Structural geomorphology of the eltanin fault system and adjacent transform faults of the Pacific-Antarctic plate boundary

Abstract

Seabeam and SeaMARC II swath-mapping surveys, with ancillary magnetic and gravity profiling, describe the fast-slipping (84–86 mm year⁻¹) 380 km-long Heezen transform (56° S) and the 145 km-long Raitt transform (54° S), together with the youngest parts of their rise-flank fracture zones. Archived seismicity, satellite altimetry, and older geophysical traverses extend these descriptions, constrain the structural interpretations, and allow preliminary interpretations of the adjacent Tharp, Hollister, and Udintsev transforms. At Heezen transform, Pacific-Antarctic plate motion is partitioned between the principal strike-slip fault zone in a deep transform valley and a marginal zone of rifting 30–40 km north of the transform axis, where a zone of secondary Riedel shearing evolved into a belt of crustal extension following a Pliocene change in relative plate motion. Crustal extension and lithospheric rupture along this belt has opened rift valleys, allowed the eruption of high volcanic ridges, and suppressed uplift of a transverse ridge along the north side of the transform valley. The south side has a high transverse ridge that is probably a flexural response to the mass deficiency of the valley; it subsides and vanishes along the eastern part of the valley, which has been infilled with recent volcanism. At the eastern risecrest intersection is another uplift of old lithosphere, an intersection high raised by transfer of heat from a curved and transform-parallel overshot ridge that prolongs the axial ridge of the East Pacific Rise (EPR). Tharp transform appears to be a mirror-image of Heezen transform, but with less evidence of volcanism at the marginal rifting site. Raitt transform responded differently to the Pliocene change in plate motion: a single strike-slip zone was replaced with anen echelon pair of newly oriented faults, connected by a 10 km-long mid-Raitt spreading axis which has accreted rough, obliquely lineated crust. Transverse ridges have been raised along both sides of the transform, probably in response to the mass deficiency of the strip of mid-Raitt crust and to heating at the mid-Raitt axis. The intersections of Raitt transform with the EPR crest lack long overshot ridges, but periodically have tall, narrow intersection highs probably raised mainly by intrusion across the transform into old lithosphere. Udintsev transform adjusted to the change in slip direction by segmenting like Raitt transform, but the mid-Udintsev spreading axis grew within a widened transtensional transform valley bordered on both sides by high transverse ridges. Volcanism at the intersections with the rifted crest of the Pacific-Antarctic Ridge does not infill and close off the transform valley, so the Udintsev transverse ridges extend past the intersections to become part of the rise-flank fracture zones. At faster separating parts of the Pacific-Antarctic boundary, and on most of the rest of the EPR, fracture zone structure is mainly inherited from the variable arrangement of volcanic ridges and tectonic uplifts at the risecrest intersections, rather than from structures formed at the transform valley.