An inverted continental Moho and An inverted continental Moho and serpentinization of the forearc serpentinization of the forearc

mantlemantle

M.G. Bostock, R.D. Hyndman, S. Rondenay, and S.M. PeacockM.G. Bostock, R.D. Hyndman, S. Rondenay, and S.M. Peacock

Presented by: Lauren MattatallPresented by: Lauren Mattatall

March 26, 2007March 26, 2007

► To explore structure of the mantle wedge To explore structure of the mantle wedge in the southern Cascadia subduction zone in the southern Cascadia subduction zone (CSZ) (CSZ)

► To understand the observed variations in To understand the observed variations in velocity structure in the forearc regionvelocity structure in the forearc region

► Relief map of the Relief map of the CSZ CSZ White squares White squares

represent represent broadband stations broadband stations

► Collected data Collected data from a dense from a dense broadband array of broadband array of 69 stations across 69 stations across Central OregonCentral Oregon high-quality P-wave high-quality P-wave

seismograms from seismograms from 31 events31 events

Bostock et al., 2002Bostock et al., 2002

1) Applied formal, multichannel inversions of:Applied formal, multichannel inversions of:-forward scattered P to S wave conversions -forward scattered P to S wave conversions -back-scattered waves caused by free surface -back-scattered waves caused by free surface

reflectionsreflections

2) Plotted perturbations in velocity structure2) Plotted perturbations in velocity structure

Bostock et al., 2002Bostock et al., 2002

Bostock et al., 2002Bostock et al., 2002

3) Plotted a thermal 3) Plotted a thermal model for Central model for Central OregonOregon-based on teleseismic -based on teleseismic profile, heat flow, and profile, heat flow, and other geophysical dataother geophysical data

► Note that the Note that the temperatures in the temperatures in the forearc are 400forearc are 400°°-600-600° ° (200°-400° colder than arc (200°-400° colder than arc temperatures)temperatures)

► Sharp increase in dip angle Sharp increase in dip angle (from ~10(from ~10°° to ~30 to ~30°°) ) argues for eclogitization of argues for eclogitization of the slabthe slab

► Eclogitization implies Eclogitization implies hydration and hydration and serpentinization of the serpentinization of the forearcforearc

► Thermal model Thermal model temperatures are within temperatures are within the range that serpentine the range that serpentine minerals are thought to be minerals are thought to be stablestable

► With a high enough % With a high enough % composition, composition, serpentinization of the serpentinization of the forearc would significantly forearc would significantly reduce S-wave velocities reduce S-wave velocities

Bostock et al., 2002Bostock et al., 2002

► As presence of As presence of serpentinite increases, serpentinite increases, S-wave velocities S-wave velocities decreasedecrease

► Observed S-wave Observed S-wave velocities in the forearc velocities in the forearc indicate a possible indicate a possible composition of composition of serpentinite as high as serpentinite as high as 50-60% 50-60%

Bostock et al., 2002Bostock et al., 2002

Bostock et al., 2002Bostock et al., 2002

► Serpentine and it’s alteration products are though Serpentine and it’s alteration products are though to exhibit stable sliding properties and will impede to exhibit stable sliding properties and will impede rupture into the forearc mantlerupture into the forearc mantle

► Mantle flow into the wedge may be modified by the Mantle flow into the wedge may be modified by the presence of the serpentinized forearc mantlepresence of the serpentinized forearc mantle The weak rheology and positive buoyancy will The weak rheology and positive buoyancy will

cause isolation from the mantle-wedge corner cause isolation from the mantle-wedge corner flow systemflow system

► To what depth are the perturbation plots reliable?To what depth are the perturbation plots reliable?

► Are there other possible explanations for such a Are there other possible explanations for such a decreased velocity within the forearc?decreased velocity within the forearc?