Description: Highlights • The Lofoten/Vesterålen margin has less Early Cenozoic lava flows than believed. • Breakup of the L/V margin is delayed ∼1 m.y. from the Vøring Plateau to the south. • Late arrival of the Iceland Plume may explain delayed breakup and prolonged extension. The Early Eocene continental breakup was magma-rich and formed part of the North Atlantic Igneous Province. Extrusive and intrusive magmatism was abundant on the continental side, and a thick oceanic crust was produced up to a few m.y. after breakup. However, the extensive magmatism at the Vøring Plateau off mid-Norway died down rapidly northeastwards towards the Lofoten/Vesterålen Margin. In 2003 an Ocean Bottom Seismometer profile was collected from mainland Norway, across Lofoten, and into the deep ocean. Forward/inverse velocity modeling by raytracing reveals a continental margin transitional between magma-rich and magma-poor rifting. For the first time a distinct lower-crustal body typical for volcanic margins has been identified at this outer margin segment, up to 3.5. km thick and ∼50. km wide. On the other hand, expected extrusive magmatism could not be clearly identified here. Strong reflections earlier interpreted as the top of extensive lavas may at least partly represent high-velocity sediments derived from the shelf, and/or fault surfaces. Early post-breakup oceanic crust is moderately thickened (∼8. km), but is reduced to 6. km after 1. m.y. The adjacent continental crystalline crust is extended down to a minimum of 4.5. km thickness. Early plate spreading rates derived from the Norway Basin and the northern Vøring Plateau were used to calculate synthetic magnetic seafloor anomalies, and compared to our ship magnetic profile. It appears that continental breakup took place at ∼53.1. Ma, ∼1. m.y. later than on the Vøring Plateau, consistent with late strong crustal extension. The low interaction between extension and magmatism indicates that mantle plume material was not present at the Lofoten Margin during initial rifting, and that the observed excess magmatism was created by late lateral transport from a nearby pool of plume material into the lithospheric rift zone at breakup time.

Description: The continuation of the Caledonides into the Barents Sea has long been a subject of discussion, and two major orientations of the Caledonian deformation fronts have been suggested: NNW-SSE striking and NE-SW striking. A regional NW-SE oriented ocean bottom seismic profile across the western Barents Sea was acquired in 2014. In this paper we map the crust and upper mantle structure along this profile in order to discriminate between different interpretations of Caledonian structural trends and orientation of rift basins in the western Barents Sea. Modeling of P-wave travel times has been done using a ray-tracing method, and combined with gravity modeling. The results show high P-wave velocities (4 km/s) close to the seafloor, as well as localized sub-horizontal high velocity zones (6.0 km/s and 6.9 km/s) at shallow depths which are interpreted as magmatic sills. Refractions from the top of the crystalline basement together with reflections from the Moho give basement velocities from 6.0 km/s at the top to 6.7 km/s at the base of the crust. P-wave travel time modeling of the OBS profile indicate an eastwards increase in velocities from 6.4 km/s to 6.7 km/s at the base of the crystalline crust, and the western part of the profile is characterized by a higher seismic reflectivity than the eastern part. This change in seismic character is consistent with observations from vintage reflection seismic data and is interpreted as a Caledonian suture extending through the Barents Sea, separating Barentsia and Baltica. Local deepening of Moho (from 27 km to 33 km depth) creates “root structures” that can be linked to the Caledonian compressional deformation or a suture zone imprinted in the lower crust. Our model supports a separate NE-SW Caledonian trend extending into the central Barents Sea, branching off from the northerly trending Svalbard Caledonides, implying the existence of Barentsia as an independent microcontinent between Laurentia and Baltica.

Description: The Møre Margin in the NE Atlantic represents a dominantly passive margin with an unusual abrupt transition from alpine morphology onshore to a deep sedimentary basin offshore. In order to study this transition in detail, three ocean bottom seismometer profiles with deep seismic reflection and refraction data were acquired in 2009; two dip-profiles which were extended by land stations, and one tie-profile parallel to the strike of the Møre–Trøndelag Fault Complex. The modeling of the wide-angle seismic data was performed with a combined inversion and forward modeling approach and validated with a 3D-density model. Modeling of the geophysical data indicates the presence of a 12–15 km thick accumulation of sedimentary rocks in the Møre Basin. The modeling of the strike profile located closer to land shows a decrease in crustal velocity from north to south. Near the coast we observe an intra-crustal reflector under the Trøndelag Platform, but not under the Slørebotn Sub-basin. Furthermore, two lower crustal high-velocity bodies are modeled, one located near the Møre Marginal High and one beneath the Slørebotn Sub-basin. While the outer lower crustal body is modeled with a density allowing an interpretation as magmatic underplating, the inner body has a density close to mantle density which might suggest an origin as an eclogized body, formed by metamorphosis of lower crustal gabbro during the Caledonian orogeny. The difference in velocity and extent of the lower crustal bodies seems to be controlled by the Jan Mayen Lineament, suggesting that the lineament represents a pre-Caledonian structural feature in the basement.

