1982 Theses Doctoral

Rifting of the Norwegian Margin and young ocean basin accretion dynamics in the Norwegian-Greenland Sea

Mutter, John Colin

This dissertation presents studies of the stratigraphy and structural geology of the Norwegian passive continental margin, and of the crustal structure of the adjacent oceanic crust. It is based primarily on analysis of single and two-ship multi-channel data collected by R/V Conrad, together with R/V Sverdrap during the latter part of 1978.

Sedimentary rocks were studied to establish a stratigraphy of the margin. Ties to DSDP drilling on the outer part of the margin allow the Cenozoic stratigraphy to be determined fairly accurately; Mesozoic stratigraphy is more difficult to assess. By studying sediment distribution patterns throughout the margin and by inferring broad facies divisions, an assessment is made of relative subsidence patterns following breakup. It is suggested that parts of the margin such as the Voring Plateau suffered a much attenuated subsidence compared with the margin off the Lofoten Islands, and compared with that expected from a simple model of thermal decay of the Iithosphere. AlI parts of the margin appear to have commenced subsiding from the same depth, near to sea level, and various parts followed different subsidence curves.

These changing patterns appear to be related to similarly variable patterns of subsidence and facies development in the period leading up to breakup of the margin. In particular, It is concluded that pronounced subsidence, and the development of a deep marine fan sequence preceeded breakup on the parts of the margin that subsided least following breakup. No extensional tectonics accompanied this "initial" subsidence. We used rough estimates of water depths during depositon to compute the amount of lithospheric extension that could give rise to the observed subsidence and find them to be smalI, consistent with the observed lack of extensional tectonics associated with breakup. It is clear that this margin did not undergo massive continental stretching as part of its formational history.

AIthough less well constrained than Cenozoic events,. the Mesozoic tectonics also appear to vary along the marg!n in association with the subsidence pattern. On the Vøring segment of the margin, some very large uplift structures are present that may have resulted from halokinetic movements in the Late Mesozoic-Early Tertiary. We also observe evidence of Mesozoic volcanism in this region. We note that the region of most developed Mesozoic tectonism corresponds to the region where we have evidence for an "initial subsidence" preceeding breakup, and to the region of most attenuated margin subsidence. We suggest that these associations are causal and may be linked through the long-term thermal development of the margin.

In studying multi-channel seismic profiles in the oceanic crust adjacent to the margin, we were able to map a remarkable set of seaward-dipping reflectors in association with the oldest magnetic anomaly sequence. In detail, they have arcuate shapes, upwardly convex and dip ubiquitously seaward increasing dip both down-dip and down-section. From detailed analysis of the velocity structure in these dipping sequences, we find that the increase in velocity with depth is much I like that in normal oceanic crust but with lower overalI velocities. We relate this to greater porosity in a basaltic, sub-aerial flow sequence that makes up the dipping layers.

Expanded Spread Profiles and sonobuoy data were reduced by automatic Tau-p mapping and Tau-sum inversion. Crustal structure In the Lofoten Basin is not significantly different from normal oceanic crust. In approaching the margin basement surface shoals and we find that the velocity structure changes progressively. There is a distinct change to lower overalI velocities in the upper and lower parts of the crust. The elevated basement is underlain by a much expanded extrusive complex of lower velocity due to high porosities and this comprises the seaward-dipping sequences. Gravity anomalies and crustal structure suggest that the isostatic mechanism is not pure Airy-type. A Pratt-type mechanism cannot be excluded.

The development of the seaward-dipping sequences is variable along the margin, being most spectacularly developed in the region where the subsidence, as determined from stratigraphic studies, is most attenuated; least where we inferred most rapid subsidence.

To explain these observations, we develop a model of early spreading which draws on studies of laterally accreted lava sequences in Eastern Iceland, DSDP drilling on the Outer Vøring Plateau, the stratigraphic data and velocity structures. We suggest that early spreading is characterized by abnormally voluminous extrusive outpourings which buiId an edifice to sea level such that spreading occurs in a sub-aerial eruption environment leading to the development of unusually large outflow of basalts. The temporal and spatial variation in eruption environment at the spreading center that is governed by the subsidence pattern which varies along the margin leads to a variable development of the seaward-dipping sequences.