Breakup Magmatism on the Vøring Margin: Insights from Sub-Basalt Imaging and Ocean Drilling Program Hole 642E

Abstract:

The seismic volcanostratigraphic distribution of breakup magmatic complexes and sub-basalt imaging around ODP Hole 642E provide new constrains on initial breakup processes and the subsequent Vøring Margin evolution. The recovered samples at Hole 642E where interpreted as two distinct volcanic series. The Upper Series, represents a seaward dipping reflector wedge (SDR), of transitional tholeiitic lava flows and thin volcaniclastic sediments. In contrast, the Lower Series is built of evolved dacitic/andicetic flows, some highly depleted sills and thicker interbedded sediments. Downhole geochemical variations based on recent analyses define a decreasing continental crustal influence from continental crustal anatectic melts in the Lower Series to purely MORB-like rocks in the Upper Series.

The deep imaging of the Lower Series on reprocessed seismic data allows the definition of a new seismic facies unit characterized by wavy to continuous subparallel reflections with an internal disrupted and hummocky shape. Some borehole samples from this seismic facies unit show hydrothermal alteration and devitrification. This unit is interpreted as subaqueous flows and sills intruded and extruded in wet sediment. The top boundary is defined as a negative polarity reflection interpreted as the K reflector and representing a transition between upper tholeiitic and lower dacitic/andesitic lavas. The base of the facies unit corresponds to the base of the breakup extrusive complexes with a thickness of up to 2 km toward the continent ocean boundary.

A revised model is suggested, including (1) the pre-breakup Lower Series crustal anatectic melt volcanism, (2) widespread explosive, subaerial volcanism and pyroclastic rock deposits defining the K reflection, (3) the main volcanic breakup stage with intense MORB-like volcanism and large subsidence leading to the built of the SDR wedge, and (4) late breakup volcanism accompanied by jumping of the volcanic activity, the accretion of newly formed oceanic crust with decreasing subsidence. The thicker basaltic rocks correspond to the SDR with local depocenters of more than 6 km documenting spatial variations of melt accommodation. We suggest that the inherited rift geometry controls not only the accommodation space but also the growth of the SDR wedge.