Intra-segment Geologic Variations along the Arc-Proximal Fonualei Spreading Center
Abstract
When backarc spreading centers (BASCs) are greater than ~150 km away from the arc volcanic front, their geologic, geophysical, and geochemical characteristics are similar to those at mid-ocean ridges (MORs). However, within ~150 km of the arc, slab-derived chemical heterogeneities (particularly water) alter the rheology and melting characteristics of the mantle wedge, controlling spreading center characteristics to an increasing degree as they approach the arc. Thus, the characteristics of arc-proximal BASCs provide insight into how chemical heterogeneities affect seafloor spreading, and how these effects vary both spatially and temporally. This study examines variations in axial morphology and volcanism at the second- and third-order scale along the arc-proximal Fonualei Spreading Center (FSC) in the northeast corner of the Lau backarc basin, primarily using hull-mounted multibeam bathymetry and backscatter data, along with magnetics and gravity. The northernmost portion of this system includes the southern branch of Mangatolu Triple Junction (MTJ-S), ~80-100 km from the Tofua arc volcanic front, and is characterized by a deep, flat, faulted axis. Across the step-over with the northern end of FSC, there is an abrupt change in axial morphology to a broad axial high, and as the axis further approaches the arc, it becomes shallower and higher relief, silica and water content in axial lavas increase, and finally at the southern end, ~20 km from the arc, the axis abruptly transitions to a large isolated conical seamount similar to an arc volcano with rift zones aligned with the spreading axis. Local topographic highs along the axis correlate with the projected locations of arc volcanoes, and others have suggested that much of the FSC axis is directly capturing the arc melt, causing the nearby arc volcanoes to become extinct. The FSC also provides an interesting comparison with the Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR) in the southern portion of the Lau basin. The ELSC and VFR constitute a ~400 km long first-order spreading segment twice the length of the FSC/MTJ-S system, but has both a similar range in arc proximity and a similar pattern in axial morphology correlated with the projected locations of arc volcanoes. The observations along both systems support a model of the mantle wedge with an abrupt transition between a more hydrous wedge corner and a less hydrous distal zone. Within the more hydrous wedge corner, topographic and geochemical correlations between the arc and axis suggest that there is a smaller scale of mantle wedge structure with more hydrous zones extending from the arc to the BASC, controlling both the spatial distribution of arc volcanoes and local along-axis topographic highs. Contrasts between these two spreading centers provide insight into the effects of subduction on back-arc spreading.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V21C2745S
- Keywords:
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- 3001 MARINE GEOLOGY AND GEOPHYSICS Back-arc basin processes;
- 3060 MARINE GEOLOGY AND GEOPHYSICS Subduction zone processes;
- 3045 MARINE GEOLOGY AND GEOPHYSICS Seafloor morphology;
- geology;
- and geophysics;
- 3075 MARINE GEOLOGY AND GEOPHYSICS Submarine tectonics and volcanism