Hydrothermal Circulation and the Thermal Structure of Shallow Subduction Zones GEOSPHERE, V

Research Paper THEMED ISSUE: Subduction Top to Bottom 2

GEOSPHERE Hydrothermal circulation and the thermal structure of shallow subduction zones GEOSPHERE, v. 13, no. 5 Robert N. Harris1, Glenn A. Spinelli2, and Andrew T. Fisher3 1College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Administration Building, Corvallis, Oregon 97330, USA doi:10.1130/GES01498.1 2Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA 3Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Earth and Planetary Sciences, Room A232, Santa Cruz, California 95064, USA 9 figures ABSTRACT and Sclater, 1977). In detail however, hydrothermal circulation and other envi- CORRESPONDENCE: [email protected] ronmental processes such as sedimentation can modify the thermal state of Hydrothermal circulation within oceanic basement can have a profound the incoming igneous crust sufficiently so as to have a significant impact on CITATION: Harris, R.N., Spinelli, G.A., and Fisher, influence on temperatures in the upper crust, including those close to the sub- plate boundary temperatures. Therefore, understanding the nature and impact A.T., 2017, Hydrothermal circulation and the thermal structure of shallow subduction zones: Geosphere, duction thrust and in the overlying plate. Heat flow evidence for hydrothermal of hydrothermal circulation on the incoming lithosphere temperature struc- v. 13, no. 5, p. 1425–1444, doi:10.1130/GES01498.1. circulation in the volcanic basement of incoming plates includes: (1) values ture is a prerequisite for accurate thermal models of the shallow subduction that are well below conductive predictions due to the advection of heat into zone. In thermal models, a portion of the effect of hydrothermal circulation can Received 18 January 2017 the ocean, and (2) variability about conductive predictions that cannot be ex- be represented by modifying the geothermal gradient (geotherm) within the Revision received 8 May 2017 Accepted 24 July 2017 plained by variations in seafloor relief or thermal conductivity. In this review incoming plate, which serves as either the seaward boundary condition for Published online 1 September 2017 we summarize evidence for hydrothermal circulation in subducting oceanic steady-state models or the initial condition for transient models. basement from the Nankai, Costa Rica, south-central Chile, Haida Gwaii, and In this review we focus on the influence hydrothermal circulation within Cascadia margins and explore its influence on plate boundary temperatures. incoming and subducting oceanic basement has on thermal conditions of the Models of these systems using a high Nusselt number proxy for hydrother- shallow subduction zone. Many of the parameters important to hydrothermal mal circulation are used to illustrate the influence of this process on seafloor circulation during subduction are also of interest in studies of ridge-flank hy- observations and thermal conditions at depth. We show that at these sub- drothermal systems more generally, including crustal permeability, basement duction zones, patterns of seafloor heat flow are best explained by thermal relief, and sediment thickness and continuity (e.g., Davis et al., 1989, 1999; models that include the influence of hydrothermal circulation. Fisher et al., 2003a; Hutnak et al., 2007). It is worth noting that the subduction zones receiving much attention over the past two decades have tended to be subducting young igneous basement (e.g., Nankai, Costa Rica, central Chile, INTRODUCTION Cascadia), in part because warm and buoyant conditions make these systems logistically easier to characterize and sample through ship-based studies and A significant emphasis over the past two decades of subduction zone research drilling. Similar to ridge flanks, youthfulness of subducting oceanic basement has been on understanding transitions in frictional sliding behavior along the facilitates hydrothermal circulation because of high permeability, large ther- plate boundary (e.g., Hyndman, 2007; Lay et al., 2012). Temperature along plate mal gradients, and thin or incomplete sediment cover. boundary faults has emerged as an important parameter because the thermal We start with a general discussion of the shallow thermal regime during state of this interface influences its rheology, including the updip and downdip subduction, and how this thermal regime can be perturbed by hydrothermal limits of seismicity, which can vary as a result of thermally driven rock-fluid reac- circulation in the upper oceanic basement. We present and discuss examples tions and differences in friction (Hyndman and Wang, 1993; Oleskevich et al., 1999; of thermal modeling studies from several convergent margins that illustrate Moore and Saffer, 2001). Measuring temperature at the plate interface, especially key processes. We summarize these results, and review how thermal models at depths of the seismogenic zone, is impractical in most settings because of the of subduction can incorporate characteristics of hydrothermal circulation to time requirements, expense, and technical difficulty of drilling deep enough to assess where this process occurs. reach this part of an active subduction zone. Instead temperatures at depth are estimated through a combination of surface heat flow measurements and modeling. A primary influence on subduction zone temperature is the thermal state of THERMAL MODELS OF THE SHALLOW SUBDUCTION ZONE the incoming plate (e.g., Dumitru, 1991; Molnar and England, 1995). In broad For permission to copy, contact Copyright terms the thermal state of the incoming lithosphere is a function of age that is Figure 1 shows a generic thermal model and the important parameters in- Permissions, GSA, or [email protected]. largely controlled by conductive cooling (e.g., Davis and Lister, 1974; Parsons fluencing surface heat flow and temperature along the subduction thrust. We

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0 Shear heating

Thermal structure of incoming-50 plate (age, hydrothermal circ.) Mantle wedge flow field

100

Convergence rate

Depth (km) 150

Slab geometry 200 Figure 1. Subduction zone thermal model. (A) Generic thermal model of a subduction zone showing important param- 0 eters for thermal modeling (circ.—circulation). Although not depicted, thermal physical rock properties are also import- A 20 B ant. For the thermal regime of the shallow subduction zone 250 and plate boundary fault, the least-well-known parameters 40 are the seaward geotherm and magnitude of shear heating 250 on the plate interface. (B) Uncertainties in seaward geotherms based on GDH1 model (Stein and Stein, 1992) for 60 C 10 Ma (red) and 100 Ma (blue) oceanic crust. Lines show Depth (km) ) 200 mean and shaded areas show one standard deviation. (C) -2 80 Heat flow across forearc for 10 (red) and 100 Ma (blue) crust. Shaded area shows uncertainty due to uncertainty in geo- 150 100 therm. Magenta and cyan lines show modeled heat flow resulting from insulated hydrothermal circulation in subduct- 120 0500 1000 1500 2000 ing oceanic crust. (D) Temperature along the plate interface 100 Temperature (°C) for 10 (red) and 100 Ma (blue) crust. Shaded area shows uncertainty due to uncertainty in geotherm. Magenta and cyan lines show modeled heat flow resulting from insulated 50 hydrothermal circulation in subducting oceanic crust. Heat Flow (mW m 0

