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An extensive seismic survey using ocean‐bottom seismographs (OBS) was performed in the area across the Jan Mayen Basin, North Atlantic, from the Jan Mayen Ridge to the Iceland Plateau. The Jan Mayen Ridge and surrounding area are considered to be a fragment of a continent which was separated from Greenland just prior to magnetic anomaly 6. This study presents the crustal structure of the Jan Mayen microcontinent and the ocean/continent transition to the west of the Jan Mayen Ridge. The crustal structures from the centre of the Jan Mayen Ridge to the Jan Mayen Basin are characterized by a deep sedimentary basin, a thin basaltic layer within the sedimentary section and extreme thinning of the continental crust towards the Iceland Plateau. The OBS data indicate that a continental upper crust (V p=5.8–6.1 km s−1) and lower crust (V p=6.7–6.8 km s−1) underlie the deep sedimentary basin. The thickness of the continental lower crust varies significantly from 12 km beneath the Jan Mayen Ridge to almost zero thickness beneath the northwestern part of the Jan Mayen Basin. An ocean/continent transition zone is found at the western edge of the Jan Mayen Basin. Within the 10 km wide transition zone, crustal velocities increase towards the Iceland Plateau, and approach the velocities of the oceanic crust obtained at the Iceland Plateau, that is 3.8–5.1 km s−1 (oceanic layer 2A), 5.9–6.5 km s−1 (oceanic layer 2B) and 6.8–7.3 km s−1 (oceanic layer 3). The crustal model indicates very thin oceanic crust (5 km) immediately oceanwards of the ocean/continent transition zone. Beneath the Iceland Plateau, the oceanic crust is thicker (9 km) than the typical thickness of normal oceanic crust. This might imply that the oceanic crust at the Iceland Plateau has been generated by asthenospheric material slightly hotter than normal. From the crustal structure obtained by the present study, it is proposed that the western part of the Jan Mayen Ridge may be referred to as a non‐volcanic continental margin, generated by a long duration of rifting. Even if the asthenospheric material upwelling along the margin were hotter than normal, only small amounts of magmatic intrusions and extrusions would have been generated because of significant conductive cooling under the long duration of rifting.

We present a revised tectonostratigraphy of the Jan Mayen microcontinent (JMMC) and its southern extent, with the focus on its relationship to the Greenland–Iceland–Faroe Ridge area and the Faroe–Iceland Fracture Zone. The microcontinent's Cenozoic evolution consists of six main phases corresponding to regional stratigraphic unconformities. Emplacement of Early Eocene plateau basalts at pre-break-up time (56–55 Ma), preceded the continental break-up (55 Ma) and the formation of seawards-dipping reflectors (SDRs) along the eastern and SE flanks of the JMMC. Simultaneously with SDR formation, orthogonal seafloor spreading initiated along the Ægir Ridge (Norway Basin) during the Early Eocene (C24n2r, 53.36 Ma to C22n, 49.3 Ma). Changes in plate motions at C21n (47.33 Ma) led to oblique seafloor spreading offset by transform faults and uplift along the microcontinent's southern flank. At C13n (33.2 Ma), spreading rates along the Ægir Ridge started to decrease, first south and then in the north. This was probably complemented by intra-continental extension within the JMMC, as indicated by the opening of the Jan Mayen Basin – a series of small pull-apart basins along the microcontinent's NW flank. JMMC was completely isolated when the mid-oceanic Kolbeinsey Ridge became fully established and the Ægir Ridge was abandoned between C7 and C6b (24–21.56 Ma).

SynopsisDuring the spawning season the operational sex ratio of the large benthivorous (LB) arctic charr morph in Thingvallavatn is skewed in favour of males which compete intensely on the spawning site. The skewness is caused by males staying longer on the spawning ground than females. LB males employ two mating tactics. Either they guard the female or they attempt sneak-matings with guarded females. The tactics seem to obey a pure conditional strategy where relative size is the important criterion. Generally, the larger males use the guarding tactic and the smaller the sneaking tactic. Assessment of size is a part of the strategy. Relative size influences male behaviour irrespective of which tactic they employ. Guarding is more successful when courting is considered, both with respect to relative frequency and duration of courting acts. Females are aggressive towards the sneakers especially when they are small. This could be interpreted as female choice in favour of large males, but also as a defence against egg predation. A small dwarflike benthivorous morph (SB) is found in the same area. It is mainly active during night and on rare occasions SB-males are seen sneaking into the nest of LB-females. In such cases they are attacked vigorously by both LB sexes.

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