Large-magnitude intraplate earthquakes within the ocean basins are not well understood. The M w 8... more Large-magnitude intraplate earthquakes within the ocean basins are not well understood. The M w 8.6 and M w 8.2 strike-slip intraplate earthquakes on 11 April 2012, while clearly occurring in the equatorial Indian Ocean diffuse plate boundary zone, are a case in point, with disagreement on the nature of the focal mechanisms and the faults that ruptured. We use bathymetric and seismic reflection data from the rupture area of the earthquakes in the northern Wharton Basin to demonstrate pervasive brittle deformation between the Ninet-yeast Ridge and the Sunda subduction zone. In addition to evidence of recent strike-slip deformation along approximately north-south–trending fossil fracture zones, we identify a new type of deformation structure in the Indian Ocean: conjugate Riedel shears limited to the sediment section and oriented oblique to the north-south fracture zones. The Riedel shears developed in the Miocene, at a similar time to the onset of diffuse deformation in the central Indian Ocean. However, left-lateral strike-slip reactivation of existing fracture zones started earlier, in the Paleocene to early Eocene, and compartmentalizes the Wharton Basin. Modeled rupture during the 11 April 2012 intraplate earthquakes is consistent with the location of two reactivated, closely spaced, approximately north-south–trending fracture zones. However, we find no evidence for WNW-ESE–trending faults in the shallow crust, which is at variance with most of the earthquake fault models.
Le traitement sismique sur la grille et sa communauté d'utilisateurs EGEODE Matthias Delescl... more Le traitement sismique sur la grille et sa communauté d'utilisateurs EGEODE Matthias Delescluse(1), Marc Schaming(2), Laure Schenini(3), Jean‐Bernard ... locale sera donc limitée à la puissance de la machine UI, alors qu'une exécution sur la grille permet de passer outre cette ...
ABSTRACT In July-August 2011, the Alaska Langseth Experiment to Understand the megaThrust (ALEUT)... more ABSTRACT In July-August 2011, the Alaska Langseth Experiment to Understand the megaThrust (ALEUT) program acquired deep penetration multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data along a part of the western Alaska subduction zone, from the freely slipping Shumagin gap to the locked regions in the Semidi segment and the western Kodiak asperity. More than 3500 km of MCS data were collected along a series of strike and dip profiles that span the entire locked zone on the megathrust (as indicated by GPS data and estimated rupture zones of past earthquakes), the updip and downdip transitions to stable sliding, bending of the downgoing plate, and preexisting structures in the oceanic crust. These data were acquired using the Langseth's 6600 cu. in. air gun array and two 8-km-long streamers. The source and one of the streamers were towed at a depth of 12 m to maximize low frequencies (and deep imaging) while the second streamer was towed at 9 m for better imaging of the sediments and upper crust. Refraction data were acquired using the same source and short period OBS spaced at ~15 km along two ~400 km-long profiles coincident with MCS data across the Shumagin Gap and Semidi segment. Eight seismometers were deployed onshore from mid June to mid August, which recorded the entire offshore experiment plus local and regional seismicity. Supporting underway multibeam, sub-bottom profiler, gravity and magnetic data were also collected along all MCS/OBS profiles. Here we present preliminary results from MCS and OBS data analysis regarding the structure of the Pacific plate seaward of the western Alaska trench. The initial images reveal marked variations in the structure of the downgoing plate, including changes in the Moho transition zone and oceanic crustal thickness, bending related faulting and hydration, and sediment input into the subduction zone, some of which occur at oceanic plate suture zones. Abundant normal faulting, thin sediments and lower mantle velocities are observed on the plate approaching the weakly coupled Shumagin Gap, while less bending-related deformation appears to be associated with the oceanic plate approaching the Semidi segment. Much of the focus of this presentation is on the extraordinary view that the collected data provide into the structure of a fossil triple junction that once separated the Pacific, Kula and Farallon plates. Reflections are captured from depths as great as some 15 km into the lithosphere surrounding this fossil triple junction. These reflections could possibly be caused by gabbroic melts that were once percolating upward but had become frozen when the triple junction was abandoned during a plate reorganization in Early Tertiary. These preliminary observations may provide key new insight into the deep structure of triple junctions, how they operate, and what happens when they fail.