800 ) D 600

400

200 Temperature (°C

0 020406080100 120140 160180 Distance from deformation front (km)

use the algorithm of Wang et al. (1995) that solves the heat transfer equation often has a low value (μ′ < 0.2) compared to a higher value (μ′ ~ 0.6) inferred assuming steady-state conditions, as applied to a two-dimensional trench nor- from early laboratory experiments (Byerlee, 1978; Di Toro et al., 2004; Fulton et mal profile at a subduction zone, al., 2013; Gao and Wang, 2014; Wang and He, 2008). Numerical models show that frictional heating has a large effect on fault zone temperatures but may ()T – cv T + A = 0, (1) have a small effect on seafloor heat flow, particularly in the presence of hydrothermal circulation that can redistribute or extract excess heat (e.g., Rotman where λ is thermal conductivity, T is temperature, ρc is volumetric heat capac- and Spinelli, 2014; Hass and Harris, 2016). As a result, observations of seafloor ity, v is the convergence rate of the subducting plate, and A is heat production. heat flow can provide weak constraints on frictional processes at depth. Mantle corner flow is incorporated by assigning an isoviscous rheology to the mantle wedge and coupling it to the subducting slab (Peacock and Wang, 1999). The kinematic parameters, the convergence rate and slab dip, control how THERMAL REGIME OF THE OCEANIC CRUST AND LITHOSPHERE quickly cold crust and mantle material are advected downward. The convergence rate is usually well determined from geodetic measurements or models Conductive Models of plate motion, and the dip of the subduction thrust is usually well known from active and/or passive seismic experiments. The thermal field is influ- Seafloor bathymetry and heat flow furnish the primary evidence for the enced by thermophysical rock properties, the most important of which are thermal state of oceanic lithosphere. Under conditions of isostatic equilibrium, the thermal conductivity and heat production. These properties can be deter- large-scale bathymetry provides an estimate of the heat content of the plate mined by dredging or drilling to collect representative material, or estimated and an integrated temperature structure through the lithosphere. These data from catalogues of measurements coupled with knowledge of site lithology have been used to develop plate-scale cooling models of the lithosphere (e.g., and structure. Our emphasis is on shallow subduction thrusts, an area that is Davis and Lister, 1974; Parsons and Sclater, 1977; Stein and Stein, 1992; Hast- largely unaffected by the mantle wedge flow field, a process that mainly influ- erok, 2013). Heat flow data are highly sensitive to the shallow thermal gradient ences thermal conditions landward of sites considered in the present review and can be used to aid in the estimation of lithospheric physical parameters, (e.g., Currie et al., 2004; Wada and Wang, 2009). especially in old seafloor where heat transfer is thought to be dominated by Less well determined parameters in these systems are the geotherm at conduction. However, heat flow data can present challenges for determination the seaward edge of the model, and the effective coefficient of friction that of lithospheric physical and cooling parameters, particularly for younger base- controls the magnitude of shear heating across the main thrust and along sec- ment, because local and shallow processes, including environmental distur- ondary fault systems. In the absence of heat flow data from the incoming plate, bances associated with sedimentation and mass wasting, changes in bottom the geotherm is often assumed to follow a conductive lithospheric reference water temperatures, and hydrothermal circulation in the underlying basaltic model such as GDH1 (Stein and Stein, 1992). The geotherm governs the mag- basement can perturb the data. nitude of the incoming heat flow and the heat content of the plate; this heat is Figure 2 summarizes marine bathymetric and heat flow data used to con- advected downdip with the subducting plate and governs the thermal budget struct a cooling-plate reference model for the lithosphere, GDH1 (Stein and of the shallow subduction zone. As shown by case examples herein, uncertain- Stein, 1992). These data are from the North Pacific and northwest Atlantic, and ties in the geotherm, including property variations in and out of the plane of are generally considered to be broadly representative of the global data set. two-dimensional simulations, can lead to significant uncertainties in the state Other analyses have led to somewhat different fitting parameters, but follow of the shallow thermal regime. similar trends. Superimposed on the data are cooling plate model predic-

Shear heating, Qf, along the plate interface is a function of the effective tions and their uncertainty based on the standard deviation of the basal plate

coefficient of friction,μ′ , and can be expressed as (e.g., Wang et al., 1995), temperature, Tm, and lithospheric thickness, L, for conductively cooling lithosphere. These plots illustrate how the bathymetry and heat flow change in v Q = v = μ , (2) response to cooling. Seafloor depth increases from the ridge, near 2.5 km at f n w zero age, to ~5.6 km at 160–170 Ma. For ages younger than ca. 80 Ma, the depth

Qf = (3) increases linearly with the square root of age as predicted by simple boundary layer cooling (Parker and Oldenburg, 1973; Davis and Lister, 1977). For older for the brittle and ductile regimes, respectively, where τ is the shear stress, is seafloor, depth approaches an asymptotic value of ~5.65 km (Stein and Stein,

the strain rate, σn is the normal stress across the fault, v is the sliding velocity, 1992). Heat flow also generally decreases with increasing age, but the mean of and w is the width over which shearing takes place. Evidence from thermal observations for a given age is generally less than the lithospheric prediction modeling, borehole measurements, and geomechanical laboratory tests sug- to ages of ca. 65 Ma (Stein and Stein, 1994). The global heat flow data set com- gests that the effective coefficient of friction along shallow subduction thrusts bined with well-established models of plate cooling provides a robust estimate

temperature (Tm = 1450 ± 250 °C) and plate thickness (L = 95 ± 15 km) as predicted by GDH1 (Stein and Stein, 1992). We show plate models having ages of 3 Tm = 1450± 250° C L = 95 ± 15 km 10 and 100 Ma, which bracket a large number of subduction zones and show

) key differences between young and old plates. We use these geotherms to construct thermal models of a generic subduction zone (Fig. 1C). The decrease 4 in heat flow from the deformation front to ~20 km landward is dictated by the convergence rate, slab geometry, and landward thickening of the margin. The heat flow uncertainty in these calculations is based on uncertainty in the geo- 5 therm and decreases with distance from the boundary condition. In the generic thermal model, this leads to an uncertainty in surface heat flow of ~15 and ~5 –2

Bathymetry (km mW m and uncertainties in plate boundary temperatures of ~90 °C and 40 °C 6 at seismogenic depths, for the 10 and 100 Ma models, respectively (Fig. 1D). These differences are resolvable with high-quality heat flow data on the incoming plate and margins. The uncertainties, especially for young plates, likely 7 represent minimum values for three reasons. First, these models represent a 050100 150 global average and specific regions need not conform to the global mean. Sec- Age (Ma) ond, in young oceanic crust, heat flow values can be substantially (>1σ) lower than the mean (i.e., departures from the global mean are greater than modeled 300 here). Third, these models are highly idealized, neglecting local variability in system properties and processes occurring out of the plane of the simulations.

) 250 -2 m 200 Hydrothermal Circulation

Figure 2 shows that heat flow on young oceanic lithosphere is below con- 150 ductive model predictions and its uncertainty based on established cooling models. This discrepancy is a measure of the advective heat loss of the oceanic 100 crust (e.g., Wolery and Sleep, 1976; Stein and Stein, 1994). The discrepancy comes about because the vast majority of marine heat flow measurements Heat Flow (mW 50 rely on inserting a probe into sediment to measure conductive heat loss from the seafloor. In areas where there is considerable volcanic rock exposure and 0 heat flow has been measured in sediment ponds between outcrops, heat flow 050100 150 values are typically low because a significant fraction of lithospheric heat is advected laterally by fluids and discharged through areas of basement expo- Age (Ma) sure. Advective cooling of the volcanic crust lowers the thermal gradient in Figure 2. Bathymetry and heat flow data as a function of crustal age showing the overlying sediments, biasing even randomly distributed heat flow obser- mean observed data (circles) and their standard deviation (black lines) in 2 m.y. bins (from Stein and Stein, 1992). Red line shows best-fitting plate model using vations toward lower values. This sampling bias is exacerbated because con- GDH1 model parameters and its uncertainty based on the uncertainty in basal ventional heat flow measurements cannot be made in areas of exposed base- temperature, T , and plate thickness, L. m ment where fluid discharge normally occurs. In a few cases where researchers have targeted areas of thin sediment cover adjacent to basement highs in regions where there is evidence for advective heat extraction, elevated heat of the total heat content in the lithosphere. Less well constrained by these flow values close to rock outcrops have been interpreted to be indicative of models is the shallow geotherm that is susceptible to shallow environmental rapid fluid discharge (e.g., Davis et al., 1992; Villinger et al., 2002; Fisher et al., processes, particularly fluid flow. 2003a, 2003b). To illustrate how uncertainties in conductive model predictions map to In addition to this bias, there is a relatively large standard deviation asso- uncertainties in temperatures along the subduction thrust, we show the un- ciated with the heat flow discrepancy within young seafloor (Fig. 2). Fluid flow certainty in the geotherm (Fig. 1B) due to the standard deviation in the basal can homogenize temperatures within the basaltic aquifer such that observed

