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Understanding the thermal structure of the Cascadia subduction zone is important for understanding megathrust earthquake processes and seismogenic potential. Currently our understanding of the thermal structure of Cascadia is limited by a lack of high spatial resolution heat flow data and by poor understanding of thermal processes such as hydrothermal fluid circulation in the subducting basement, sediment thickening and dewatering, and frictional heat generation on the plate boundary. Here, using a data set of publically available seismic lines combined with new interpretations of bottom simulating reflector (BSR) distributions, we derive heat flow estimates across the Cascadia margin. Thermal models that account for hydrothermal circulation predict BSR-derived heat flow bounds better than purely conductive models, but still over-predict surface heat flows. We show that when the thermal effects of in-situ sedimentation and of sediment thickening and dewatering due to accretion are included, models with hydrothermal circulation become consistent with our BSR-derived heat flow bounds. [ABSTRACT FROM AUTHOR]

We investigate the depth distribution of smectite clay transformation and its along-strike variability at the Middle America Trench offshore Costa Rica. We take advantage of recent well-constrained thermal models that refine our understanding of the margin's thermal structure and which capture significant along-strike variability. Using these thermal models, together with sediment compositions defined by drilling, we compute the distribution of smectite transformation and associated fluid production. We show that the hypocenters of large ( M > 6.9) well-located megathrust earthquakes lie consistently downdip of peak fluid production. We suggest that silica cementation associated with smectite transformation promotes lithification and slip-weakening behavior that, in combination with declining fluid pressures, facilitate the initiation of unstable slip. The earthquake ruptures extend updip into the region of peak reaction, possibly due to excess pore pressures that facilitate their propagation. These results are consistent with the hypothesis that smectite transformation contributes to the onset of stick-slip behavior and acts as an important control on earthquake nucleation and propagation. [ABSTRACT FROM AUTHOR]

Eleven recently collected multichannel seismic (MCS) profiles from the Cascadia Open-Access Seismic Transects experiment offshore Washington State are used to characterize the distribution of bottom-simulating reflectors (BSRs) from seaward of the deformation front onto the continental shelf of the Cascadia Subduction Zone. The 11 MCS lines consisted of nine lines perpendicular and two lines parallel to the Cascadia margin covering a 100 km along-strike region of the accretionary wedge. From these MCS profiles we generated a 3-D view of the Cascadia margin thermal structure by interpreting 40,232 individual BSR picks in terms of temperature and heat flow. Overall BSR-derived heat flow values decrease from approximately 95 mW m−2 10 km east of the deformation front to approximately 60 mW m−2 located 60 km landward of the deformation front. Anomalously low heat flow values near 25 mW m−2 on a prominent midmargin terrace indicate recent sediment failure within the accretionary prism. Localized differences between BSR heat flow and numerical models reflect an estimated regional mean vertical fluid flow of +0.53 cm yr−1 for the survey area, with localized fluid flow approaching a maximum of +3.8 cm yr−1. Distinct finite element models for the nine MCS profiles perpendicular to the deformation front reproduce BSR heat flow values, producing an overall root-mean-square misfit of 10.2 mW m−2. At the deformation front, the incoming oceanic sediment/crust interface temperatures vary from 164°C to 179°C, indicating the updip limit of the Cascadia seismogenic zone. [ABSTRACT FROM AUTHOR]

We investigate differences in the thermal structure of the subduction thrust between accretionary and erosive margins using a finite element model. Global averages of plate margin geometries, sediment thickness, plate age, and convergence rate are used to construct generic models of accretionary and erosive margins. Of these parameters, our analysis shows that the largest uncertainty in these models is the geotherm for the incoming oceanic plate. Despite these uncertainties, the subduction thrust of similarly aged accretionary margins is slightly warmer than erosive margins primarily due to the effect of sediment insulating the subduction thrust at accretionary margins. If the updip limit of seismicity is thermally controlled, warmer accretionary margins suggest shallower seismogenic updip limits, counter to the observation that erosional margins are more likely to generate tsunamigenic earthquakes. This discrepancy can be reconciled if frictional heat generation at erosive margins is larger than at accretionary margins. [ABSTRACT FROM AUTHOR]