Implications d'un saut de rift et du fonctionnement d'une zone transformante sur les déformations du Nord de l'Islande. Approches structurale, sismotectonique et radiochronologique

Abstract : In Northern Iceland, the active rift is located 120 km eastward with respect to the Kolbeinsey Mid-Atlantic Ridge. The Tjörnes Fracture Zone connects these two rifts and accommodates a dextral transform motion. A major active fault of the Tjörnes Fracture Zone is the WNW-ESE trending Húsavík-Flatey Fault (HFF). The seismic activity associated to the HFF defined a nearly vertical fault surface that cuts through the 12 km thick seismogenic crust. Analyses of major structures combined with inversion of fault slip data allow us to discuss the kinematics and mechanics of the HFF since the Late Tertiary. For the present-day kinematics, we also use inversion of earthquake focal mechanisms provided by the Icelandic Meteorological Office. The main state of stress along the HFF corresponds to a dextral transtension, with an ENE-WSW trending extension due to the obliquity of the transform fault relative to the E-W direction of plate divergence. This overall mechanism is subject to slip partitioning that include an extension trending NW-SE, parallel to the HFF, and an extension trending NE-SW, perpendicular to the fault. These three regimes do not reflect a succession in time of tectonic events, but occur simultaneously at different places and in various chronological orders. They are thought to express the geometric accommodation of the transform motion in the oblique transform zone. Near the connection of the HFF to the Kolbeinsey Ridge, most dextral transform faults that trend parallel to the HFF are replaced by normal faults with a dextral component. On the opposite side of the HFF, at its junction with the northern Icelandic rift, two main transform fault segments have been mapped out. The northern fault directly connects to the northern Icelandic rift as a triple point junction, whereas the southern one progressively evolves from a WNW-ESE trending dextral transform fault to a N-S trending normal fault, parallel to the direction of the rift structures. One hypothesis, which would explain these structural differences, involves the presence of pre-existing structures, like volcanoes, that affect the direction of propagation of the southern transform fault segment. Another hypothesis would be that the southern transform fault, the oldest between these two transform faults, developed and then was connected to the northern Icelandic rift before the rift propagated northward. After this northward propagation of the northern Icelandic rift, the northern transform fault segment propagated linearly to the rift. Moreover, near the junction of the HFF with the northern Icelandic rift, the behaviour of the normal faults associated with the rift appears to be strongly influenced by the dextral transtension characterising the HFF. On other hand, seismic faults have been mapped out near the eastern border of the Flateyjarskagi shelf. Inversions of related focal mechanisms suggest that movements associated with the present-day unlocking of the eastern part of the HFF, considered as locked until recently because of its low seismic activity, are compatible with the proposed model of dextral transtension. The present-day offset between the Kolbeinsey Ridge and the northern Icelandic rift results from an eastward rift jump of the Mid-Atlantic Ridge in Iceland. As the North American-Eurasian plate boundary drifts westward, eastward rift jumps relocate the rift zones above the Icelandic hotspot. Using 40Ar/39Ar dating of dykes along a profile parallel to the direction of plate motion in Northern Iceland, a paleo-rift axis has been recognised 60 km eastward to the previously accepted location. This paleo-rift, localised in the continuation of fjord of Skagafjördur and referred to as, remained active until approximately 3 Ma ago. The activity of the northern Icelandic rift started about 8-8.5 My ago, intruding the presently dated 9-9.5 Ma old eastern flank of the Skagafjördur paleo-rift. This age of 1 Ma for the intruded paleo-rift rocks at the rift jump time is considered as a minimal value, resulting to hypotheses assumed in the theoretical model for the dating data interpretation. An important plio-pleistocene glacial erosion, inducing a decrease of the lava pile thickness and thus a diminution of the area covered by the production of the northern Icelandic rift, would explain the presence of dykes related to the northern Icelandic rift as far as 25 km outside of its borders. The two rifts were thus simultaneously active during 5-5.5 Ma. During this period, the accretion rates along the two rift zones were nearly equal. However, accretion was asymmetrical along each rift, with the higher rate of accretion situated on the external flank. The supposed absence of drift from the northern Icelandic rift since its initiation implies a strong asymmetry of accretion, favouring the Northern American plate. A decrease of the hotspot activity would release the northern Icelandic rift and allow it to drift westward with the Mid-Atlantic Ridge. Then, an increase of the hotspot activity would facilitate the shift of the accretion zone above to the hotspot by an eastward rift jump. The tectonic changes related to the last rift jump explains the observed deformations in Northern Iceland. The Skagafjördur paleo-rift shrank, after it became extinct, both towards the Kolbeinsey Ridge and towards the active volcanic zone of central Iceland. This volcanic zone of central Iceland probably corresponds to an "en échelon" zone between the northern and the southern Icelandic rifts. The weight of the lava flows spread out by the volcanic zone of central Iceland since 3 Ma has induced a flexuring of the tertiary lava flow pile produced by the Skagafjördur paleo-rift. In a similar way, the lava flows erupted by the Skagafjördur paleo-rift have been flexured by those erupted by the northern Icelandic rift. These flexures are directed towards the volcanic zone of central Iceland and towards the northern Icelandic rift, respectively. They define a continuous flexure belt through Northern Iceland. An important extensive fracturing is related to the flexure of the lava flows. Such a flexure would explain that the synform-like arrangement of the lava flows, considered as a characteristic of an Icelandic rift axis, is absent along the southern part of the Skagafjördur paleo-rift axis, whereas such a structural shape is recognised along its northern part. The synform that was initially thought to localise the axis of the paleo-rift would be the axis of the flexural zone produced by the weight of plio-pleistocene lava flows erupted from the south, in the volcanic zone of central Iceland. Despite the limited number of radiochronological data available as time constraints in Southern Iceland, a global model of the tectonic evolution of the island, based on the model proposed for the evolution of Northern Iceland, is presented. This model highlights the importance of the volcanic zone of central Iceland. This volcanic zone would accommodate the differences in the tectonic evolution, like diachronisms in rift jump process and consequently shifts of the accretion zones, between the northern and southern parts of Iceland.
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Géologie appliquée. Université Pierre et Marie Curie - Paris VI, 2003. Français
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Sebastian Garcia. Implications d'un saut de rift et du fonctionnement d'une zone transformante sur les déformations du Nord de l'Islande. Approches structurale, sismotectonique et radiochronologique. Géologie appliquée. Université Pierre et Marie Curie - Paris VI, 2003. Français. 〈tel-00009796〉

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