heat flow is proportional to sediment thickness, leading to considerable lo- that are systematically below lithospheric predictions of conductive cooling cal variability (e.g., Davis et al., 1989; Davis and Becker, 2004; Langseth et al., on a regional basis. However, local heat flow values near areas of discharge 1992). Studies show that when the heat flow variability is larger than can be can be larger than lithospheric predictions (e.g., Langseth and Herman, 1981; attributed to heat refraction or sedimentation effects due to variations in sea- Fisher and Wheat, 2010). This style of hydrothermal circulation is referred to floor relief or thermal conductivity, the variability can be confidently attributed as ventilated. to hydrothermal circulation (e.g., Davis et al., 1992; Von Herzen, 2004; Fisher As sediment thickness increases and there are fewer pathways between the and Von Herzen, 2005). Thus hydrothermal circulation can result in two pri- crustal aquifer and overlying ocean, hydrothermal advection of lithospheric mary seafloor heat flow signatures, one or both of which may be found at in- heat becomes restricted, decreasing the discrepancy between observed and dividual sites: measured values that are systematically below the lithospheric predicted heat flow. In this case, hydrothermal circulation may be manifested prediction, and measured values that have high local variability. as local variability in surface heat flow, due to the pattern of convecting cells Regional and local modeling of fluid flow based on detailed heat flow -sur and/or the homogeneity of upper basement temperatures in areas of base- veys provides insight into hydrothermal circulation. Modeling studies con- ment relief. Regional and local studies show that spatial variability in heat flow strained by seafloor heat flow data point to the importance of permeability, can be related to the roughness of the upper basement as the sediment-base- sediment thickness, completeness of sediment cover, and basement relief ment interface approaches an isothermal state (Davis et al., 1989; Fisher and on the vigor, pattern, and degree of ventilation of hydrothermal circulation Becker, 1995; Wang et al., 1997). This style of hydrothermal circulation is re- (Sclater et al., 1974, 1976; Lister, 1972; Anderson and Hobart, 1976; Davis and ferred to as insulated. Davis et al. (1989) documented insulated circulation in Lister, 1977; Anderson et al., 1977; Davis et al., 1989, 1999; Fisher et al., 2003b). the igneous crust of the Juan de Fuca Ridge flank where circulation is vigorous The style of hydrothermal circulation is primarily dictated by the permea- enough to homogenize the sediment-basement interface temperature over lat- bility structure of the basaltic oceanic crust and overlying sediment. The upper eral distances of kilometers. igneous crust is composed of extrusive pillow basalts, flows, and breccia that The concepts summarized here can be used to estimate the geotherm can have very high permeability (~10−9–10−14 m2) because of interconnected forming the seaward boundary and initial conditions for thermal models of porosity largely resulting from coarse clast size, fractures, lava flow contacts, subduction. Key parameters in determining the shallow crustal geotherm are and faults (Fisher, 1998; Becker and Davis, 2004; Fisher et al., 2014). Below the depth and thickness of the aquifer, the degree of thermal homogenization this extrusive basalt aquifer, permeability measurements in sheeted dikes within the aquifer, heat input below the aquifer, and the thermal properties of are low (10−17–10−18 m2) (Becker and Davis, 2004). Where the basement aquifer the rock-fluid system. These same parameters are important for estimating the is not exposed to the ocean, it is capped by sediment typically many orders structure of the geotherm below the aquifer, but also include the history and of magnitude less permeable (10−19–10−13 m2) that depends on lithology and nature of hydrothermal circulation (Spinelli and Harris, 2011). porosity (Spinelli et al., 2004). For example, siliceous oozes have relatively high permeability and fine-grained turbidites have relatively low permeability. For layered sediment, the effective vertical permeability is dominated by the Hydrothermal Circulation in Subducting Oceanic Crust low-permeability layers, and interbedded high-permeability layers have little influence on vertical flow (e.g., Giambalvo et al., 2000). Spinelli and Wang (2008) argued that hydrothermal circulation within the Resistance to flow can be characterized by hydraulic impedance, a func- basaltic basement aquifer should not be expected to stop at the deformation tion of permeability and thickness. In marine sediments hydraulic impedance front of a subduction zone, but likely continues as long as there is sufficient can vary over five orders of magnitude for sediment thicknesses of 10–100 m, permeability in volcanic crust. Following Davis et al. (1997), Spinelli and Wang increasing greatly as sediment thickens and permeability decreases due to po- (2008) applied a computationally efficient way of exploring hydrothermal cir- rosity loss. Because the buoyancy forces driving fluid flow is limited to a few culation within the subducting basaltic aquifer using a Nusselt number ap- hundred kilopascals, there is typically little flow through sediments extract- proximation. The Nusselt number, Nu, is the ratio of the total heat transport to ing significant quantities of heat where sediment is greater than a few tens of heat that would be transferred by conduction alone, meters thick (Spinelli et al., 2004). Slow seepage through marine sediments

is geochemically detectable at rates of millimeters per year, but much higher Nu = qt , (4) flow rates are required to allow advective heat extraction from the volcanic

ocean crust. Thus sediment plays a key role in insulating the oceanic basement where qt is the total heat transport, λ is the thermal conductivity, and Γ is the and limiting the advective loss of lithospheric heat. conductive thermal gradient. In this approach the aquifer is given an effective Where the upper basaltic crust is exposed at the seafloor, fluid flow can- by thermal conductivity that is increased by a factor of Nu, simulating the ther- pass sediments and advect heat directly from the volcanic aquifer to the ocean. mal impact of efficient fluid mixing. The vigor of hydrothermal circulation in an The strongest evidence for this process is regional heat flow measurements idealized porous medium is described by the Rayleigh number, Ra,

gk qh2 Ra = f , (5) 0 μ Sediment where α is the coefficient of thermal expansion,g is gravitational acceleration 2 Crustal Aquifer (9.81 m/s ), k is permeability, ρf is the fluid density,h is aquifer thickness, μ is viscosity, q is the vertical conductive heat flow at the base of the aquifer, and 1 κ is thermal diffusivity (κ = λ/ρc), where ρc is the specific heat of the aquifer. Plate In practice, k is set a priori (or modified as part of a modeling study to achieve

a particular result), h is specified in the model geometry, andα , ρf, μ, and κ are calculated as functions of pressure and temperature (Harvey et al., 1997). 2 Application of Ra calculations to assess stability in hydrothermal systems nec- essarily simplify geological conditions. Aquifer permeability can be parameterized as a function of depth, z (Spinelli and Wang, 2008), 3

log(k) = –9 –5.5(z –0.5zaq ), (6) Depth (km bsf)

where zaq is the depth of the aquifer before subduction. Nu is computed using an empirical relationship with Ra (Wang, 2004; Spinelli and Wang, 2008), 4

Nu = 0.08Ra0.89. (7)

