Seismic images of a collision zone offshore NW Sabah/Borneo

Multichannel reflection seismic data from the southern South China Sea, refraction and gravity modelling were used to investigate the compressional sedimentary structures of the collision-prone continental margin off NW Borneo. An elongated imbricate deepwater fan, the toe Thrust Zone bounds the Northwest Borneo Trough to the southeast. The faults separating the individual imbricates cut through post-Early Miocene sediments and curve down to a carbonate platform at the top of the subsiding continental Dangerous Grounds platform that forms the major detachment surface. The age of deformation migrates outward toward the front of the wedge. We propose crustal shortening mechanisms as the main reason for the formation of the imbricate fan. At the location of the in the past defined Lower Tertiary Thrust Sheet tectonostratigraphic province a high velocity body was found but with a much smaller extend than the previously defined structure. The high velocity structure may be interpreted either as carbonates that limit the transfer of seismic energy into the sedimentary layers beneath or as Paleogene Crocker sediments dissected by remnants of a proto-South China Sea oceanic crust that were overthrust onto a southward migrating attenuated continental block of the Dangerous Grounds during plate convergence.     

A 3D gravity and magnetic model for the Entenschnabel area (German North Sea)

International Journal of Earth Sciences - In this study, we focus on structural configuration of the Entenschnabel area, a part of the German exclusive economic zone within the North Sea, by means...     

The trace of the Pacific‐Cocos‐Nazca triple junction in the Central Pacific and the formation of an overlapping spreading centre

The `plate tectonic mirror image' to the region of the Cocos and Nazca plates, which are currently being subducted beneath Central America, is preserved in the Central Pacific around 120°W just south of the equator. Cruise SO‐180 investigated this remote area during project CENTRAL and acquired new magnetic and bathymetric data. A plate tectonic model for the ‘mirror image’ is presented based on the newly acquired as well as reprocessed existing data. Discordant magnetic anomaly patterns and bathymetric structures indicate at least two major reorganization events (19.5 and 14.7 Myr), which can be detected both in the Cocos‐Nazca spreading system and in the East Pacific Rise. Irregularities in the anomaly pattern and curvilinear structures on the sea floor of the survey area are interpreted in terms of a fossil overlapping spreading centre at the location where the Farallon break‐up originated.     

The continent‐ocean transition on the northwestern South China Sea

Rifted margins are created as a result of stretching and breakup of continental lithosphere that eventually leads to oceanic spreading and formation of a new oceanic basin. A cornerstone for understanding what processes control the final transition to seafloor spreading is the nature of the continent‐ocean transition (COT). We reprocessed multichannel seismic profiles and use available gravity data to study the structure and variability of the COT along the Northwest subbasin (NWSB) of the South China Sea. We have interpreted the seismic images to discern continental from oceanic domains. The continental‐crust domain is characterized by tilted fault blocks generally overlain by thick syn‐rift sedimentary units, and underlain by fairly continuous Moho reflections typically at 8–10 s twtt. The thickness of the continental crust changes greatly across the basin, from ~20 to 25 km under the shelf and uppermost slope, to ~9–6 km under the lower slope. The oceanic‐crust domain is characterized by a highly reflective top of basement, little faulting, no syntectonic strata and fairly constant thickness (over tens to hundreds of km) of typically 6 km, but ranging from 4 to 8 km. The COT is imaged as a ~5–10 km wide zone where oceanic‐type features directly abut or lap on continental‐type structures. The South China margin continental crust is cut by abundant normal faults. Seismic profiles show an along‐strike variation in the tectonic structure of the continental margin. The NE‐most lines display ~20–40 km wide segments of intense faulting under the slope and associated continental‐crust thinning, giving way to a narrow COT and oceanic crust. Towards the SW, faulting and thinning of the continental crust occurs across a ~100–110 km wide segment with a narrow COT and abutting oceanic crust. We interpret this 3D structural variability and the narrow COT as a consequence of the abrupt termination of continental rifting tectonics by the NE to SW propagation of a spreading centre. We suggest that breakup occurred abruptly by spreading centre propagation rather than by thinning during continental rifting. We propose a kinematic evolution for the oceanic domain of the NWSB consisting of a southward spreading centre propagation followed by a first narrow ridge jump to the north, and then a younger larger jump to the SE, to abandon the NWSB and create the East subbasin of the South China Sea.     

The relationship of the Cocos and Carnegie ridges: age constraints from paleogeographic reconstructions

Paleogeographic restorations for the oceanic crust formed by the Cocos-Nacza spreading center and its precursors were performed to reconstruct the history and ages of the submarine aseismic ridges in the Eastern Pacific Basin, the Carnegie, Coiba, Cocos, and Malpelo ridges. The bipartition of the Carnegie ridge reflects the shift from a precursor to the presently active Cocos-Nazca spreading center. The Cocos ridge is partly composed of products from the Galápagos hotspot but may also contain material from a second center of volcanic activity which is located approximately 600 km NE of Galápagos. The Malpelo ridge is a product of this second hotspot center, whereas the Coiba ridge probably formed at the Galápagos hotspot. The geometric relationship of the Cocos and Carnegie ridges indicates symmetric spreading and a constant northward shift of the presently active Cocos-Nazca spreading center.     

Extinct spreading on the Cocos Ridge

Using recently acquired marine magnetic data and existing magnetic and bathymetric data sets together with ODP Leg 170 age determinations we present a revised plate tectonic model for the southern Cocos and northern Nazca plate area. According to this model the formation of the southern Cocos plate was governed by spreading at different ridge axes with alternations between spreading ridges producing a complex magnetic anomaly pattern. In the Cocos and Malpelo ridge area we have identified two precursors of the recently active Cocos–Nacza spreading system which were active from 22.8 to 14.7 Ma, with a change in spreading direction from NW–SE to ENE–WSW at 19.5 Ma. The oceanic crust of these abandoned spreading systems was subsequently thickened and overprinted by hotspot volcanism that formed the Cocos and Malpelo ridges. The centre of this hotspot volcanism is about 500 km away from, but most probably related to, the Galapagos hotspot.     

Seafloor spreading anomalies and crustal ages of the Clarion-Clipperton Zone

The Clarion-Clipperton Zone (CCZ) of the central Pacific is one of the few regions in the world’s oceans that are still lacking full coverage of reliable identifications of seafloor spreading anomalies. This is mainly due to the geometry of the magnetic lineations’ strike direction sub-parallel to the Earth’s magnetic field vector near the equator resulting in low amplitude magnetic anomalies, and the remoteness of the region which has hindered systematic surveying in the past. Following recently granted research licenses for manganese nodules in the CCZ by the International Seabed Authority, new magnetic data acquired with modern instrumentation became available which combined with older underway data make the identification of seafloor spreading anomalies possible for large parts of the CCZ and adjacent areas. The spreading rates deduced from the seafloor spreading patterns show a sharp increase at the end of Chron 21 (47.5 Ma) which corresponds to the age of the bend in the Hawaii-Emperor seamount chain and an associated plate tectonic reorganisation in the Central Pacific. An accurate map of crustal ages for the central-eastern Pacific based on our anomaly picks may provide a basis for improved plate tectonic reconstructions of the region.