Detailed velocity models of the earth subsurface can be obtained through waveform tomography, a m... more Detailed velocity models of the earth subsurface can be obtained through waveform tomography, a method that relies on using information from the full wavefield. Such models can be of significantly higher resolution than the corresponding models formed by more generic traveltime tomography methods, which are constrained only by the wave arrival times. However, to derive the detailed subsurface velocity, the
We present tomographic models for closely spaced (~200 m) multichannel seismic (MCS) profiles col... more We present tomographic models for closely spaced (~200 m) multichannel seismic (MCS) profiles collected in 2008 along the axis of the East Pacific Rise (EPR) at 9°41' - 9°57' N during cruise MGL0812. The investigated profiles target the locations of documented hydrothermal venting as well as the area where layer 2A is believed to thin north of 9°52' N. We
Oceanic lithosphere is strong and continental lithosphere is weak. As a result, there is relative... more Oceanic lithosphere is strong and continental lithosphere is weak. As a result, there is relatively little deformation in the oceanic domain away from plate boundaries. However, the interior of oceanic lithosphere does deform when highly stressed. We review here places where intraoceanic compression is at work. In the more than 30 years since the first observations of active compressive intraplate deformation in the Central Indian Ocean through seismic profiling (Eittreim et al., 1972), compressive deformation has been identified in a variety of other oceanic tectonic settings: as a result of small differential motion between large plates (between North America and South America in the Central Atlantic; between Eurasia and Nubia offshore Gibraltar; between Macquarie and Australia plates in the Southern Ocean), within back-arcs (northwest Celebes Sea, Okushiri Ridge in the Japan Sea, on the eastern border of the Caroline plate), and ahead of subduction (Zenisu Ridge off Nankai Trough). Deformation appears to be more diffuse when larger plates are involved, and more localized for younger plates, perhaps in relation with the increasing rigidity of oceanic plates with age. The best example of diffuse deformation studied so far remains the Central Indian Ocean. Numerous marine data have been collected in this area, including shallow and deep seismic, heat flow measurements, multibeam bathymetry. The present-day deformation field has been modeled using GPS and earthquakes as far field and near field constraints respectively. Reactivation of the oceanic fabric (including for portions of the Indo-Australian plate which are now in subduction as evidenced by the September 2009 Padang earthquake), selective fault abandonment (Delescluse et al., 2008) and serpentinization (Delescluse and Chamot-Rooke, 2008) are some of the important processes that shape the present-day pattern of deformation. These rare intraplate deformation areas constitute excellent natural laboratories to investigate the very early stages of formation of faulted oceanic bodies that may further be incorporated into mountain belts as ophiolites. They allow to discuss rates and duration of deformation, diffuse vs localized deformation, re-activation vs neo-formed faults, serpentinization and thermal regime, spacing of minor and major thrust faults. Delescluse, M., L. G. J. Montesi, and N. Chamot-Rooke (2008) Fault reactivation and selective abandonment in the oceanic lithosphere. Geophys. Res. Lett., v. 35. Delescluse, M., and N. Chamot-Rooke (2008) Serpentinization pulse in the actively deforming Central Indian Basin. Earth Planet. Sci. Lett., v. 276, p. 140-151. Eittreim, S. L., and J. Ewing (1972), Mid-Plate Tectonics in the Indian Ocean, J. Geophys. Res., 77(32), 6413-6421.
Large-magnitude intraplate earthquakes within the ocean basins are not well understood. The M w 8... more Large-magnitude intraplate earthquakes within the ocean basins are not well understood. The M w 8.6 and M w 8.2 strike-slip intraplate earthquakes on 11 April 2012, while clearly occurring in the equatorial Indian Ocean diffuse plate boundary zone, are a case in point, with disagreement on the nature of the focal mechanisms and the faults that ruptured. We use bathymetric and seismic reflection data from the rupture area of the earthquakes in the northern Wharton Basin to demonstrate pervasive brittle deformation between the Ninet-yeast Ridge and the Sunda subduction zone. In addition to evidence of recent strike-slip deformation along approximately north-south–trending fossil fracture zones, we identify a new type of deformation structure in the Indian Ocean: conjugate Riedel shears limited to the sediment section and oriented oblique to the north-south fracture zones. The Riedel shears developed in the Miocene, at a similar time to the onset of diffuse deformation in the central Indian Ocean. However, left-lateral strike-slip reactivation of existing fracture zones started earlier, in the Paleocene to early Eocene, and compartmentalizes the Wharton Basin. Modeled rupture during the 11 April 2012 intraplate earthquakes is consistent with the location of two reactivated, closely spaced, approximately north-south–trending fracture zones. However, we find no evidence for WNW-ESE–trending faults in the shallow crust, which is at variance with most of the earthquake fault models.
Le traitement sismique sur la grille et sa communauté d'utilisateurs EGEODE Matthias Delescl... more Le traitement sismique sur la grille et sa communauté d'utilisateurs EGEODE Matthias Delescluse(1), Marc Schaming(2), Laure Schenini(3), Jean‐Bernard ... locale sera donc limitée à la puissance de la machine UI, alors qu'une exécution sur la grille permet de passer outre cette ...