Because there is a nonlinear feedback between temperature and Nu, tem- 5 0 20 40 60 80 100 perature is computed using a multistep method. The iteration begins using a Temperature (°C) simulation with Nu = 1 and Ra is calculated for each aquifer element in the model and a new Nu is determined from Equation 7. Then the thermal conductivity is Figure 3. Geotherms for defining the seaward boundary condition (bsf—below seafloor). The conductive geotherm (red) corresponds to a conductive plate increased in each aquifer element by a factor of Nu, the simulation is rerun, Ra cooling model appropriate for the age of the crust. In this figure the age of the and Nu are recalculated until temperature changes are less than 1 °C between oceanic basement is 24 Ma, such that the basal heat flow is 102 mW –2m . The iterations. Values of Nu typically range from 1 to 1000. Nu values of 1000 are change in gradient through the 400 m of sediment is due to the contrast in thermal conductivity between the sediment (1.1 W m–1 K–1) and the crust (2.9 W large enough to homogenize temperatures within the aquifer; larger values have m–1 K–1). The intermediate geotherm (green) has the same gradient through the only a small additional impact on the thermal structure (e.g., Wang et al., 2015). sediments as the plate cooling geotherm but includes an isothermal aquifer Simulations are run for either insulated or ventilated conditions. Example 0.5 km thick. This geotherm is coupled to the plate cooling geotherm at depth and is used for insulated models. The cool geotherm (blue) is parameterized geotherms are shown in Figure 3. The conductive geotherm (red line) shows with a low thermal gradient (10 °C km–1) appropriate for observed ventilated a change in trend at the sediment-basement interface reflecting the change conditions through the sediments, an isothermal aquifer, and plate cooling at in thermal conductivity. The geotherm reflecting insulated conditions (green depth. This geotherm is used for ventilated models. line, Fig. 3) has the same thermal gradient through the sediments, but an isothermal aquifer. The geotherm through the sediments sets the temperature of the aquifer. Below the aquifer the geotherm is based on conductive cooling Thermal models simulating ventilated conditions set the geotherm at the models with a net heat deficit reflecting the increased heat transfer into the deformation front. In these simulations all heat transported from depth along aquifer relative to conductive conditions. A ventilated geotherm (blue line, Fig. the subducting aquifer is removed so that the incoming plate is not warmed 3) using a thermal gradient through the sediment of 10 °C/km is also shown. seaward of the deformation front. In contrast, thermal models simulating in- Like the insulated case, the temperature of the aquifer is based on the thermal sulated conditions set the geotherm well seaward (150 km or more) of the gradient through the sediments, and below the aquifer the geotherm is based deformation front, and the incoming plate is allowed to warm seaward of the on conductive cooling models. For both ventilated and insulated circulation, deformation front. Hybrid cases where the crust transitions from ventilated to the geotherm through the aquifer can be adjusted to mimic the degree of ther- insulated conditions can be simulated by modeling ventilated hydrothermal mal homogenization; in the ventilated case, the geotherm through sediments circulation to a time corresponding to a distance in front of the deformation can be adjusted to indicate the efficiency of heat extraction from the volcanic front and then imposing insulated conditions at that point. crustal aquifer. With insulated models all heat transfer from the aquifer to the The pastel lines in Figure 1C show the impact of hydrothermal circulation ocean is along the conductive thermal gradient through the sediments. on heat flow across the trench and forearc corresponding to ventilated and

insulated hydrothermal circulation. Two conspicuous features include higher sediment thickness increases. Nankai Trough sediment consists of turbidite fill heat flow at the trench than can be explained with conductive cooling mod- overlying hemipelagic sediment (Ike et al., 2008). The sediment thickness in els (anomalous magnitudes ~50–100 mW m–2), and a more abrupt landward the trench is typically 200–600 m greater than the basement relief (Ike et al., decrease in heat flow (by ~50–100 mW –2m over ~10–20 km) than can be ex- 2008). The increase in sediment thickness (such that it exceeds basement re- plained with reasonable variations in the convergence rate and slab dip. In lief) as the plate approaches the trough could facilitate a switch from ventilated these models the high effective thermal conductivity transports heat from the to insulated hydrothermal circulation. deeper, warmer portion of the aquifer to the shallower, cooler portion of the Harris et al. (2013) assessed the likelihood of hydrothermal circulation aquifer at and seaward of the trench. within Shikoku Basin basement by computing the heat flow fraction defined as Hydrothermal circulation within subducting oceanic crust is manifested by the ratio of observed heat flow to that predicted by conductive cooling models. a change in temperature along the plate boundary relative to models without They found that the heat flow fraction is 0.8 with a standard deviation of 0.4 fluid flow (Fig. 1D). In general temperatures are increased near the deforma- and is remarkably similar to the global heat flow fraction for this age of crust. tion front and decreased landward, consistent with the transport of heat from The low mean and large standard deviation of fractional heat flow is character- depth to the trench region. The landward distance affected by hydrothermal istic of ventilated hydrothermal circulation and consistent with its young age circulation is a function of how the aquifer permeability decreases with dis- and exposed basement. tance from the deformation front, a parameter that is not well known. If the Along the Nankai Trough, where sediment is thicker and more continuous, updip and downdip extent of the seismogenic zone is between ~100 °C and values of heat flow are 20% higher than predicted by conductive cooling mod- 350 °C, then the impact of fluid flow is to increase the width of the seismogenic els based on basement age (Yamano et al., 2003). The incoming sediment is zone. This increase in width has implications for seismic and tsunami hazard overlain by coarse turbidites that may support lateral flow of fluids, but if the analysis, patterns of interseismic locking, and the progress of diagenetic and/ flow is parallel to isotherms then little heat is transported. The high mean frac- or metamorphic reactions that, in turn, affect the distribution of fluid sources tional heat flow suggests an additional source of heat relative to conductive from subducting material. predictions. Locally, the Kinan seamounts show evidence for volcanism that persisted to as late as 7.8 Ma with many samples in the 10–8 Ma range (Sato et al., 2002; Ishizuka et al., 2009). Although this late-stage volcanism may add CASE STUDIES heat in the vicinity of the Kinan Seamount Chain, it was not enough to over- print the observed magnetic lineations and does not explain the regionally Heat flow surveys designed to understand hydrothermal circulation in oce- higher than predicted heat flow along the Nankai Trough (Yamano et al., 2003). anic crust approaching and entering subduction zones are rare; instead, scat- Figure 4 shows heat flow along three drilling transects illustrating features tered heat flow measurements are often combined to make inferences about discussed in previous compilations (Yamano et al., 1984; Kinoshita and Ya- the thermal state and hydrothermal circulation of the incoming oceanic crust. mano, 1986; Yamano et al., 1992, 2003; Harris et al., 2013): (1) relatively large Similar to ridge flank studies, hydrothermal circulation in subduction zones variability between closely spaced heat flow measurements, (2) heat flow can be understood in terms of insulated and ventilated conditions. Case stud- higher than conductive predictions within and just seaward of the Nankai ies chosen for this review are either regions that are relatively data rich or have Trough, and (3) a decrease in heat flow just landward of the deformation front. data that help to highlight important aspects of hydrothermal circulation. These features are best expressed in the Muroto transect where heat flow just seaward (< 25 km) of the deformation front has a value of ~200 mW m–2, much higher than the predicted value for 15 m.y. old lithosphere, 130 ± 22 mW m–2. Nankai These high values decline steeply landward to ~60 mW m–2 over a distance of 65 km. The landward decrease in heat flow is less abrupt along the Kumano The Nankai Trough (Fig. 4) has been the site of numerous heat flow surveys transect, where the crust is 20 Ma and the predicted heat flow is 100 ± 20 mW and thermal studies (e.g., Yamano et al., 1992, 2003; Kinoshita et al., 2008; see m–2. Here heat flow values decline from ~140 mW m–2 30 km seaward of the review in Harris et al., 2013), although none has specifically targeted hydro- deformation front to values of ~60 mW m–2 30 km landward of the deformation thermal circulation on the incoming plate. Nevertheless, the regional environ- front, a distance of 60 km. Sparse data from the Ashizuri transect (Fig. 4C) pre- ment is conducive to hydrothermal circulation as indicated by the heat flow clude a definitive assessment of heat flow variations, but the general pattern is data. Here, the Shikoku Basin (Philippine Sea plate) is subducting beneath the consistent with those at the other two transects. Eurasian plate at a rate of 60 mm/yr. The Shikoku Basin basement varies in In Spinelli and Wang (2008) a number of potential sources for the elevated age between ca. 14 and 30 Ma. Well seaward of the trough, sediment cover is heat flow near the deformation front and the steep decrease landward of this discontinuous but can be as thick as 250 m in basement depressions (Higuchi area at the Muroto transect were considered, the most likely being hydrother- et al., 2007). As the Philippine Sea plate approaches the Nankai Trough, the mal circulation within the subducting oceanic aquifer. They applied a model

