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Subduction along a semicircular trough

It can be inferred that a strong, elastic, oceanic lithosphere would experience great difficulty in subducting down a semicircular trench. Such subduction is possible only because the ocean plate contains a number and variety of fractures. The important fractures in this context are the major and minor strike-slip, parallel shear fractures in the S Atlantic, which trend parallel to the E-W boundaries of the Scotia Plate. These fractures will permit the ocean lithosphere to be subducted in sections (Figure 7.18). In the central portion of the arc, the subduction is relatively easy, but becomes progressively more difficult at the northern and southern portions of the arc, where the flexure is required to develop at an acute angle to the transform fractures. Figure 7.19 shows the displacement of the magnetic anomalies along strike-slip, shear faults. The preponderance of seismic activity related to the subduction of the arc coincides with the areas where the arc joins the transform faults, where, it is inferred, the stresses are highest.

Grunow (1993) states that the 'Northern and southern mid-Cretaceous (palaeomagnetic) poles of Graham Land are very much alike, suggesting that the "S" shape of the Antarctic Peninsula is not due to oroclinal bending since (approximately) 110 Ma'. In his paper, the description of the various sites from which the samples were taken and the difficulties of obtaining good rock samples are emphasised. The spread of results inherent in the palaeomagnetic data obtained from these sites could easily hide a modest amount of 'bending' to have taken place since 110 Ma. However, this is not incompatible with his remark, except for the northernmost part of N Graham Land and the S Shetland Islands. Indeed, the 'curvature' may be more

Figure 7.19 Plan of Scotia Arc and break-up of internal band of basalt by vertical shear fractures.

apparent than real, in that it may well be the result of differential shear displacements on strike-slip faults rather than bending.

Figure 7.19 Plan of Scotia Arc and break-up of internal band of basalt by vertical shear fractures.

apparent than real, in that it may well be the result of differential shear displacements on strike-slip faults rather than bending.

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