Description: The Murray Ridge/Dalrymple Trough system forms the boundary between the Indian and Arabian plates in the northern Arabian Sea. Geodetic constraints from the surrounding con- tinents suggest that this plate boundary is undergoing oblique extension at a rate of a few millimetres per year. We present wide-angle seismic data that constrains the composition of the Ridge and of adjacent lithosphere beneath the Indus Fan. We infer that Murray Ridge, like the adjacent Dalrymple Trough, is underlain by continental crust, while a thin crustal section beneath the Indus Fan represents thinned continental crust or exhumed serpentinized mantle that forms part of a magma-poor rifted margin. Changes in crustal structure across the Murray Ridge and Dalrymple Trough can explain short-wavelength gravity anomalies, but a long-wavelength anomaly must be attributed to deeper density contrasts that may result from a large age contrast across the plate boundary. The origin of this fragment of continental crust remains enigmatic, but the presence of basement fabrics to the south that are roughly parallel to Murray Ridge suggests that it separated from the India/Seychelles/Madagascar block by extension during early breakup of Gondwana.

Description: Earthquake history shows that the Sunda subduction zone of the Indonesian margin produces great earthquakes offshore Sumatra, whereas earthquakes of comparable magnitude are lacking offshore Java and the Lesser Sunda islands. Morphological structures in multibeam bathymetric data across the forearc relate with the extent of the seismogenic zone. Its updip limit corresponds to the slope break, most distinct off Java and Lesser Sunda islands, where we find coincident narrow, uniform, continuous outer arc ridges. Their landward termination and a shallow upper plate mantle mark the downdip limit of the seismogenic zone. In contrast the outer arc ridges off Sumatra are wider and partly elevated above sea level forming the forearc islands. The downdip limit of the seismogenic zone coincides with a deeper upper plate mantle. Sunda Strait marks a transition zone between the Sumatra and Java margins. We find the differences along the Sunda margin, especially the wider extent of the seismogenic zone off Sumatra, producing larger earthquakes, to result from the interaction of different age and subduction direction of the oceanic plate. We attribute a major role to the sediment income and continental/oceanic upper plate nature of Sumatra/Java influencing the composition and deformation style along the forearc and subduction fault.

Description: A temporary passive seismic network of 31 broad-band stations was deployed in the region around Talca and Constitución between 35°S to 36°S latitude and 71°W to 72.5°W longitude. The network was operated between March and October 2008. Thus, we recorded data prior the magnitude Mw=8.8 earthquake of 27 February 2010 at a latitude of the major slip and surface uplift. The experiment was conducted to address fundamental questions on deformation processes, crustal and mantle structures, and fluid flow. We present first results of a teleseismic P receiver function study that covers the coastal region and reaches to the Andes. The aim is to determine the structure and thickness of the continental crust and constrain the state of hydration of the mantle wedge. The P-wave receiver function technique requires large teleseismic earthquakes from different distances and backazimuths. A few percent of the incident P-wave energy from a teleseismic event will be converted into S-wave (Ps) at significant and relatively sharp discontinuities beneath the station. A small converted S phase is produced that arrives at the station within the P wave coda directly after the direct P-wave. The converted Ps phase and their crustal multiples contain information about crustal properties, such as Moho depth and the crustal vp/vs ratio. We use teleseismic events with magnitudes mb 〉 5.5 at epicentral distances between 30° and 95° to examine P-to-S converted seismic phases. Our preliminary results provide new information about the thickness of the continental crust beneath the coastal region in Central Chile. At most of the stations we observed significant energy from P to S converted waves between 4 and 5 s after the direct P-wave within a positive phase interpreted as the Moho, occurring at 35 to 40 km. Thus, the great Maule earthquake of 27 February 2010 nucleated up-dip of the continental Moho and hence ruptured along a plate contact between subducted sediments and continental crust. Further, the Moho reflection show a positive polarity, indicating that the mantle is either dry or only moderately hydrated.