Heat flow -2 Muroto Kumano (mW m ) Transect 240 Transect 220 200 180 Ashizuri 160 Transect 140 120 300 100 Muroto 80

) 60 500 -2 250 Kinan 40 Muroto smts. 20 Shikoku 0 400 200 Basin 300 150 Figure 4. Heat flow data from the Nan- 200 kai Trough (modified from Harris et 100 al., 2013) (smts.—seamounts). All dis- mperature (°C) Heat Flow (mW m tances are relative to the deformation 50 Te 100 front with landward distance being positive. Inset shows heat flow data A D color and location of thermal mod- 0 0 els. Marine probe (circles), boreholes (stars), and BSR (bottom simulating 300 Kumano reflector; small circles) are color coded by heat flow. On land, circles show ) Heat Flow Determinations 500 -2 250 Kumano borehole values of heat flow. (A–C) BSR Heat flow data and thermal models Marine Borehole 400 along the Muroto (Spinelli and Wang, 200 Marine Probe 2008), Kumano (Spinelli and Harris, Land Borehole 2011), and Ashizuri transects (Harris 150 300 et al., 2013). Thermophysical rock properties and other model details are given in the original studies. The per- 100 200 meability evolution of the subducting mperature (°C) Heat Flow (mW m aquifer is based on Equation 6. Solid Te 100 and dashed black lines show thermal 50 models with and without fluid flow, B E respectively. (D–F) Temperature along 0 0 the subduction thrust for Muroto, Ku- 300 mano, and Ashizuri. Ashizuri ) -2 250 500 Ashizuri

200 400

150 300

100 200 Heat Flow (mW m mperature (°C)

50 Te 100 C F 0 0 -100 -50 0 50 100 150 200 250 0 50 100 150 200 250 Distance (km) Distance (km)

of insulated hydrothermal circulation in the incoming crust and interpreted Fracture zone and that fluid flow in the subducting oceanic crust leads to temperatures that are 12˚ triple junction trace generally 25 °C higher near the toe of the margin wedge, and 50–100 °C lower Ridge jump near the downdip limit of the seismogenic zone, relative to models without Propagator trace fluid flow. Both the Kumano and Azshizuri transects are simulated using similar style of models (Spinelli and Harris, 2011; Harris et al., 2013). Collectively, BGR99-44 Nicoya Peninsula these studies suggest that there is a wider seismogenic zone than would be present if there were not hydrothermal circulation in the subducting plate, an 10˚ SO81-8 interpretation that is generally consistent with geodetic estimates of the geometry or the seismogenic zone (Sagiya and Thatcher, 1999; Baba et al., 2002, EPR Generated Crust 2006; Ito et al., 2007). 87 mm/yr

8˚ Costa Rica e The region offshore the Nicoya Peninsula was surveyed specifically to un-

derstand the thermal state of the oceanic crust just prior to subduction (Fisher 96 mm/yr et al., 2003a; Hutnak et al., 2007, 2008). A significant along-strike thermal tran- A Cocos Ridg sition allows comparisons between models of hydrothermal circulation in sub- 6˚ CNS Generated −90˚ −88˚ −86˚ −84˚ ducting crust. Crust Offshore of western Costa Rica, the 24–16 Ma Cocos plate is subducting under the Caribbean plate at a rate of ~87 mm/yr (Fig. 5). Here the lithology 60 consists of hemipelagic mud overlying pelagic carbonate (Spinelli and Under- B EPR Crust 50 wood, 2004). The bathymetry and sediment thickness of the incoming Cocos plate varies substantially along strike and is a function of the plate’s origin 40 and subsequent history. The Cocos plate was formed at two ridges, the fast spreading East Pacific Rise (EPR) and the intermediate-spreading Cocos-Nazca 30 spreading center (CNS). The EPR and CNS derived crust is juxtaposed across a Number 20 plate suture that is a combination of a triple junction trace and a fracture zone (Barckhausen et al., 2001). After the plate formed it was intruded by Galapa- 10 gos magmatism that generated the Cocos Ridge and many seamounts that 0 contribute to large variations in bathymetric relief. Sediment on the EPR crust, 0 0.5 11.5 2 where it is not disrupted by seamounts, is generally 350–450 m thick (Spinelli and Underwood, 2004); sediment is thinner on CNS crust, and thins toward the Heat Flow Fraction Cocos Ridge. Exposed basement rock on the Cocos plate is more common on 60 CNS crust than on EPR crust. C CNS Crust 50 Heat flow offshore of Costa Rica, both on the incoming plate and margin, is well characterized and was summarized in Hutnak et al. (2007) and Harris et 40 30 Figure 5. Heat flow along Costa Rica margin. (A) Data come from a variety of sources including Number older data (black circles). Red circles are from a regional study of the incoming plate (Hutnak et 20 al., 2007), and data crossing the margin are shown as white circles (Harris et al., 2010a). Ocean Drilling Program and Integrated Ocean Drilling Program heat flow values are shown as yellow 10 stars. EPR—East Pacific Rise; CNS—Cocos-Nazca spreading center. (B) Histogram of heat flow data seaward of the deformation front for EPR shown as the ratio of observed heat flow to 0 predicted heat flow determined from GDH1 model (Stein and Stein, 1992). The gray line and red 0 0.5 11.5 2 area show the global mean and standard deviation of observed values. (C) Histogram of heat flow data for CNS. Heat Flow Fraction

