Transtensional tectonics during the break-up of Svalbard and Greenland thinned the crust fsom 35 km below inner Svalbard, to 18-28 km as observed along the profile. The deep structure of the southern section of the profile can be related to the basement prov- ince West of the Raudfjorden Fault building up the central tesrane of Svalbard. The char- acter of the stretched continental crust remains similar on the northem Yermak Plateau, but the actual lithology of the middle and lower crust remains speculative.
North of Svalbard's coastline the velocity structure supports the existente of a detach- ment fault, although the fault plane is not resolved in the seismic data itself. Above this, the Tertiary Danskeya Basin is underlain by a Paleozoic sedimentary basin, of suggested Late Silurian/Devonian age. The Paleozoic sedimentary origin of that basin is con- strained by low seismic velocities of 5.1-5.8 krn/s, which are typical for Devonian rocks
44 CRUSTAL STRUCTURE OF NORTHWESTERN SVALBARD AND THE ADJACENT YERMAK PLATEAU:
EVIDENCE FOR OLIGOCENE DETACHMENT TECTONICS AND NON-VOLCANIC BREAK-UP
of Greenland and Svalbard. Paleozoic sedimentary rocks may occur also in the upper section (6 km) between the H.U. Sverdrup Bank and the Danskgya Basin. The onset of extension in the Danskgya Basin above the detachment was in the Oligocene, while its duration is speculative. Due to the obvious asymmetric constsuction, simple shear rifting is supposed. This might have driven an uplift of maximum 300 m in the Cenozoic.
The final velocity model reveals no evidence for a large-scale influence and magma- tism of a suggested hot spot during the break-up of Svalbard from northern Greenland (Yermak Hot Spot). The boundary conditions for a widespread distribution of mantle derived melts are generally given since thin lithosphere can control the evolution of the plurne along the major fractures and would provide ideal paths.
We propose, that the observed crustal structure is typical for the entire westem plateau up to 82ON, which is thus a stretched spur of the continental central tessane of Svalbard.
The volcanism that created the northeastern Yermak Plateau and the Morris Jesup Rise seems to have been local and short-lived, and is therefore unrelated to mantle plume activity as proposed for the North Atlantic margins. Therefore non-volcanic continental margins for northern and western Svalbard may be expected. Known volcanic events fur- ther south are, according to our interpretation, related to melt generation caused by sim- ple shear tectonics combined with a slightly increased asthenospheric temperature.
CT IN NORTHWESTERN
Oliver Ritzmaiinl, Wilfried Jokatl, Wojciech Czuba2, Alexander ~uterch', Rolf Mjelde3& Yuichi Nishimura4
'
Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany Institute of Geophysics, Polish Academy of Sciences, Warsaw, Poland Institute of Solid Earth Physics. University of Bergen, Bergen, Norway Institute of Seismology and Volcanology, Hokkaido University, Sapporo, Japan Submitted to Geophysical Journal International, Blackwell Science UK. September 20023.1 Abstract
New seismic refraction data were collected across the westem Svalbard continental margin off Kongsfjorden (Ny Alesund) during the cruise leg ARK1512 of RV Polarstern.
The use of on- and offshore seismic receivers and a dense airgun shot Pattern provide a detailed view of the velocity stmcture of Svalbard's continental interior, the conti- nent-ocean transition, and oceanic crust related to the northern Knipovich Ridge and the Molloy Ridge.
The Caledonian central and western terranes of Svalbard are not distinguishable on the basis of seismic velocity structure. Below a 7 to 8 km thick Paleozoic sedimentary Cover the crystalline crust reveals a three-layer structure with seismic velocities ranging between 6.1 and 6.9 km/s. The geological suture between the terranes is imperceptible.
The middle and upper crust below the Tertiary Forlandsundet Graben shows striking low velocities. This can be related to the Early Paleozoic convergent transpressive move- ments between Svalbard and northern Greenland, followed by an extensional (relaxing) phase. We argue that a brittle-fractured rock foimation is present below the graben, which also buries a sedimentary Paleozoic core.
The continent-ocean transition can be classified as a sheared margin formed at the continental part of the Spitsbergen Fracture Zone. Moderate crustal thinning is achieved only to the west of the low velocity Zone below the Forlandsundet Graben. This leads to the assumption that transtensional rift movements since Oligocene were decoupled from the central terrane of Svalbard. The Moho dips with an angle of 45' eastwards at the con- tinent-ocean transition that exhibits higher seisn~ic velocities of 7.2 km/s on the conti- nental side. These can be interpreted as minor mantle-derived intrusions, probably
46 A DEEP SEISMIC TRANSECT IN NORTHWESTERN SVALBARD AT KONGSFJORDEN (NY ALESUND) AND THE IMPLICATIONS FOR THE CENOZOIC BREAK-UP FROM GREENLAND: A SHEARED MARGEN STUDY
induced by convection due to the juxtaposition of cool continental and hot oceanic litho- sphere.
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 cliaracterised by the absence of oceanic layer 3. These observations can be ascribed to conductive cooling of the ascending mantle due to the extremely low divergence rate and the neighbouring cool continental crust. The underlying mantle is slightly serpentinised below the Knipovich Ridge Segment, reflected by low seismic velocities of -7.7 km/s. A thicker sequence of syn- and post rift sediments and sedimentary rocks are observed on the Molloy Ridge oceanic segment, which results from greater subsidence relative to the Knipovich Ridge segment.