Читать книгу Iceland Within the Northern Atlantic, Volume 1 - Группа авторов - Страница 24

The full insularization of Iceland began about 16 My ago (Figures 1.2 and 1.8). The evolution of Iceland until around 9 My was mainly controlled by the tectonic and magmatic processes related to both the opening of the Atlantic Ocean and the evolution of the hot spot. They built a classically evolving volcanic island, even if its formation was unusual (Chapter 3). The last rift jump (Chapter 2) in the north of the island started about 8 My ago. Then a rift jump occurred in the south of the island around 3 My, when the great glaciations started. At the same time, Iceland’s shape was also influenced by global eustatic evolution, which allowed the development of marine abrasion surfaces around the island (Figures 1.8 and 1.9).

The development of radiometric dating (section 2.4 of Volume 2) allowed us to better constrain both the chronology of the progressive extinction of the paleorifts and also that of the glaciations with respect to the tectonic and paleo-oceanographic evolution of the North Atlantic. The impact of the glacial loading of rifts during periods of extended ice caps could explain pro parte this cyclic evolution linked to rift-jump.

Figure 1.8. W-E topographic section of Iceland with its various morphological and geological dated constituents. The geology is partially hypothetical, adapted and completed from (Hjartason et al. 2007). K-Ar dates are in red. NADW: North Atlantic Deep Water

The upper plateau of the island, raised at about 600 m in altitude, is a relatively recent construction (<7 My) as far as the age of dyke injection and subaerialvolcanism is concerned. The peninsulas of Flateyjarskagi, Tröllaskagi and Tjörnes are mainly constituted by lava flows dated between 5 and 3 My, interspersed with glacial and marine formations, as in the sector of Husavik. The peninsulas of the northwest and east consist of much older lavas. The ages of these basaltic series usually range between 7 and 6 My very locally reaching 16 My at their base (in the west).

Strandflat is a marine abrasion surface that can be found along the whole North Atlantic. It has sometimes been attributed to the action of glaciers only, but is scoured both on the edge of continents and in Iceland by large glacial valleys that reuse a pre-existing hydrographic network. Between +20 and −50 m, the platform of the Icelandic strandflat contains mostly lava flows dated between 7 and 10 My. It is strongly shaped by the action of glaciers. Several ages, obtained from interglacial flows fossilizing the strandflat, are distributed between 1.8 My on Skagi to 0.13 My for Snæfellnes: it is therefore polygenetic. It is also continuous over several dozen square kilometers between the three great peninsulas of the west coast. In the rest of the island, it forms a relatively narrow terrace.

Figure 1.9. Global sea level evolution from the Eocene to the present day in relation to glaciations and the development of thermohaline circulation

COMMENT ON FIGURE 1.9.– (1) Compilation of sea level changes (based on (Abreu et al. 1998)). (2) Curve δ18O compiled by Cramer et al. (2008). (3) Eustatic curves defined by Miller et al. (2005, 2011, in black and blue) and Kominz et al. (2008, in brown). The blue background reflects the intensification of the thermohaline circulation. SH: Southern hemisphere, NH: Northern hemisphere. It can be compared with Figures 3.6. and 3.3(B) in Volume 2.

The submerged platform that lies between −200 m and −50 m around the island (Figures 1.1 and 1.3) truncates the Miocene and Oligocene effusive formations of the GIR (Hjartason et al. 2007; Chapter 3 of this volume). This platform is itself scoured by glaciers (Figures 1.3 and 1.10) as is the West Coast of Spitsbergen or the coast of Newfoundland (Figure 3.1(A)), which are free of volcanic activity.

The evolution of the eustatic signature (Miller et al. 2005, 2011; Kominz et al. 2008) on long time scales indicates that the tectono-eustatic component has not exceeded 50 m since 60 My (Rowley 2013), mainly as a result of variations in seafloor production and mantle evolution.

Figure 1.10. The Northwest Peninsula, western Iceland, and the strandflat

COMMENT ON FIGURE 1.10.– Subsident morphology with a very large strandflat (<50 m, clear) south of the Northwest Peninsula and west of Iceland with fossilized coastal cliffs, flooded glacial valleys up to the upstream cirque (red arrow) and eroded platform in the Vestfirðir sector. Neogene and Quaternary glacial overdeepenings are indicated by black arrows.

In fact, this surface could have been shaped in approximately the Lower Oligocene and especially at the end of this epoch (Chattian). The first evidence of the glaciations of the northern hemisphere dates back to 9 My (beginning of the Tortonian) thus trace since this period a thermal subsidence and a tilting of the platform of at least 150 m west of Iceland, allowing an accentuation of the thermohaline circulation (section 3.1 of Volume 2). The flooded glacial valleys of the southern part of the Northwest Peninsula (Figure 1.10) also illustrate this evolution.

The eastern coast of the island, on the other hand, has a different history. It includes an older surface, located today at a depth of 1,000 m, which may have been shaped in the Bartonian age, a key period for the tectonics of the Atlantic margins (Chapter 3).