al. (2010a), respectively. These measurements come from a variety of sources, directly associated with the plate suture, but instead appears to be influenced including probe and downhole measurements and estimates from bottom most strongly by the proximity of basement highs that penetrate the otherwise simulating reflectors. Values seaward of the deformation front are used to pa- thick sediment cover. rameterize the geotherm of thermal models and values across the margin are We show the influence of ongoing hydrothermal circulation on thermal used to assess model performance. We discuss each set of values in turn. models for this area by focusing on two relatively data-rich profiles, one lo- Early measurements (Von Herzen and Uyeda, 1963; Vacquier et al., 1967) cated on each side of the thermal transition (Fig. 6). Thermal conditions within were made without the benefit of modern bathymetric or seismic reflection EPR-generated crust are consistent with conductive heat transfer through the records and tend to be widely spaced (Fig. 5). These measurements indicated 350 m of sediment, an isothermal aquifer 500 m thick, and half-space cooling that EPR-generated igneous crust of the Cocos plate is associated with very beneath the aquifer. We use a thermal gradient of 10 °C/km through the sed- low heat flow relative to the global mean for crust of the same age. In contrast, iment, consistent with the regional heat flow in the area. The temperature of the mean of early heat flow values on CNS-generated igneous crust is close to the aquifer is set to the temperature at the sediment-basement interface, and the conductive prediction, but above the observed global mean for crust of this the model is run to steady state assuming that these thermal conditions have age. These values exhibit large variability indicative of fluid flow with a mean occurred for much of the history of this ridge flank. On the CNS side of the and standard deviation of 110 and 60 mW m–2, respectively (Fig. 5). thermal transition, we use the same approach, but with higher (lithospheric) In 2000 and 2001, heat flow studies specifically designed to investigate the heat flow through the sediments, and commensurately warmer conditions in nature of the cool EPR crust and its transition with warm CNS crust were com- the crustal aquifer (Fig. 6). pleted (Fisher et al., 2003a; Hutnak et al., 2007). All heat flow measurements For both sides of the thermal transition, models incorporating hydrother- are closely spaced and collocated with seismic reflection lines, providing an mal circulation in the subducting oceanic crust fit the data better than models environmental context for interpreting the data. Heat flow values on CNS that do not include advective heat transport in the basalt aquifer. On the EPR crust have a mean of 103 ± 74 mW m–2 corresponding to a fractional mode of side of the thermal transition, heat flow values are significantly lower than con- 1.0. This mode corresponds to the conductive prediction; the large variability ductive predictions (Fig. 6). Heat flow increases landward of the deformation suggests that fluid flow is active, but it redistributes heat within the crustal front, showing the impact of bringing heat from depth. On the CNS side of the aquifer rather than removing it to the ocean. Most measurements on CNS thermal transition, heat flow increases seaward to values greater than can be crust come from a triangular area between the fracture zone and ridge jump accounted for with a conductive model. (Fig. 5) where sediment is continuous and basement is not exposed. However, These calculations indicate significant differences in the depth to the up- the older values, which are not in this triangular area, have a similar mean, dip limit of seismicity on either side of the thermal transition (Newman et al., which is surprising given that basement is more commonly exposed south- 2002). Histograms of hypocenters (Kyriakopoulos et al., 2015) along the sub- east of the Nicoya Peninsula, but may result from a sampling bias. In spite of duction thrust indicate that the updip limit of seismicity is ~10 km deeper on the basement exposure, these heat flow values are consistent with insulated the cold side of the transition than the warm side (Fig. 6). We use the thermal fluid flow. The variability in heat flow values suggests active redistribution of models to assign a temperature to each hypocenter and find that for both sides heat within the crust by rapidly convecting fluids, but limited advective heat the updip limit of seismicity corresponds to ~100 °C. Because the sediment and extraction. All of the predicted lithospheric heat is accounted for in this area. fault properties are likely similar over this short along-strike section of the Mid- In contrast, heat flow measurements on adjacent EPR crust have typical dle America Trench, these results are consistent with the idea that thermally values of 20–40 mW m−2, 60%–90% lower than conductive predictions. The controlled processes likely affect the updip limit of seismicity. Processes that fractional heat flow mode is 0.2, suggesting efficient advective extraction of may affect frictional behavior of plate interface material and/or the pore fluid geothermal heat, and probably requiring very high basement permeability at pressure at temperatures ~100 °C include the opal to quartz and smectite to a regional scale (Fisher et al., 2003a; Hutnak et al., 2008). Thus heat flow on illite transitions (e.g., Moore and Saffer, 2001; Spinelli et al., 2006). this part of EPR crust is consistent with ventilated hydrothermal circulation. Numerical models suggest that basement cooling may be limited to the upper 100–600 m of basement, and application of a one-dimensional analytical South-Central Chile model (Langseth and Herman, 1981) suggests an effective permeability of 10−10–10−8 m2 (Hutnak et al., 2007). A third example of hydrothermal circulation from subducting oceanic crust Closely spaced heat flow measurements across the warm and cool parts comes from the south-central Chile Trench. Four heat flow transects across this of the seafloor show a sharp transition (<5 km), indicating that the difference region were analyzed (Rotman and Spinelli, 2014; Fig. 7). Offshore south-cen- in thermal state is consistent with advective heat extraction from the upper tral Chile, the Nazca plate is subducting under the South American plate at an oceanic crust within the cool part of the plate (Fisher et al., 2003a). The thermal approximately uniform rate of 60 mm/yr (Fig. 7). Sediments seaward of the transition between warmer and cooler parts of the plate in this region is not trench comprise mainly siliceous red clay, ≤200 m thick, and exposed base-

200 )

-2 A BGR99-44 m 100 H F (mW 0 0 20 40 60 Distance (km)

B Temp (°C) 100 150 1200 Figure 6. Thermal models from Costa 0 80 CDRica across the thermal transition (Har- 1000 100 ris et al., 2010b) combined with new 60 800 hypocenter relocations (Kyriakopoulos 50 et al., 2015). (A) Heat flow (HF) data

Number 40 600 50 and thermal models along seismic profile BGR99–44. Solid and dashed lines 400 20 100 Cool EPR show thermal models with and with- Depth (km) out fluid flow, respectively. (B) Thermal crust 200 0 0 01020304050 0 100 200 300 model showing hypocenters (red cir- 150 Depth (km) Temperature (°C) cles) along plate boundary. EPR—East 0 100 200 Pacific Rise. Temp—temperature. (C, D) Distance (km) Histograms showing depth and temperature distribution of plate boundary hypocenters. (E) Heat flow data and thermal models along seismic profile 200 SO81–8. (F) Thermal model showing ) SO81-8 hypocenters (red circles) along plate -2 E boundary. CNS—Cocos-Nazca spread- m ing center. (G, H) Histograms showing 100 depth and temperature distribution of plate boundary hypocenters. Ther- mophysical rock properties and other model details are given in Harris et al. H F (mW 0 (2010b). The evolution of permeability 0 20 40 60 in the subducting aquifer is based on Distance (km) Equation 6 (see text). Temp (°C) F 100 150 0 1200 GH 1000 80 100 50 800 60

600 Number 40 50 400 100 20 Depth (km) Warm CNS 200 crust 0 0 150 01020304050 0 100 200 300 0 100 200 Depth (km) Temperature (°C) Distance (km)

−78˚ −76˚ −74˚ −72˚ −70˚ 200

) 150 BF31.5 Ma 31.5 Ma -2 100 −36˚ 100 0

w (mW m Thermal effects of: Friction 50 emp Diff (°C) -100 T Fluid flow Friction and fluid flow −38˚ Heat Flo 0 -200 200

) 150 27.5 Ma 27.5 Ma -2 CG 100 100 −40˚ 0 w (mW m 50 emp Diff (°C) -100 T

Heat Flo 0 -200 −42˚ 200

) 13.7 Ma 150 D 13.7 Ma H -2 100

100 0 −44˚ (mW m

50 emp Diff (°C ) -100 T

Heat Fl ow 0 -200 A 200

−46˚ ) 150 1.2 Ma 1.2 Ma -2 EI Explanation for panels b-e 100 No fluid flow, no friction 100 Ventilated hydrothermal circulation, no friction 0 Ventilated hydrothermal circulation, friction (mW m Insulated hydrothermal circulation, no friction 50 emp Diff (°C ) -100 ODP borehole T Shallow probe BSR Heat Fl ow 0 -200 -100 0100 200 0100 200 Distance landward of trench (km) Distance landward of trench (km)

