The Structure and Dynamics of an Active Arc-Backarc System: A Notional
Experiment
Douglas A. Wiens (Dept of Earth and Planetary Sci., Washington University, St. Louis,
MO, email: doug@seismo.wustl.edu)
An active arc-backarc system poses many interesting questions in geodynamics.
For example, one may expect a complex flow pattern in such a region, governed
variously by coupling to the downgoing slab, slab advance or rollback, tears in the slab,
and upwelling beneath the backarc spreading center. In addition, there may be a complex
interaction between the backarc and arc magmatic systems, as suggested by their
geochemistry and petrology. An arc-backarc system has many advantages over midocean ridge sites from an imaging perspective, due to the array of energetic, high
frequency sources provided by the seismicity in the downgoing slab.
I propose a notional experiment along the Eastern Lau Spreading Center (ELSC)
in the Lau Basin. The ELSC and its southern extension, the Valu Fa ridge, show strong
along-strike variations in geochemical slab signatures, indicating a complex interplay
with a slab fluid source and the arc magmatic system. In addition, the ELSC shows
extreme variations in ridge elevation and morphology, suggesting a variable magma
supply. We propose to image this system with approximately 50 ocean bottom
seismographs, deployed in a larger sparse array and two denser 3-D arrays along very
different segments of the ELSC. This array would be used to investigate the mantle
structure of the backarc spreading center, and its variation as a function of ridge
spreading rate and morphology. The array would span the Tonga magmatic arc, and thus
allow imaging of the spatial relationship and interconnection between the backarc and
magmatic arc along strike. In addition, the magmatic arc would be imaged in detail,
providing important constraints on the depth of arc magma genesis and its spatial
relationship to the slab. Finally, the array would allow detailed imaging of the
downgoing oceanic crust and identification of its velocity structure as a function of depth
for this extremely rapid subduction system. The seismic velocity of the downgoing crust
can be related to phase transformations, which may be important in liberating fluids
important in magmatism, as well as intermediate depth earthquakes.
The South Sandwich arc also provides a fast backarc spreading system above a
seismically active slab, but suffers from more difficult logistics. Geochemical
observations suggest that South Atlantic mantle material is flowing around the South
Sandwich slab and interacting with the Pacific mantle along the Sandwich backarc
spreading center. A more limited experiment along the South Sandwich spreading center
would allow mapping of this flow pattern using seismic anisotropy and comparison of
geochemical and seismological flow indicators.
Experiments at active arc-backarc systems are an important component of Ocean
Mantle dynamics and allow interaction with focus sites for the MARGINS (Mariana Is.)
and RIDGE (Lau basin) programs.