Metamorphic Reaction Controls on ETS via Tomographic Imaging

Abstract:

Observations of tremor and LFEs in Cascadia have shown that ETS occur at depths of 20-40 km along the plate interface and are facilitated by near-lithostatic pore pressures. However, the structural controls responsible for the elevated pore pressures and the restricted depth range of slow slip remain debated. Our objective is to clarify the relationship of ETS with structure and metamorphism using tomographic imaging. ETS may be controlled or influenced by three metamorphic reactions, each with a distinct seismic signature: serpentinization of peridotite within the mantle wedge; eclogitization of oceanic crustal metabasalt/gabbro; and silica precipitation within the overriding plate from fluids released at greater depths. Audet et al. (2009) have proposed that the large solid-phase volume decrease accompanying eclogitization is important in rupturing the plate boundary seal, facilitating rise of fluids into the wedge corner, and so limiting the downdip limit of overpressure conducive to ETS. Audet and Burgmann (2014) provide evidence that silica enrichment in the crustal forearc modulates ETS recurrence intervals. Hyndman et al. (2015) further suggest that rising silica-rich fluids are focussed by an impermeable, serpentinized mantle wedge toward and updip of the wedge corner. This latter interpretation does not consider evidence that the plate boundary up-dip of the corner behaves as a seal and acknowledges uncertainty in the precise location of the mantle wedge corner in northern Cascadia. Hence there is a need to improve resolution of structure in the mantle wedge region. Our LFE catalogue in southern Vancouver Island comprises 130 template locations within the forearc that are spatially complementary to regular earthquake hypocenters and sampling of the SHIPS refraction experiment. In addition, because LFE templates are generated using waveform data spanning more than a decade, each template includes waveforms for a complete ~60 station complement. Thus LFE templates afford an important new traveltime data set to be exploited in tomographic studies of forearc velocity structure. We have compiled traveltime data from previous tomographic studies along with LFE picks to image the mantle wedge region below SVI at high resolution and address the influence of metamorphic reactions on tremor occurrence.