Figure 7. Results from south-central Chile. ODP—Ocean Drilling Program. (A) Tectonic map showing location of trench (barbed line) between the subducting Nazca plate and overriding South America plate. Arrows show the convergence direction and rate. Black lines show location of thermal models and age of crust at trench. (B–E) Measured and modeled heat flow along the profiles. Bottom simulating reflector (BSR) measurements are from Grevemeyer et al. (2005), and the shallow probe measurements are from Cande et al. (1987), Flueh and Grevemeyer (2005), Grevemeyer et al. (2005), and Contreras-Reyes et al. (2007). Heat flow peaks seaward of the deformation front show the modeled change from ventilated (seaward) to insulated (landward) conditions (gray and black curves). (F–I) The impact of hydrothermal circulation and frictional heating on plate boundary temperatures. Thermophysical rock properties and other model details are given in Rotman and Spinelli (2014). Temp Diff—temperature difference.

ment is common. Turbidites are common within and near the trench, and the the presence of hydrothermal circulation: hydrothermal circulation can both sediment thickness generally increases to the south, from <2.0 to ~2.4 km. For extract and redistribute excess (frictional) heat. this study, it is important that crustal age at the deformation front decreases Models with no hydrothermal circulation show that the distance along from ca. 1.2 Ma at 45°S, close to where an actively spreading ridge is subduct- the plate boundary between the 100 °C and 350 °C isotherms should increase ing, to 31.5 Ma at 36°S. Conductive models of crustal cooling predict that heat with incoming plate age. Instead, it was found (Rotman and Spinelli, 2014) flow should decrease from ~450 to 90 mW m–2 over this age range, and global that hydrothermal circulation proportionately cools younger and warmer marine heat flow data show an even more abrupt decrease with age (Fig. 2D). crust, homogenizing thermal conditions such that the distance between the Although heat flow data near the south-central Chile Trench are limited, val- 150 °C and 350 °C isotherm is relatively constant along strike. ues from regions where the sediment thickness is <2 km are generally 70% less than conductive predictions (based on crustal age), and there is high spatial variability, with measurements that are 6%–148% of conductive models (Rotman Haida Gwaii and Spinelli, 2014). Contreras-Reyes et al. (2007) reported heat flow values associated with exposed basement varying between 150 and 7 mW m–2 over a dis- Wang et al. (2015) explored the implications of hydrothermal circulation tance of <10 km. Observations of seafloor heat flow that tend to be suppressed offshore western Canada at Haida Gwaii. Here, the 8–6 Ma Pacific plate is con- well below lithospheric cooling predictions are characteristic of ventilated hy- verging obliquely and underthrusting the North American plate at a rate of drothermal circulation, and high variability can be explained by a combination ~20 mm/yr (Fig. 8). The incoming plate is covered by ~1.0–1.5 km of sediment of local convection and proximity to sites where fluids enter or exit the volcanic that promotes conditions for insulated hydrothermal circulation. Although crust. Other evidence for widespread hydrothermal circulation includes variable there are seamounts well offshore, there are no known areas of basement ex- heat flow over plate-bending normal faults (Grevemeyer et al., 2005), and lower posure on the incoming plate near the Queen Charlotte fault. than expected seismic velocities at depth that might result from hydration of the Heat flow measurements on the incoming plate in this region are scarce, oceanic crust and uppermost mantle (Contreras-Reyes et al., 2007). but near the margin tend to indicate values ~250 mW m–2, ~75 mW m–2 higher Similar to examples from Nankai and Costa Rica, heat flow data across the than conductive predictions (Hyndman et al., 1982). Landward of the Queen Chile Trench are better replicated with thermal models incorporating the effects Charlotte fault observed heat flow drops to ~75 mW m–2 landward of the Queen of ventilated fluid flow than those not incorporating fluid flow (Fig. 7). All mod- Charlotte fault (Yorath and Hyndman, 1983). This decrease in heat flow is more els use the same permeability structure (Equation 6), but vary the position of rapid than can be explained with the plate geometry, convergence rate, and the transition from ventilated to insulated hydrothermal circulation as a func- landward thickening of the margin. Wang et al. (2015) showed that the data tion of sediment thickness and basement relief. Rotman and Spinelli (2014) gave can be fit well with models that incorporate hydrothermal circulation using more weight to lower values of heat flow on the margin, arguing that high and a high-Nu proxy. Because of the lack of heat flow data on the incoming plate, scattered values likely reflected areas of focused sediment dewatering; they Wang et al. (2015) estimated the geotherm using GDH1. Figure 8B shows heat also investigated tradeoffs between hydrothermal circulation and frictional flow models corresponding to Nu = 1 (no fluid flow), and Nu = 1000 (vigor- heating, applying a coefficient of friction ′µ = 0.03, consistent with mechanical ous flow). The model that represents the thermal influence of vigorous hydro- studies of subduction thrusts (Wang and He, 2008). These simulations showed thermal circulation clearly provides a better fit to the data, allowing prediction that friction could account for ≤7 mW m–2 of surface heat flow, illustrating that of temperatures along the subduction thrust (Fig. 8C). The intersection of the these measurements are relatively insensitive to frictional heating at depth. Queen Charlotte fault with the subduction thrust occurs at a temperature of Differences in plate temperature for these models illustrate a number of ~350 °C, ~50 °C cooler than the no-fluid-flow model. key points (Fig. 7). First, the efficacy of hydrothermal circulation in cooling The implication of these data and models is that, even though the incoming plate boundary temperatures depends in part on the increase in temperature plate is well sealed by a thick accumulation of sediment, heat advected from with distance from the deformation front (ΔT/ΔL) in the absence of fluid flow depth within the subducting crust can substantially increase the surface heat (Rotman and Spinelli, 2014). The age of the subducting oceanic basement has flow near the deformation front. This process is important for accurately pre- a strong control on this ratio. Second, although modest frictional heating does dicting the distribution of temperatures along the subduction thrust, and thus not have a large impact on surface heat flow, it does have a strong influence the behavior of the fault. on plate boundary temperatures. Because of the dependence on the normal stress, maximum frictional heating increases with depth to the brittle-ductile transition (at the 350 °C isotherm in these models), which deepens with in- Cascadia creasing basement age. Models combining hydrothermal circulation and frictional heating show tradeoffs in the plate boundary temperature that depend One area where thick sediment cover may mask the heat flow signal from on age, and illustrate the difficulty of resolving the coefficient of friction in insulated circulation is along the Cascadia margin of Washington, Oregon, and

mW m-2

300 ) 300 A -2 200 200 Figure 8. (A) Location map showing heat flow measurements from Hynd- man et al. (1982) and Lewis et al. (1991). 100 QCF—Queen Charlotte fault; Pac—Pa- 100 cific; JdF—Juan de Fuca; CSZ—Cas- 0 cadia subduction zone. Deformation front is shown as red line. The model

Heat flow (mW m B profile is shown as a black line. (B) Heat 0 flow observations (circles) projected along model profile and predicted heat flow from model (lines) for model with North 400 no hydrothermal circulation (Nusselt America number, Nu = 1) and vigorous circulation (Nu = 1000). The predicted heat QCF flow just seaward of the deformation front is 175 mW m–2. (C) Temperatures 200 along the subduction thrust. Figure CSZ QCF modified from Wang et al. (2015). Pac Nu = 1 mperature (°C) Nu = 1000

