Mantle structure and flow patterns beneath active back-arc basins inferred from passive seismic and electromagnetic methods

Passive seismic and electromagnetic (EM) imaging methods can provide strong constraints on mantle processes associated with active back-arc spreading systems. Seismic velocity and attenuation structures are strongly sensitive to temperature, melt content, melt-pore geometry, and water content. In contrast, EM imaging is sensitive to the presence of fluids and melt when there is high connectivity and is relatively insensitive to temperature. Both seismic and EM methods can detect anisotropy, which may be caused by lattice-preferred orientation (LPO) or oriented melt or fluid pockets. EM studies of the Mariana Trough show a low-conductivity region in the shallow mantle at depths of less than 70 to 150 km, indicating dry conditions resulting from basaltic melt extraction in the uppermost mantle. Regional seismic waveform studies of four active backarc basins suggest that the Lau back arc is characterized by the slowest upper mantle velocity, and the Mariana Trough shows the fastest velocity, with primary differences occurring between 40 and 100 km depth. These findings are consistent with petrological evidence that suggests higher mantle temperatures for the Lau system. Seismic tomographic images show large low-velocity and high-attenuation anomalies beneath the Lau Basin, indicating that a broad zone of magma production feeds the back-arc spreading center. Slow velocities extend to at least 250 km depth, and these deep anomalies may result from the release of volatiles transported to depth by hydrous minerals in the slab. Shear wave splitting data from the Lau Basin show southward along-strike mantle flow, in agreement with geochemical data. The existence of along-arc flow patterns in many subduction zones suggests that viscous coupling does not exert a strong control on the mantle flow pattern in back-arc regions.