Subduction of the Juan de Fuca Plate: Thermal consequences

Abstract

Terrestrial heat flux was measured in fjords, in boreholes, and in offshore wells at sites across the convergent margin of southwestern British Columbia from the continental shelf landward to the Garibaldi Volcanic Belt. Temperatures in the offshore wells were corrected for drilling disturbances, and formation thermal conductivities were modeled using measurements on cuttings and downhole geophysical logs. Marine measurements in the fjords were corrected for the large effects of refraction as well as aperiodic temperature variations in the bottom waters. There was excellent agreement between marine measurements and those from nearby onshore boreholes. The heat flux above the subducting Juan de Fuca plate steadily decreases landward from over 50 mW m−2 on the shelf to 25 mW m−2 seaward of the Garibaldi Volcanic Belt. An abrupt increase to 80 mW m−2 over a distance of 20 km is centered 30 km seaward of the volcanic zone. Very large variations in heat flux occur locally within the Pleistocene volcanic area, the result of advective cooling of intrusive magmas. The measured heat generation of crustal samples along the entire profile is low, 0.6–0.8 μW m−3. A landward dipping, seismically reflective zone above the subducting oceanic plate beneath Vancouver Island appears to be nearly profile is low, 0.6–0.8 μW m−3. A landward dipping, seismically reflective zone above the subducting oceanic plate beneath Vancouver Island appears to be nearly isothermal. It is postulated that dehydration of the subducting oceanic crust at and above approximately 450°C absorbs heat and produces water which flows updip along this zone in the overlying subduction complex, effectively redistributing the heat seaward to where the water is reabsorbed in hydration processes. A relatively cool crustal wedge lies above the deeper subducting oceanic crust, and at its thick, landward side the abrupt increase in surficial heat flux must be caused by a shallow (10 km depth) heat source produced by ascending magma.

Subduction zone forearc serpentinites as incubators for deep microbial life

“…5C). These depth estimates are based on surface heat flow values of 0.03-0.04 W·m −2 that agree with the observed depression of isotherms in most forearc mantle wedges, even those of relatively hot origin such as the Cascadia subduction zone (38,39). Moreover, our thermal calculations are in agreement with more complex geodynamic models (40,41), confirming that the 122°C isotherm is reached at ∼10,000 mbsf in forearcs.…”

Transient thermal regimes in the Sierra Nevada and Baja California extinct outer arcs following the cessation of Farallon subduction

“…These shear zones correlate with the location of the extinct Farallon magmatic arc during the subduction period, where sharp changes in the heat flow regimes were observed in the present counterparts [Ziagos et al, 1985;Lewis et al, 1988;Blackwell et al, 1990aBlackwell et al, , 1990b. Formation of shear zones along the volcanic/magmatic arcs of subduction zones can be expected even in active subduction zones if the convergence of the plates is oblique.…”

“…[4] Convergent plate margins are characterized by low heat flow in their outer arc and high heat flow in their back-arc regions (see, e.g., Honda [1985] for Japan; Lewis et al [1988] for northern Cascadia; Blackwell [1971] and Blackwell et al [1990aBlackwell et al [ , 1990b for southern Cascadia; Ziagos et al [1985] for northern Cocos arcs; and Currie and Hyndman [2006], in general). The cold outer arc block of a subduction zone is surrounded by two dynamically sustained thermal-mechanical boundaries.…”

Tomographic image of low P velocity anomalies above slab in northern Cascadia subduction zone

“…In this area, heat flow increases by a factor of two over a distance of 20 km. Such a rapid heat flow transition requires heat sources within the upper crust (Lewis et al, 1988). In addition to the large reflective bands beneath Vancouver Island, smaller but very "bright" reflectors have been observed at shallower depths beneath the mainland (Cook et al, 1988).…”