The Jan Mayen microcontinent (JMMC) in the NE Atlantic was created through two Cenozoic rift episodes. Originally part of East Greenland, the JMMC rifted from NW Europe during the Early Eocene under extensive magmatism. The eastern margin is conjugate to the Møre-Faeroes volcanic margin. The western JMMC margin underwent prolonged extension before it finally separated from East Greenland during the Late Oligocene. Here we present the modelling by forward/inverse ray tracing of two wide-angle seismic profiles acquired using Ocean Bottom Seismometers, across the northern and the southern JMMC. Early Eocene breakup magmatism at the eastern JMMC produced an igneous thickness of 7-9 km in the north, and 12-14 km in the south. While the continent is clear in the north, the southern JMMC appears to be affected by later Icelandic magmatism. Reduced seismic velocity and increased crustal thickness are compatible with continental crust adjacent to the volcanic margin in the south, but the continental presence towards the Iceland shelf is less clear. Our magnetic track off the southern JMMC gives seafloor spreading rates comparable to that of the conjugate Møre Margin. Transition to ultraslow seafloor spreading occurs at ~43 Ma, indicating onset of major deformation of the JMMC. Calculating the igneous thickness-mean V P relationship at the eastern volcanic margin gives the typical positive correlation seen elsewhere on the NE Atlantic margins. The results indicate temperature driven breakup magmatism under passive mantle upwelling, with a maximum mantle temperature anomaly of ~50 °C in the north and 90-150 °C in the south.
Abstract The Logi Ridge, located north of the West Jan Mayen Fracture Zone, is E‐W oriented and 140–150 km long. The seafloor surrounding the Logi Ridge is ∼0.65 km shallower than the conjugate seafloor east of the Mohn's Ridge, attributed to asymmetry in the regional NE Atlantic dynamic uplift. Eight reflection seismic lines across the Logi Ridge constrain its development. Both the western and eastern parts have flat tops, indicating erosion at sea level. Three different basement types surrounding the Logi Ridge are observed: rough basement represents abyssal hills original or reactivated by later magmatic/tectonic events; smooth basement caused by basalt flows overprinting early sediments; and irregular basement formed by later basalt flows and intrusions. The surrounding sediments have two distinct units, where the unit boundary is Middle Miocene (12–14 Ma). Mass transport from the Logi Ridge appears episodically throughout Late Oligocene to Middle Miocene, when development ends. The end of erosion age can also be estimated from seamount height, or present top seamount depth. In the west there is agreement with the age constrained by the sedimentation, proving little dynamic topography change. In the east, discrepancies between the methods are explained by 0.15–0.3 km dynamic uplift after submergence. Hence, most of the regional dynamic uplift occurred before the end of the Logi Ridge development in the Middle Miocene, suggesting a causative relationship. Minor recent magmatic growth and seafloor uplift over a ∼100 km wide zone southeast of the Logi Ridge may be tied to the younger dynamic uplift in the east.
New seismic refraction data were collected across the western Svalbard continental margin off Kongsfjorden (Ny Ålesund) during the cruise leg ARK15/2 of RV Polarstern. The use of onshore and offshore seismic receivers and a dense air-gun shot pattern provide a detailed view of the velocity structure of Svalbard's continental interior, the continent–ocean transition, and oceanic crust related to the northern Knipovich Ridge and the Molloy Ridge. The proposed Caledonian central and western terranes of Svalbard are not distinguishable on the basis of seismic velocity structure. Below a 7 to 8 km thick Palaeozoic sedimentary cover the crystalline crust reveals a three-layer structure with seismic velocities ranging between 6.1 and 6.9 km s−1. The geological suture between the terranes is imperceptible. The middle and upper crust below the Tertiary Forlandsundet graben shows low velocities. This can be related to faulting during the Early Palaeozoic movements between Svalbard and northern Greenland, followed by the continental break-up. Moreover, a sedimentary Palaeozoic core is may be buried below the Forlandsundet graben. The continent–ocean transition can be classified as an obliquely sheared (transform) continental margin. The Moho dips with an angle of 45° eastwards at the continent–ocean transition that exhibits higher seismic velocities of more than 7.2 km s−1on the continental side. The sheared margin evolution is linked to the Spitsbergen Transform Fault, today located north of the Molloy Ridge spreading segment. During a later evolutionary stage the Molloy Ridge passed the continental margin. The separating boundary between continental and oceanic crust off northwestern Svalbard is today part of the inactive Spitsbergen Fracture Zone. The high seismic velocities at the continent–ocean boundary can be interpreted as minor mantle-derived intrusions, probably induced by interaction of the passing spreading ridge during the sheared margin evolution. The oceanic crust generated at the Knipovich Ridge and the Molloy Ridge is thin (2 to 4 km), compared to the global mean, and is thinner as previously observed. The oceanic crust is characterized by the absence of oceanic layer 3. These observations can be ascribed to conductive cooling of the ascending mantle as a result of the extremely low divergence rate. The underlying mantle is slightly serpentinized below the Knipovich Ridge segment, reflected by low seismic velocities of ∼7.7 km s−1. A thicker sequence of syn- and post-rift sediments and sedimentary rocks are observed on the Molloy Ridge oceanic segment, which most likely results from greater subsidence relative to the Knipovich Ridge segment.
