Matthias Delescluse

April 2012 intra-oceanic seismicity off Sumatra boosted by the Banda-Aceh megathrust

Large earthquakes nucleate at tectonic plate boundaries, and their occurrence within a plate’s interior remains rare and poorly documented, especially offshore. The two large earthquakes that struck the northeastern Indian Ocean on 11 April 2012 are an exception: they are the largest strike-slip events reported in historical times and triggered large aftershocks worldwide. Yet they occurred within an intra-oceanic setting along the fossil fabric of the extinct Wharton basin, rather than on a discrete plate boundary. Here we show that the 11 April 2012 twin earthquakes are part of a continuing boost of the intraplate deformation between India and Australia that followed the Aceh 2004 and Nias 2005 megathrust earthquakes, subsequent to a stress transfer process recognized at other subduction zones. Using Coulomb stress change calculations, we show that the coseismic slips of the Aceh and Nias earthquakes can promote oceanic left-lateral strike-slip earthquakes on pre-existing meridian-aligned fault planes. We further show that persistent viscous relaxation in the asthenospheric mantle several years after the Aceh megathrust explains the time lag between the 2004 megathrust and the 2012 intraplate events. On a short timescale, the 2012 events provide new evidence for the interplay between megathrusts at the subduction interface and intraplate deformation offshore. On a longer geological timescale, the Australian plate, driven by slab-pull forces at the Sunda trench, is detaching from the Indian plate, which is subjected to resisting forces at the Himalayan front.

2D waveform tomography applied to long-streamer MCS data from the Scotian Slope

Detailed velocity models of the earth’s subsurface can be obtained through waveform tomography. The accuracy of the long-wavelength component of such velocity models, which is the background velocity field, is particularly sensitive to modeling low-frequency refracted waves that have long paths through target structures. Thus, field examples primarily have focused on the analysis of long-offset wide-angle data sets collected using autonomous receivers, in which refractions arrive at significantly earlier times than reflections. Modern marine acquisition with long streamers now offers the ability to record refracted waves with high spatial density and uniform source, both in shallow and deep water. We used 2D multichannel seismic (MCS) data acquired with a 9-km-long streamer over the Scotian Slope in water depths of ∼1600 m. The refracted arrivals, although mostly restricted to far-offset receivers, provided sufficient information to successfully invert for a high-resolution background velocity field. Usin...

Successive Rifting Events in Marginal Basins: The Example of the Coral Sea Region (Papua New Guinea)

Reactivation of extensional structures is commonly inferred during rift evolution. In that context, we present original seismic interpretation to explore the geometry and interactions of three successive rifting events in the Coral Sea region, Papua New Guinea. The first event (R 1), poorly documented, occurred during the Triassic along an older N-S Permian structural fabric. During the Jurassic, extensional faults were reactivated through a second extensional episode (R 2), which formed small (~10/20 km) basins bounded by N-S, NE-SW, and E-W listric faults. Extension prolonged during the Lower Cretaceous with seafloor spreading in the Owen Stanley Oceanic Basin, now incorporated in the Papuan fold and thrust belts. A third Late Cretaceous extensional phase (R 3) gently reactivated some of the faults with very limited landward tilt in most basins and deformation located along the present continent-ocean transition. Seafloor spreading in the Coral Sea followed from Danian to Ypresian. This extensional system is sealed by unequally preserved Eocene strata that mark the onset of postrift thermal subsidence prior to the margin inversion from Oligocene onward. This overall evolution suggests various extensional systems that are geographically and temporarily defined the one another. The early rifting of the crust is controlled by preexisting continental features resulting in the local Pangaea breakup. In contrast, the Coral Sea propagator cuts through the rifted margin and is controlled by a subduction complex in accordance with the Tasman Sea opening. This evolution underlines the interactions existing between two extension modes in agreement with variations of the regional geodynamical setting around Australia.

