Linear ridge groups: Evidence for tensional cracking in the Pacific Plate

Abstract

Abstract. A new class of oceanic bathymetric i•ature, discovered in 1987, consists of en-echelon groups of linear ridges. There are now at least three such sets of ridges known, and their remarkable similarity supports the notion that they result from a widespread tectonic process taking place in the interior of plates. In two of these major linear ridge groups, the Crossgrain and Puka Puka ridges, numerous morphologic features indicate that they originated as tension cracks. The form of the individual volcanic structures that make up the ridges correlates with the degree of tension that formed them, and the similar orientation of all ridges in a group indicates that the stress that formed them is relatively widespread geographically and temporally. The ridges show a characteristic sequence of developxnent, beginning with a swath of small volcanoes followed by larger domical volcanoes where the ridge will eventually develop. This paper examines proposed mechanisms of formation and concludes that no single source of tension in the lithosphere could plausibly have caused both the Crossgrain and Puka Puka ridges. The similarity of the orientations of all the groups of en-echelon linear ridges on the Pacific plate suggests that the individual ridges form normal to a least compressive stress direction that is geographically variable in the crust. Parallel ridges constrained to a narrow band then result in an en-echelon arrangement, though some closely spaced, synchronously formed sets of two or three ridge segments may have influenced each other's form.

Fissure control on volcanic action in the Pacific

“…1), which are perpendicular to the East Pacific Rise in the south Pacific (EPR in Fig. 1), their easterly age progression, which is faster than that of the Hawaiian chain (Sandwell et al, 1995), and their relationship to adjacent gravity lineaments observed from satellites (Sandwell et al, 1995;Lynch, 1999;Sandwell and Fialko, 2004).…”

“…The westnorthwest cross-trend ridges date from ca. 70 Ma (Davis et al, 2002), and are parallel to the much younger, clearly nonplume Pukapuka ridges (Sandwell et al, 1995;Lynch, 1999, Janney et al, 2000 and to the current direction of Pacific plate motion. Samples Davis et al (2002).…”

“…However, although Dana recognized the age progressions along the Hawaiian, Society, and Samoan Islands, he knew nothing about sliding lithospheric plates. Nevertheless, modern ideas about the control of lithospheric contraction on seamount distribution (e.g., Lynch, 1999;Sandwell and Fialko, 2004) are very similar to Dana's. In addition, the spreading ridges change orientation and geometry in response to consumption of ridge axes at trenches by migration of triple junctions along continental margins or by cessation of spreading near trenches.…”

On the shallow origin of hotspots and the westward drift of the lithosphere

“…A few authors (Smith, 1993;Smith and Lewis, 1999;Anderson, 2000;Janney et al, 2000;Green et al, 2001), argued against the deep mantle plume paradigm (e.g., Schubert et al, 2001). Some authors also propose crack propagation in the lithosphere along hotspot tracks as the reason for Pacific intraplate magmatism (Winterer and Sandwell, 1987;Sandwell et al, 1995;Lynch, 1999). Moreover, some seismic tomography results do not show lowered velocity beneath the Hawaiian hotspot (Wolfe et al, 2002;Foulger et al, 2003).…”

“…This evidence casts doubt on both the notion of absolute plate motions computed in the hotspot reference frame, and the nature of the magmatism itself. Different models have been proposed on the origin of these magmatic overproductions, such as deep plume, or alternatively, shallow plumes generated by cracks, or boudins of the lithosphere (Winterer and Sandwell, 1987;Sandwell et al, 1995;Lynch, 1999;Natland and Winterer, 2003). Compositional heterogeneity in the mantle has also been invoked, particularly because no significant positive thermal anomaly relative to normal mid-ocean ridge has been observed (e.g., Stein, 1992, 1993).…”