Zusammenfassung
Wide-angle shear wave arrivals, converted from compressional to shear
waves at crustal interfaces, enable crustal Vp/Vs ratios to be determined
which provide valuable constraint on geological interpretations.
Analysis of the converted shear wave phases represents the next logical
step in characterizing the crustal structure and composition following
multichannel seismic structural imaging and tomographic inversion
of the wide-angle compressional wave phases. In this offshore study
across two passive margins extending from stretched continental to
fully oceanic crust, the high-data density (2-10 km ocean bottom
seismometer, OBS, spacing) and a consistent, efficient conversion
interface produced shear wave data sets suitable for traveltime inversion.
The shear waves were recorded by three orthogonal geophones in each
OBS. Arrival phases, visible to 180 km offset, were identified using
their arrival times, moveout velocities and particle motions. Across
the North Atlantic volcanic rifted continental margins studied, breakup
was accompanied by the eruption of large volumes of basalts of the
North Atlantic Igneous Province. The interface between post-volcanic
sediments and the top of the basalts provides the dominant conversion
boundary across the oceanic crust and the continent-ocean transition.
However, the shear wave data quality was significantly diminished
at the continental ends of the profiles where the thick basalt flows
and hence this conversion interface feathers out and crustal attenuation
increases. Initial modelling of the converted shear wave phases was
carried out using a layer-based approach with arrivals converted
on the way up used to constrain the Vp/Vs ratio of the post-volcanic
sedimentary sequence beneath each OBS. To produce a model with continuous
crustal S-wave velocities, the compressional wave velocities beneath
the sediment-top basalt interface were transformed into starting
shear wave velocities using a constant value of Vp/Vs and the inversion
carried out by specifying the appropriate ray path. Once the data
set had been fully interpreted, correction of the traveltimes to
effective symmetric ray paths enabled us to apply a regularized grid
inversion. Such inversions are less subjective than the layer-based
approach and yield more robust minimum structure results with quantifiable
errors, except in the vicinity of a known subbasalt low-velocity
zone encountered on the Faroes margin. Monte Carlo analyses were
performed for this approach; the average model from multiple inversions
using randomized starting models and traveltimes shows the structure
required by the traveltimes and the model standard deviation gives
an estimate of uncertainty. Model and inversion parametrizations
were fully tested and optimum parameters chosen for compressional
and shear wave inversions. This allows, after appropriate model smoothing,
an estimate to be made of the spatial variation of the Vp/Vs ratio
within the crust. There are marked gradients in Vp, Vs and Vp/Vs
ratio across the continent-ocean transition, which may result from
intrusion of high magnesium mafic igneous material into the crystalline
continental crust. The Vp/Vs ratio, used in conjunction with Vp,
also provides constraints on the subbasalt lithologies forming the
low-velocity zone. We conclude from such an analysis that this zone
is unlikely to be composed entirely of igneous hyaloclastite material;
some proportion of clastic sedimentary rocks is likely to be present.
The Vp/Vs and Vp properties of the units underlying the low-velocity
zone are inconsistent with crystalline continental basement and this
unit is likely to represent a sill-intruded Mesozoic sedimentary
sequence from a pre-breakup sedimentary basin.
Nutzer