Architecture and geodynamic evolution of the Svalbard Archipelago, the Yermak Plateau and the Fram Strait oceanic Province from deep seismic experiments

oritzmann [ at ]


Between 1997 and 1999 new seismic refraction data were acquired in the region of the Svalbard Archipelago. These experiments were carried out by a German, Polish, Norwegian and Japanese cooperation. The resultant seismic velocity profiles give new insight into the general crustal structure of Svalbard, its western continental margin, the Yermak Plateau and the adjacent Fram Strait in the northern Atlantic oceanic realm.A fundamental observation of this study is that the Yermak Plateau north of the Svalbard Archipelago exhibits no evidence for elevated magmatic activity due to the presence of a mantle plume. The seismic velocity structure reveals none of the characteristic features of a rifted-volcanic margin. The southern Yermak Plateau shows instead a mid-crustal detachment that supports theories of Oligocene extensional movements and parallel development of pull-apart-like basins. Only slight amounts of melt have been intruded due to crustal thinning and subsequent decompressive melting.Svalbard's different Caledonian terranes cannot be distinguished on the basis of their seismic structure. Stretching of continental crust, associated with the Cenozoic rifting of western Svalbard, is confined to the western terrane. The boundary between the western and central Caledonian terranes is overprinted by the Cenozoic Western Spitsbergen Orogenic Belt. This belt exhibits remarkably low seismic velocities within the upper brittle crust down to a depth of ~20 km, that leads to the assumption that the rock fabric is intensively sheared and faulted. Proposed flower structure models, in a transpressive tectonic regime, seem plausible for the evolution of the orogenic belt. The coincidence of Caledonian and Cenozoic (sinistral and dextral) shear zones may indicate the possible reactivation of old sutures.The continental margin off western Svalbard is more segmented than previously thought. Off Van Mijenfjorden a rifted margin is observed. The continent-ocean transition off Kongsfjorden reveals a steep and abrupt change in Moho-depth that is interpreted as a sheared margin. The evolution of this sheared margin is associated with the Spitsbergen Fracture Zone of the southern Fram Strait. A rifted continental margin segment is observed off the northern margin of western Svalbard.Oceanic crust off western Svalbard is very thin compared to the global mean. This is associated with slow spreading rates in the North Atlantic and the Fram Strait. The absence of oceanic layer 3, and low mantle seismic velocities, attributed to slight serpentinisation of upper mantle peridotites, are characteristics of the entire observed region.The continental margins presented in this study are characterised by slight amounts of melt within the continent-ocean transition zone, regardless of having a rifted or sheared nature. At the sheared segment of the western Svalbard margin a possible source of melts is the large thermal contrast between cool continental crust and the hot oceanic mantle, which may have enhanced convective partial melting. The injection of melts is supposed to have occurred during the northward propagation of the Knipovich Ridge, or the passage of the Molloy Ridge along the (sheared) continental margin. The ancient Tertiary shear zones between Svalbard and Greenland (Spitsbergen Shear Zone) provide possible pathways for channelling and distribution of these melts along the western margin.The zones of slight intrusion within the continent-ocean transition coincide with magnetic anomalies along Svalbard's western coast. At least one further anomaly is observed at the western Yermak Plateau, at the southern termination of a transform fault in the Fram Strait (Lena Trough). With regard to the proposed segmentation of the Svalbard margin, recent plate tectonic reconstructions, and the observed magnetic anomaly pattern, it follows, that the segmentation of the continental margin continues further north. The injection of melts is suggested to accompany the break-up. Sheared and non-sheared segments alternate along the margin.

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Thesis (PhD)
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Ritzmann, O. (2003): Architecture and geodynamic evolution of the Svalbard Archipelago, the Yermak Plateau and the Fram Strait oceanic Province from deep seismic experiments , PhD thesis, Universität Bremen.

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