Heat flow variations from bottom simulating reflectors on the Cascadia margin

Abstract

Abstract A bottom simulating reflector (BSR), representing the base of the gas hydrate stability field, was observed over a closely spaced grid of seismic lines on the continental slope of the northern Cascadia margin. A simple conductive model was used to calculate heat flow from the depth of the BSR. A regional trend was observed, in which heat flow decreased landward across the margin from an average of ∼80 mW/m2 at a distance of 15 km from the deformation front to ∼65 mW/m2 at a distance of 40 km. This trend reflects the processes of tectonic thickening of accretionary wedge sediments and subduction of the Juan de Fuca plate, and contrasts with the near-constant heat flow across the Oregon margin where the dip of the subducting plate and thus the rate of sediment thickening are slightly smaller. Consistent local variations were also observed, notably low heat flow values over prominent topographic highs and high heat flow values over the flanks of the topographic highs. At some localities over a horizontal distance of 1–2 km, heat flow increased by as much as 50%, from typical values of 65 to 100 mW/m2. Much of this variation may be due to focusing and defocusing effects of the topography alone, and indicates the importance of carrying out topographic corrections to heat flow in regions of significant relief such as the continental slope. However, a component of the local heat flow patterns may be due to dynamic effects, including the displacement of isotherms by thrust faulting and the upward migration of warm fluids along permeability channels such as faults.