EARTHQUAKES CONFIRM BOOKSHELF FAULTING AT LARGE NON-TRANSFORM RIDGE OFFSETS

Mid-ocean ridge segments are commonly offset by distinct transform faults. Some offsets, however, are bridged by a diffuse zone of deformation rather than a discrete transform fault. These non-transform offsets (NTO's) vary in width from 0 to 120 km. At the largest NTO's, one ridge segment is propagating, thereby overlapping and advancing at the expense of the failing ridge segment. Bookshelf faulting has proven to be a promising model for explaining this deformation (Wetzel et al., Nature, 1993). In this model, pre-existing normal faults reactivate and rotate to relieve tectonic stresses produced by propagating rifts.

Wetzel et al. examined earthquake focal mechanisms from selected NTO's to determine if the fault rotation present was consistent with a bookshelf faulting model. As a continuation, this study examines more recent earthquakes from the locations considered in Wetzel et al. as well as earthquakes from a suite of new locations. Wetzel et al. studied NTO's from the Lau Basin, the Galapagos Rift, the Rivera microplate, and the East Pacific Rise at 2.8 and 29 degrees south. The newly examined locations include NTO's from the Australian-Antarctic Discordance, the North Fiji Basin, and the Pacific-Antarctic Ridge at 36.5 and 62 degrees south. All together, the locations represent NTO's potentially influenced by a wide variety of tectonic conditions including plate rotation, back-arc spreading, hotspot activity, and mantle downwelling.

Focal mechanisms for teleseismic earthquakes from the given locations were obtained online from the Harvard CMT database. The focal mechanisms were plotted with Generic Mapping Tools (GMT) software and visually compared to seafloor lineament maps. To quantify fault rotation, fault plane orientations were numerically compared to the orientations of the associated failing rifts.

A total of 61 focal mechanisms were studied among the different locations. Sixty-two percent are rotated between 10 and 29 degrees with respect to the ridge where they originated. All measured rotations are between 0 and 45 degrees. The observed focal mechanisms and the distributions of fault plane orientations are consistent with the results of Wetzel et al., providing further support for the bookshelf faulting model.