Description: The Hawaiian–Emperor Seamount chain records the motion of the Pacific Plate relative to the Hawaiian mantle hotspot for ~80 m.y. A notable feature of the chain is the pronounced bend at its middle. This bend had been widely credited to a change in plate motion, but recent research suggests a change in hotspot motion as an alternative. Existing paleomagnetic data from the Emperor Chain suggest that the hotspot moved south during the Late Cretaceous and Early Tertiary, but reached its current latitude by the age of the bend. Thus, data from area of the bend are important for understanding changes in plume latitude. In this study, we analyze the magnetic anomalies of five seamounts (Annei, Daikakuji-W, Daikakuji- E, Abbott, and Colahan) in the region of the bend. These particular seamounts were chosen because they have been recently surveyed to collect multibeam bathymetry and magnetic data positioned with GPS navigation. Inversions of the magnetic and bathymetric data were performed to determine the mean magnetization of each seamount and from these results, paleomagnetic poles and paleolatitudes were calculated. Three of the five seamounts have reversed magnetic polarities (two are normal) and four contain a small volume of magnetic polarity opposite to the main body, consistent with formation during the Early Cenozoic, a time of geomagnetic field reversals. Although magnetization inhomogene ties can degrade the accuracy of paleomagnetic poles calculated from such models, the seamounts give results consistent with one another and with other Pacific paleomagnetic data of approximately the same age. Seamount paleolatitudes range from 13.7 to 23.7, with an average of 19.4F7.4 (2j). These values are indistinguishable from the present-day paleolatitude of the Hawaiian hotspot. Together with other paleomagnetic and geologic evidence, these data imply that the Hawaiian hotspot has moved little in latitude during the past ~45 m.y.

Description: A 550-km-long transect across the Ninetyeast Ridge, a major Indian ocean hotspot trail, provided seismic refraction and wide-angle reflection data recorded on 60 ocean bottom instruments. About 24 000 crustal and 15 000 upper mantle arrivals have been picked and used to derive an image of the hotspot track. Two approaches have been chosen: (i) a first-arrival tomographic inversion yielding crustal properties; and (ii) forward modelling of mantle phases revealing the structure at the crust–mantle boundary region and of the uppermost mantle. Away from the volcanic edifice, seismic recordings show the typical phases from oceanic crust, that is, two crustal refraction branches (Pg), a wide-angle reflection from the crust–mantle boundary (PmP) and a wave group turning within the upper mantle (Pn). Approaching the edifice, three additional phases have been detected. We interpret these arrivals as a wide-angle reflection from the base of material trapped under the pre-hotspot crust (Pm2P) and as a wide-angle reflection (PnP) and its associated refraction branch (PN) from a layered upper mantle. The resulting models indicate normal oceanic crust to the west and east of the edifice. Crustal thickness averages 6.5–7 km. Wide-angle reflections from both the pre-hotspot and the post-hotspot crust–mantle boundary suggest that the crust under the ridge has been bent downwards by loading the lithosphere, and hotspot volcanism has underplated the pre-existing crust with material characterized by seismic velocities intermediate between those of mafic lower crustal and ultramafic upper mantle rocks (7.5–7.6 km s−1). In total, the crust is up to ≈ 24 km thick. The ratio between the volume of subcrustal plutonism forming the underplate and extrusive and intrusive volcanism forming the edifice is about 0.7. An important observation is that underplating continued to the east under the Wharton Basin. During the shield-building phase, however, Ninetyeast Ridge was located adjacent to the Broken Ridge and was subsequently pulled apart along a transform fault boundary. Therefore, underplating eastwards of the fracture zone separating the edifice from the Wharton Basin suggests that prolonged crustal growth by subcrustal plutonism occurred over millions of years after the major shield-building stage. This fact, however, requires mantle flow along the fossil hotspot trail. The occurrence of PnP and PN arrivals is probably associated with a layered and anisotropic upper mantle due to the preferential alignment of olivine crystals and may have formed by rising plume material which spread away under the base of the lithosphere.

