Heat flow in the Oregon Cascade Range and its correlation with regional gravity, Curie point depths, and geology

Abstract

New heat flow data for the Oregon Cascade Range are presented and discussed. Heat flow measurements from several deep wells (up to 2500 m deep), as well as extensive new data from industry exploration efforts in the Breitenbush and the Santiam Pass‐Belknap/Foley areas are described. The regional heat flow pattern is similar to that discussed previously. The heat flow is about 100 mW m−2 in the High Cascade Range and at the eastern edge of the Western Cascade Range, It is about 40–50 mW m−2 to the west in the outer arc block of the subduction zone. In the high heat flow zone the heat flow is low at shallow depths in young volcanic rocks due to the high permeability of the rocks and the resultant rapid groundwater flow. Below a depth of 200–400 m much of the area appears to be dominated by conductive heat transfer at least to 2–2.5 km depth. There are perturbations to the regional heat flow in the vicinity of the hot springs where values are up to twice the background. The gravity field in the Cascade Range has characteristics that can be closely related to the heat flow pattern. The relationship may be causal, and to examine the relationship in more detail, earlier two‐dimensional modeling is extended to three dimensions. Consideration of the effects of a midcrustal density anomaly, such as might be associated with a region with at least areas of partial melt, has two major consequences. The first of these is that a high‐frequency gravity gradient near the Western Cascade Range/High Cascade Range boundary is explained. Second, the negative gravity anomaly associated with the north half of the High Cascade Range can be removed, and as a result, the prominent northeast/southwest striking regional Bouguer gravity anomaly associated with the north edge of the Blue Mountains becomes continuous across the Cascade Range with a similar feature along the north side of the Klamath Mountains. Apparently, this zone is a major crustal feature upon which the negative gravity anomaly coincident with the high heat flow is superimposed. The correlation, or lack thereof, of the heat flow, depth to Curie point, gravity field, crustal electrical resistivity, crustal seismic velocity, and geology in the High/Western Cascade Ranges is summarized. Many of the data show aspects that can be interpreted in relation to possible high temperatures in the midcrust of the Cascade Range. The High Cascade Range midcrust has unusually high temperatures and contains a zone of magma staging at 10±2 km depth that can also be identified in subdued form in the Cascade Range in Washington and British Columbia.

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

“…He also discussed possible magnitudes of the shear stress, and concluded that the effect of shear heating must have been low in California outer arc. This result agrees with the results of thermal studies on the present subduction zones [Ziagos et al, 1985;Blackwell et al, 1990aBlackwell et al, , 1990bHyndman and Wang, 1993;Wang et al, 1995;McKenna, 2002] that effect of frictional heating at the slab tops are not significant. Thus, the California outer arc seems to have had a ''typical'' subduction zone thermal setting from the Laramide period up to the cessation of subduction [Dumitru, 1991].…”

“…[34] With heat flow values of 30 ± 3 mW m À2 (Figure 2a), and low paleogeothermal gradients (5 -15°C km À1 [Dumitru, 1991]), the California outer arc must have had lower heat flow values than Cascadia outer arc (40 ± 2 mW m À2 [Lewis Blackwell et al, 1990aBlackwell et al, , 1990b); even though they are genetically similar. Part of the high heat flow in the Cascadia outer arc has been explained by the thermal blanketing effect of a thick sedimentary cover in front of the outer arc [Blackwell et al, 1990b;Hyndman and Wang, 1993;McKenna, 2002].…”

“…Because the outer arc block is cold, it is internally coherent and heat transfer is dominated by conductive boundary processes. Many heat flow studies indicate that across magmatic arcs the transition from very low to very high heat flow occurs in short distances (<20 km) implying a sharp thermal transition within the crust [e.g., Blackwell et al, 1990aBlackwell et al, , 1990bZiagos et al, 1985;Honda, 1985]. With the cessation of subduction, this dynamically driven thermal equilibrium can no longer be supported and the now extinct outer arc block can potentially be heated up both from the bottom and from the back-arc side.…”

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.…”

Comment [on “Rates and patterns of groundwater flow in the Cascade Range volcanic arc and the effect on subsurface temperatures” by S. E. Ingebritsen, D. R. Sherrod, and R. H. Mariner]

“…Indeed, new data recently described by Ingebritsen et al [ 1993] further support the thermal regime in the Cascades described by Blackwell et al [1982, 1990a]. Fluid flow certainly plays a part in the heat transport, as described in detail by Blackwell et al [ 1990a]. However, the high heat flow west of 122øW cannot be explained except by conductive processes or (less likely) by very deep convection (at high temperature) below the level of any of the drilling so far.…”