ABSTRACT In July-August 2011, the Alaska Langseth Experiment to Understand the megaThrust (ALEUT)... more ABSTRACT In July-August 2011, the Alaska Langseth Experiment to Understand the megaThrust (ALEUT) program acquired deep penetration multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data along a part of the western Alaska subduction zone, from the freely slipping Shumagin gap to the locked regions in the Semidi segment and the western Kodiak asperity. More than 3500 km of MCS data were collected along a series of strike and dip profiles that span the entire locked zone on the megathrust (as indicated by GPS data and estimated rupture zones of past earthquakes), the updip and downdip transitions to stable sliding, bending of the downgoing plate, and preexisting structures in the oceanic crust. These data were acquired using the Langseth's 6600 cu. in. air gun array and two 8-km-long streamers. The source and one of the streamers were towed at a depth of 12 m to maximize low frequencies (and deep imaging) while the second streamer was towed at 9 m for better imaging of the sediments and upper crust. Refraction data were acquired using the same source and short period OBS spaced at ~15 km along two ~400 km-long profiles coincident with MCS data across the Shumagin Gap and Semidi segment. Eight seismometers were deployed onshore from mid June to mid August, which recorded the entire offshore experiment plus local and regional seismicity. Supporting underway multibeam, sub-bottom profiler, gravity and magnetic data were also collected along all MCS/OBS profiles. Here we present preliminary results from MCS and OBS data analysis regarding the structure of the Pacific plate seaward of the western Alaska trench. The initial images reveal marked variations in the structure of the downgoing plate, including changes in the Moho transition zone and oceanic crustal thickness, bending related faulting and hydration, and sediment input into the subduction zone, some of which occur at oceanic plate suture zones. Abundant normal faulting, thin sediments and lower mantle velocities are observed on the plate approaching the weakly coupled Shumagin Gap, while less bending-related deformation appears to be associated with the oceanic plate approaching the Semidi segment. Much of the focus of this presentation is on the extraordinary view that the collected data provide into the structure of a fossil triple junction that once separated the Pacific, Kula and Farallon plates. Reflections are captured from depths as great as some 15 km into the lithosphere surrounding this fossil triple junction. These reflections could possibly be caused by gabbroic melts that were once percolating upward but had become frozen when the triple junction was abandoned during a plate reorganization in Early Tertiary. These preliminary observations may provide key new insight into the deep structure of triple junctions, how they operate, and what happens when they fail.
Detailed velocity models of the earth subsurface can be obtained through waveform tomography, a m... more Detailed velocity models of the earth subsurface can be obtained through waveform tomography, a method that relies on using information from the full wavefield. Such models can be of significantly higher resolution than the corresponding models formed by more generic traveltime tomography methods, which are constrained only by the wave arrival times. However, to derive the detailed subsurface velocity, the
We present tomographic models for closely spaced (~200 m) multichannel seismic (MCS) profiles col... more We present tomographic models for closely spaced (~200 m) multichannel seismic (MCS) profiles collected in 2008 along the axis of the East Pacific Rise (EPR) at 9°41' - 9°57' N during cruise MGL0812. The investigated profiles target the locations of documented hydrothermal venting as well as the area where layer 2A is believed to thin north of 9°52' N. We
Oceanic lithosphere is strong and continental lithosphere is weak. As a result, there is relative... more Oceanic lithosphere is strong and continental lithosphere is weak. As a result, there is relatively little deformation in the oceanic domain away from plate boundaries. However, the interior of oceanic lithosphere does deform when highly stressed. We review here places where intraoceanic compression is at work. In the more than 30 years since the first observations of active compressive intraplate deformation in the Central Indian Ocean through seismic profiling (Eittreim et al., 1972), compressive deformation has been identified in a variety of other oceanic tectonic settings: as a result of small differential motion between large plates (between North America and South America in the Central Atlantic; between Eurasia and Nubia offshore Gibraltar; between Macquarie and Australia plates in the Southern Ocean), within back-arcs (northwest Celebes Sea, Okushiri Ridge in the Japan Sea, on the eastern border of the Caroline plate), and ahead of subduction (Zenisu Ridge off Nankai Trough). Deformation appears to be more diffuse when larger plates are involved, and more localized for younger plates, perhaps in relation with the increasing rigidity of oceanic plates with age. The best example of diffuse deformation studied so far remains the Central Indian Ocean. Numerous marine data have been collected in this area, including shallow and deep seismic, heat flow measurements, multibeam bathymetry. The present-day deformation field has been modeled using GPS and earthquakes as far field and near field constraints respectively. Reactivation of the oceanic fabric (including for portions of the Indo-Australian plate which are now in subduction as evidenced by the September 2009 Padang earthquake), selective fault abandonment (Delescluse et al., 2008) and serpentinization (Delescluse and Chamot-Rooke, 2008) are some of the important processes that shape the present-day pattern of deformation. These rare intraplate deformation areas constitute excellent natural laboratories to investigate the very early stages of formation of faulted oceanic bodies that may further be incorporated into mountain belts as ophiolites. They allow to discuss rates and duration of deformation, diffuse vs localized deformation, re-activation vs neo-formed faults, serpentinization and thermal regime, spacing of minor and major thrust faults. Delescluse, M., L. G. J. Montesi, and N. Chamot-Rooke (2008) Fault reactivation and selective abandonment in the oceanic lithosphere. Geophys. Res. Lett., v. 35. Delescluse, M., and N. Chamot-Rooke (2008) Serpentinization pulse in the actively deforming Central Indian Basin. Earth Planet. Sci. Lett., v. 276, p. 140-151. Eittreim, S. L., and J. Ewing (1972), Mid-Plate Tectonics in the Indian Ocean, J. Geophys. Res., 77(32), 6413-6421.
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