Description: EGU2011-12780 A temporary passive seismic network of 31 broad-band stations was deployed in the region around Talca and Constitución between 35°S to 36°S latitude and 71°W to 72.5°W longitude. The network was operated between March and October 2008. Thus, we recorded data prior the magnitude Mw=8.8 earthquake of 27 February 2010 at a latitude of the major slip and surface uplift. The experiment was conducted to address fundamental questions on deformation processes, crustal and mantle structures, and fluid flow. We present results of a teleseismic P receiver function study that covers the coastal region and reaches to the Andes. The aim is to determine the structure and thickness of the continental crust and constrain the state of hydration of the mantle wedge. The P-wave receiver function technique requires large teleseismic earthquakes from different distances and backazimuths. A few percent of the incident P-wave energy from a teleseismic event will be converted into S-wave (Ps) at significant and relatively sharp discontinuities beneath the station. A small converted S phase is produced that arrives at the station within the P wave coda directly after the direct P-wave. The converted Ps phase and their crustal multiples contain information about crustal properties, such as Moho depth and the crustal vp/vs ratio. We use teleseismic events with magnitudes mb 〉 5.5 at epicentral distances between 30° and 95° to examine P-to-S converted seismic phases. Our preliminary results provide new information about the thickness of the continental crust beneath the coastal region in Central Chile. At most of the stations we observed significant energy from P to S converted waves between 4 and 5 s after the direct P-wave within a positive phase interpreted as the Moho, occurring at 35 to 40 km. The great Maule earthquake of 27 February 2010 nucleated up-dip of the continental Moho. The rupture of this earthquake seems to have propagated down-dip of the Moho. The Moho reflection show a positive polarity, indicating that the mantle is either dry or only moderately hydrated. We observed converted energy from an intracrustal boundary at around 2 s that disappears near the coast. Further, positive polarity peaks occur that are possibly caused by the down going plate.

Description: The Chile Triple Junction (CTJ) is the place where the Chile Ridge (Nazca–Antarctic spreading center) is subducting beneath the continental South American plate. Sediment accretion is active to the south of the CTJ in the area where the northward migrating Chile Ridge has collided with the continent since 14 Ma. At the CTJ, tectonic erosion of the overriding plate narrows and steepens the continental slope. We present here a detailed tomographic image of the upper lithospheric Antarctic–South America subduction zone where the Chile Ridge collided with the continent 3–6 Ma off Golfo de Penas. Results reveal that a large portion of trench sediment has been scraped off and frontally accreted to the forearc forming a 70–80 km wide accretionary prism. The velocity–depth model shows a discontinuity at 30–40 km landward of the deformation front, which is interpreted as the contact between the frontal (poorly consolidated sedimentary unit) and middle (more compacted sedimentary unit) accretionary prism. The formation of this discontinuity could be related to a short term episode of reduced trench sedimentation. In addition, we model the shape of the continental slope using a Newtonian fluid rheology to study the convergence rate at which the accretionary prism was formed. Results are consistent with an accretionary prism formed after the collision of the Chile Ridge under slow convergence rate similar to those observed at present between Antarctic and South America (∼2.0 cm/a). Based on the kinematics of the Chile Ridge subduction during the last 13 Ma, we propose that the accretionary prism off Golfo de Penas was formed recently (∼5 Ma) after the collision of the Chile Ridge with South America.

Description: Geophysical investigations of the northern Hikurangi subduction zone northeast of New Zealand, image fore‐arc and surrounding upper lithospheric structures. A seismic velocity (Vp) field is determined from seismic wide‐angle data, and our structural interpretation is supported by multichannel seismic reflection stratigraphy and gravity and magnetic modeling. We found that the subducting Hikurangi Plateau carries about 2 km of sediments above a 2 km mixed layer of volcaniclastics, limestone, and chert. The upper plateau crust is characterized by Vp = 4.9–6.7 km/s overlying the lower crust with Vp 〉 7.1 km/s. Gravity modeling yields a plateau thickness around 10 km. The reactivated Raukumara fore‐arc basin is 〉10 km deep, deposited on 5–10 km thick Australian crust. The fore‐arc mantle of Vp 〉 8 km/s appears unaffected by subduction hydration processes. The East Cape Ridge fore‐arc high is underlain by a 3.5 km deep strongly magnetic (3.3 A/m) high‐velocity zone, interpreted as part of the onshore Matakaoa volcanic allochthon and/or uplifted Raukumara Basin basement of probable oceanic crustal origin. Beneath the trench slope, we interpret low‐seismic‐velocity, high‐attenuation, low‐density fore‐arc material as accreted and recycled, suggesting that underplating and uplift destabilizes East Cape Ridge, triggering two‐sided mass wasting. Mass balance calculations indicate that the proposed accreted and recycled material represents 25–100% of all incoming sediment, and any remainder could be accounted for through erosion of older accreted material into surrounding basins. We suggest that continental mass flux into the mantle at subduction zones may be significantly overestimated because crustal underplating beneath fore‐arc highs have not properly been accounted for.