with respect to the asthenospheric flow field. Imposed mantle flow produces either over or underpressure on the lower surface of the slab depending on the initial mantle flow pattern (horizontal or vertical, respectively). Overpressure promotes shallow dip subduction while underpressure tends to steepen the slab. Horizontal mantle flow with rates of 1–10 cm yr⁻¹ provides sufficient overpressure on a dense subducting lithosphere to obtain a subduction angle of  ∼60°  , while the same lithospheric slab sinks vertically when no flow is imposed. Vertical drag force (due to downward mantle flow) exerted on a slab can result in steep subduction if the slab is neutrally buoyant but fails to produce steep subduction of buoyant oceanic lithosphere. The strain regime in the overriding plate due to the asthenospheric drag force depends largely on slab geometry. When the slab dip is steeper than the interplate zone, the drag force produces negative additional normal stress on the interplate zone and tensile horizontal stress in the overriding plate. When the slab dip is shallower than the interplate zone, an additional positive normal stress is produced on the interplate zone and the overriding plate experiences additional horizontal compressive stress. However, the impact of the mantle drag force on interplate pressure is small compared to the influence of the slab pull force since these stress variations can only be observed when the slab is dense and interplate pressure is low.     

Comment on 'Consequences of progressive eclogitization on crustal exhumation, a mechanical study' by H. Raimbourg, L. Jolivet and Y. Leroy

Two simple end-member models of a subduction channel have been proposed in the literature: (i) the 'pressure-imposed' model for which the pressure within the channel is assumed to be lithostatic, the channel walls have negligible strength with respect to lateral pressure gradients, and the channel geometry therefore varies with time and (ii) the 'geometry-imposed' model of constant channel geometry, rigid walls and resultant lateral variation in pressure. Neither of these models is realistic, but they provide lower and upper bounds to potential pressure distributions in natural subduction zones. The critical parameter is the relative strength of the confining plates, reflected in the effective viscosity ratio between the channel fill and the walls. The assertion that the 'geometry-imposed' model is internally inconsistent is incorrect—it merely represents one bound to possible behaviour and a bound that may be approached for realistic values of the effective viscosity for weak channel fill (e.g. unconsolidated ocean-floor sediments) and relatively cold and strong subducting and overriding lithospheric plates.