Description: The Dalrymple Trough marks part of the transform plate boundary between India and Arabia in the northern Arabian Sea. Oblique extension is presently active across this portion of the boundary at a rate of a few millimetres per year, and seismic reflection profiles across the trough confirm that it is an extensional structure. We present new swath bathymetric and wide-angle seismic data from the trough. The bathymetric data show that the trough is bounded by a single, steep, 3-km-high scarp to the southeast and a series of smaller, en-echelon scarps to the northwest. Wide-angle seismic data show that a typical oceanic crustal velocity structure is present to the northwest, with a crustal thickness of ~ 6 km. There is an abrupt change in crustal thickness and velocity structure at the northwestern edge of the trough, and the trough itself is underlain by 12-km-thick crust interpreted as thinned continental crust. Therefore we infer that Dalrymple Trough is an unusual obliquely extending plate boundary at which continental crust and oceanic crust are juxtaposed. The extensional deformation is focused on a single major fault in the continental lithosphere, but distributed over a region ~ 60 km wide in the oceanic lithosphere

Description: A number of different models have been developed for the tectonic processes and theevolution of the marginal basins in Southeast Asia. Due to their complex setting at the triplejunction of three major converging plates, a definite tectonic concept has not yet been establishedfor most of the basins. In particular, the formation and history of the Celebes Sea basin is stillambiguous. New seismic wide-angle recordings along the North–Sulawesi subduction zone provideinsight into the deep structure of the accretionary wedge and the Celebes Sea crust, particularly inconjunction with multichannel reflection data. Based on the reflection seismic lines, we previouslyinterpreted prominent block structures within the accretionary wedge to be of ophiolitic character,but in most cases refraction seismic modelling could not confirm crystalline velocities; even in thedeeper parts of the wedge the velocities are relatively low. Therefore a sedimentary origin seemsmore plausible. The Celebes Sea is underlain by typical oceanic crust, except for an unusual crustalthickening from about 7 to 12 km beneath the accretionary wedge. This is observed on all threeprofiles and could represent tectonic thickening of the crust, induced by a collision of thesouthward subducting Celebes Sea plate with another slab subducting northward underNorth–Sulawesi (the still active Molucca Sea slab or the northern part of the Sula platform, thatonce subducted beneath the Celebes Sea).

Description: The Celebes Sea is an Eocene basin between Indonesia and the Philippines, located in the collision zone of three converging tectonic plates: the Pacific, the Indian-Australian and the Eurasian Plate. The tectonic processes of the Celebes Sea are still ambiguous as are for most of the numerous marginal basins and collision zones in this area. Due to the northward thrust of the Australian Plate, the Celebes Sea is subducting beneath the northern part of the Sulawesi island. Refraction seismic interpretations of the North-Sulawesi subduction zone provide insight to the subduction mechanisms, structure, and composition of the oceanic ernst in the Celebes Sea. Seismic wide-angle and multichannel-reflection data were recorded on three lines perpendicular to the strike of the North-Sulawesi subduction zone. The data processing and interpretation was improved by combining both datasets and the correlation is shown in joint illustrations. Due to the extent of the measurements and the complexity of the subsurface, the interpretation of the wide angle data is largely based upon traveltime information. In some parts of the research area dynamic modelling was also carried out, particularly for verification of strong shear wave amplitudes observed at some stations. Strang reflexions outline prominent block structures on all three reflection seismic lines. These structures were firstly interpreted as crustal splinters imbedded in the accretionary wedge, similar to those observed in the adjacent Sulu Sea. However, in the Celebes Sea this interpretation cannot be supported by increased refraction velocities. Here, the seismic velocities are relatively low even in deeper parts of the wedge. The Celebes Sea is underlain by normal oceanic crust, but beneath North-Sulawesi a crustal thickening from 7 km to about 12 km is observed on all profiles. An explanation for this thickening might be provided by thrusting or jolting of the ernst, induced by collision of the subducting Celebes Sea plate with a lithospheric slab dipping northward under Sulawesi. A hypothesis which is supported by active seismicity in the area.