The Geological Evolution of the Aden‐Owen‐Carlsberg Triple Junction (NW Indian Ocean) Since the Late Miocene

The Aden-Owen-Carlsberg triple junction is the place where Arabia, Somalia, and India meet in the Arabian Sea (NW Indian Ocean). Here we present a new seismic data set crossing a key structure of the triple junction, namely, the Beautemps-Beaupré pull-apart basin at the southern end of the Owen Fracture Zone. The seismic data set is tied to Ocean Drilling Program Leg 117 Sites at the top of the Owen Ridge, which provides a detailed tectono-stratigraphic framework since the Late Miocene. We show that the triple junction configuration has been disturbed by a major kinematic change at ~8 Ma and since then experienced a series of transient structural adjustments. A major structural episode is recorded at 2.4 Ma, expressed by the opening of the Beautemps-Beaupré Basin and the uplift of its southern flank (the Beautemps-Beaupré Ridge). This episode is coeval with the formation of the present-day Owen Fracture Zone and must be considered as a part of a major structural reorganization of the entire India-Arabia plate boundary up to the Makran subduction zone. This 2.4-Myr-old geological episode is unrelated to any significant kinematic change, leaving questions over its driving mechanism.

Thermal and seismic hints for chimney type cross-stratal fluid flow in onshore basins

When modelling onshore sedimentary basins, modellers generally assume that semi-permeable layers (aquitards) greatly restrict vertical flow between aquifers. Aquitards are therefore considered as confining media and vertical flow is assumed to take place mainly within localised permeable faults, if any. In the offshore context, however, interpretation of seismic data frequently provides evidence of fluid flow between sedimentary layers via structurally disrupted formations (pervasive fractures) recognised as zones of reduced seismic amplitude and generically called “chimneys”. Here we show that chimneys are also present onshore, and that they crosscut confining layers. In the Anglo-Paris Basin, seismic data suggest 1 to 2 km wide zones of disrupted seismic signal spatially correlated to a hitherto unexplained major temperature anomaly of 20 °C. When included in geothermal models using a five-order increase in permeabilities with respect to confining layers, we find that fluid flows vertically through aquifers and confining layers, thereby explaining this major temperature anomaly. Despite the importance of their hydrodynamic and thermal impacts, chimneys – less obvious than faults – have been overlooked as fluid flow paths in many onshore sedimentary basins exploited for their resources. This indicates a clear need for better understanding of pervasive flow paths, especially as the resources and properties of basins (i.e. conventional and unconventional hydrocarbons, geothermal potential, CO2 storage, nuclear waste repository, drinking water, etc.) are increasingly being harnessed.

Continental break-up of the South China Sea stalled by far-field compression

The outcome of decades of two-dimensional modelling of lithosphere deformation under extension is that mechanical coupling between the continental crust and the underlying mantle controls how a continent breaks apart to form a new ocean. However, geological observations unequivocally show that continental break-up propagates in the third dimension at rates that do not scale with the rate of opening. Here, we perform three-dimensional numerical simulations and compare them with observations from the South China Sea to show that tectonic loading in the direction of propagation exerts a first-order control on these propagation rates. The simulations show that, in the absence of compression in that direction, continental break-up propagates fast, forming narrow continental margins independently of the coupling. When compression is applied, propagation stagnates, forming V-shaped oceanic basins and wide margins. Changes in out-of-plane loading therefore explain the alternation of fast propagation and relative stagnation. These new dynamic constraints suggest that the west-to-east topographic gradient across the Indochinese Peninsula prevented continental break-up propagation through the 1,000-km-wide continental rift of the central and west basin of the South China Sea, until the direction of stretching changed 23 million years ago, resulting in bypassing and acceleration of continental break-up propagation.Tectonic loading in the direction of propagation exerts an important control on the propagation of continental break-up, according to three-dimensional simulations of the South China Sea.

Deep Imaging of Extensional Structures in the SW South China Sea

We process a 230-km-long MCS section and a coincident OBS refraction profile acquired across the Spratly Islands south of the SW SCS oceanic basin. By joint interpretation between MCS profile and OBS derived P-wave velocity field, we identify pre-rift units, two stages of syn-rift and post-rift sediments in the basins. Normal faults tilting pre-rift units root in a shallow decollement south of the profile. This shallow decollement is directly imaged by a thick band of reflectivity and also identified by a velocity contrast in the refraction derived P-wave velocity model. Deeper down (~7s twt), significant reflectivity may be interpreted as the top of the lower crust with ~7 km/s velocities. Even deeper reflectors around 10-11s may be related to the Moho or mantle. The images show moderate finite extension in the pre-rift sedimentary infill, although large faults penetrate the basement. Syn-rift units show that the tertiary rifting of the SW SCS may be characterized by rapid and short-lived activity of the faults or more subaerial conditions. Deeper, we infer that multiple heterogeneities present in the continental crust are used as decollement levels at different depths down to the ductile lower crust.