Direct evidence from anisotropy of magnetic susceptibility for lateral melt migration at superfast spreading centers

Abstract

[1] Rare, fault-bounded escarpments expose natural cross sections of ocean crust in several areas and provide an unparalleled opportunity to study the end products of tectonic and magmatic processes that operated at depth beneath oceanic spreading centers. We mapped the geologic structure of ocean crust produced at the East Pacific Rise (EPR) and now exposed along steep cliffs of the Pito Deep Rift near the northern edge of the Easter microplate. The upper oceanic crust in this area is typified by basaltic lavas underlain by a sheeted dike complex comprising northeast striking, moderately to steeply southeast dipping dikes. Paleomagnetic remanence of oriented blocks of dikes collected with both Alvin and Jason II indicate clockwise rotation of $61°related to rotation of the microplate indicating structural coupling between the microplate and crust of the Nazca Plate to the north. The consistent southeast dip of dikes formed as the result of tilting at the EPR shortly after their injection. Anisotropy of magnetic susceptibility of dikes provides well-defined magmatic flow directions that are dominantly dike-parallel and shallowly plunging. Corrected to their original EPR orientation, magma flow is interpreted as near-horizontal and parallel to the ridge axis. These data provide the first direct evidence from sheeted dikes in ocean crust for along-axis magma transport. These results also suggest that lateral transport in dikes is important even at fast spreading ridges where a laterally continuous subaxial magma chamber is present.Components: 2965 words, 5 figures.

Paleomagnetic constraints on deformation of superfast-spread oceanic crust exposed at Pito Deep Rift

“…Both bathymetric and magnetic anomaly lineations Naar et al, 1991;Varga et al, 2008] reveal trends that deviate from EPR-parallel trends by 020°to 045°clockwise near the northeastern wall of Pito Deep Rift (Figure 2). Trends of abyssal hill lineations approach 055°to 065°near the dive areas, and trend ∼070°across the rift to the southwest, in the interior of the microplate.…”

“…Our interpretation of the rotational history that includes inward-tilting near the ridge axis would produce the opposite sense of anomalous skewness to that observed, a distinction that could be reconciled a couple of different ways as discussed later. Although a subset of the data was reported by Varga et al [2008], the full data set and thorough analysis is presented here.…”

Crustal faults exposed in the Pito Deep Rift: Conduits for hydrothermal fluids on the southeast Pacific Rise

“…The following briefly summarizes relationships that are described in much greater detail elsewhere [Karson et al, 2005;Chutas, 2007;Perk et al, 2007;Heft et al, 2008;Varga et al, 2008]. Ascending from >4000 m below sea level (bsl) to <3000 m bsl, transects encountered extensive bedrock exposures in steep cliffs, interspersed with more gently sloping areas covered by talus.…”

Tectonics: 50 years after the Revolution

“…Gabbros are present in some places. In the Pito Deep, in situ magnetic measurements indicate that the direction of intrusion of the gabbro was horizontal, parallel to the ridge direction (Varga et al, 2008).…”

“…Hawkins (2003) summarized the complexities of the marginal basins of the western Pacifi c and elsewhere. Paleomagnetic evidence shows that some magmas in oceanic crust have been intruded horizontally (Varga et al, 2008).…”