Implications of paleomagnetic data on the collision related bending of island arcs

Paleomagnetic studies from the Marianas, central Philippines, Ryukyus, Sulawesi, Fiji‐New Hebrides all show major differences in the declination values observed from individual islands within the same arc. Comparable differences are not observed in the inclination values. A simple plate tectonic model would predict that each island arc should experience the same rotation along its entire length as the plate moves around its rotation axis. Bathymetry from each of these arcs show that the differentially rotated segments of the upper plate occur in areas where the downgoing plate carries a feature such as a seamount chain, rifted continental fragment, or an island arc. The coincidence of the bathymetric feature and the discordant paleomagnetic declination data support an earlier hypothesis of Vogt [1973] that these features are buoyant zones on the downgoing plate which interact with the margin of the upper plate. As a result of this interaction, the upper plate boundary is locally deformed. In addition to the paleomagnetic evidence, other geologic and geophysical data from these areas suggest such interactions. For example there appears to be a good correlation between the deflections in major structures around the collision zone and the paleomagnetic directions that are reported. As this deformation proceeds, eventually the stresses resulting from the collision may result in any of the following tectonic processes: 1) slip‐line style strike‐slip faults allowing for the sideward extrusion of portions of the upper plate [Tapponnier et al. 1983]; 2) changes in the polarity of the subduction zone; 3) development of strike‐slip faults around the margin of the indenter; or 4) reorganization of the entire plate margin. In addition, data from the Marianas, southern end of the RyuKyu arc, and the Lau Basin suggest that back arc basin development may result from the collision related deformation.