JdF Te C 0 0204060 Distance from trench (km)

northern California, USA (Cozzens and Spinelli, 2012). Here, the 9–7 Ma Juan found that the thick sediment cover at the deformation front results in a broad de Fuca plate subducts beneath the North American plate at ~3–4 cm/yr (Fig. 9). low-amplitude surface heat flow anomaly that may be difficult to discern from Pleistocene glaciation has resulted in a laterally continuous and thick section conductive predictions (Fig. 9). However, Cozzens and Spinelli (2012) argued of sediment that extends to near the active spreading centers. Sediment thick- for the presence of hydrothermal circulation on the basis of correlating pres- ness along the northern deformation front is as much as 3–5 km (Davis and sure-temperature conditions with the seismically estimated location of the Hyndman, 1989), approximately three times that observed at Haida Gwaii. basalt to eclogite transition within the subducting slab at depths of 40–45 km The eastern flank of the Juan de Fuca Ridge, in the FlankFlux experiment inferred from analysis of seismic velocities (Bostock et al., 2002; Rondenay et area (Fig. 9), has been the site of numerous heat flow studies designed to un- al., 2008; Abers et al., 2009). In this setting, models incorporating hydrother- derstand hydrothermal circulation (e.g., Davis et al., 1992, 1999; Fisher et al., mal circulation are in better agreement with the basalt to eclogite transition 2003b; Spinelli and Fisher, 2004; Hutnak et al., 2006). Among other findings, than models without fluid flow (Cozzens and Spinelli, 2012). these studies show that hydrothermal circulation is ventilated close to the Heat flow measurements seaward of the deformation front can be used to ridge, but the circulation system becomes insulated and heat flow approaches test for the presence of hydrothermal circulation. With the recognition that in- conductive predictions at ~35 km from the ridge at an age of 1.3 Ma. Variability sulated hydrothermal circulation leads to an inverse relationship between sed- in heat flow across sedimented regions is inversely proportional to sediment iment thickness and heat flow (e.g., Davis et al., 1989; Fisher and Becker, 1995; thickness, implying that the sediment-basement interface is nearly isothermal Wang et al., 1997), a “topo test” was proposed (Harris and Chapman, 2004) to (Davis et al., 1997). explore for the presence of hydrothermal circulation. In this test, closely spaced Heat flow measurements outside of the FlankFlux area are sparse and heat flow measurements are made over seismically imaged basement topogra- the apparent agreement between heat flow data and conductive cooling phy with continuous, but variable thickness, sediment covering the basement. models at the eastern end of the FlankFlux area are used as arguments for If advection is sufficiently vigorous to create a nearly isothermal condition at conductive conditions at the deformation front (Hyndman and Wang, 1993). the top of the permeable igneous basement, then this state will be manifested However, given the young age of the basement, it was argued (Cozzens and by heat flow that is inversely proportional to sediment thickness. High heat flow Spinelli, 2012) that insulated hydrothermal circulation might be expected, and will occur over buried topographic highs; low heat flow will occur over thickly the impact of hydrothermal circulation along this margin was explored. They buried topographic lows. If hydrothermal circulation does not occur, or if it is

50 °N ) Surface heat flux data Permeability (m2 -2 150 150 ) 10-10 10-9 Probe 0 Ocean Drilling Program borehole Bottom-simulating reflector 10 Shelf borehole

Low FF Mid 100 100 High 48 °N 350 °C Land borehole 20 Depth (km) VI 30

43 mm/yr WA 50 50

46 °N 450 °C 450

Surface heat flux (mW m B Vancouver Island C Washington Juan de Fuca North plate American 0 50 100 150 200 0 50 100 150 200 plate ) 150 150 36 mm/yr -2 OR 44 °N

Pacific 100 100 plate 42 °N 33 mm/yr CA 50 50 A 40 °N Surface heat flux (mW m D Oregon E California 130 °W 128 °W 126 °W 124 °W 122 °W 120 °W 0 50 100 150 200 0 50 100 150 200 Distance landward of trench (km)Distance landward of trench (km) Figure 9. (A) Cascadia subduction zone showing convergence rate and direction (arrows) and locations of model profiles. Figure is modified from Cozzens and Spinelli (2012). Shaded area labeled FF is the FlankFlux region (see text). VI—Vancouver Island; WA—Washington; OR—Oregon; CA—California. (B–E) Heat flow observations and model results from Cozzens and Spinelli (2012). Dashed lines are modeled with no fluid flow, solid lines show low-, mid-, and high-permeability models (inset in C). Thermophysical rock properties and other model details are given in Cozzens and Spinelli (2012).

insufficiently vigorous to thermally homogenize the upper crust, the heat flow significance of heat flow variability. Swath mapping and seismic reflection data through the sediment covering the rough topography will be constant except give information on the measuring environment and sediment thickness, allow- for relatively small thermal refraction and sedimentation effects (e.g., Fisher ing downward continuation of seafloor data into the volcanic basement. These and Von Herzen, 2005). The relationship between heat flow variability and base- survey techniques are especially important for distinguishing between devia- ment relief can be used to make estimates of aquifer permeability that can then tions in thermal conditions associated with hydrothermal circulation, conductive be fed into thermal models of the shallow subduction zone. Such a test can be refraction, and/or sedimentation, which provide rationale explanations for heat applied to the Cascadia subduction zone (and other thickly sedimented mar- flow variability and help to discriminate between competing thermal models. gins) near the trench in a location with appropriate basement topography. Relative to conductive predictions, insulated hydrothermal circulation seaward of the trench leads to high values of heat flow at the deformation front and a steeper landward decline in heat flow than can be accounted for on the DISCUSSION basis of the plate convergence rate, slab dip, and/or landward thickening of the margin. Insulated hydrothermal circulation is most likely to occur where The studies discussed in this review show evidence consistent with per- the sediment cover is greater than a few hundred meters thick and regionally sistent hydrothermal circulation within subducting oceanic crust. Many of these continuous at a length scale of 100 km or more. studies benefit from modern heat flow survey techniques that include high-pre- In contrast, ventilated hydrothermal circulation is characterized by heat cision navigation and closely spaced measurements (<1 km) on both the in- flow that is significantly lower than predicted with conductive models. Ven- coming plate and across the deformation front. Closely spaced measurements tilated hydrothermal circulation is more likely to occur where sediment is overcome issues associated with aliasing and help to constrain the extent and thin, sedimentation rates are low, and basement rocks are exposed. These

conditions may occur at island arcs (where sediment supplies are limited) faults are envisioned to act as high-permeability conduits facilitating advective or where sediments being shed from larger continental landmasses are de- heat loss from the lithosphere at focused locations (and not commonly ob- posited upslope of the trench (e.g., Vannucchi et al., 2016). Ventilated hydro- served by typical heat flow observations). If this were to occur, it would tend to thermal circulation can generate large differences in subduction thrust tem- reduce temperatures in the basement, such that heat flow observations made in peratures, relative to no flow conditions, due to depression of the geotherm seafloor sediments overlying the aquifer are anomalously low for the plate age. relative to conductive conditions (Fig. 3). If the hydrothermal circulation is An alternative hypothesis posits that plate-bending normal faults could in- ventilated, heat being transported updip along the subducting aquifer may crease the depth extent of hydrothermal circulation within the basement rocks subsequently discharge to the ocean where the aquifer is exposed at the (i.e., increase the aquifer thickness) without developing conduits through the seafloor (possibly as far landward as the deformation front), such that the overlying sediment with permeability sufficient for substantial heat advection high heat flow in the trench and the steep landward decrease in heat flow to the seafloor (i.e., the system maintains insulated hydrothermal circula- characteristic of insulated circulation are not present. tion; Kawada et al., 2014; Yamano et al., 2014). In this second scenario, the Both insulated and ventilated models of hydrothermal circulation in the plate-bending faults facilitate delivery of heat from deeper in the lithosphere subducting oceanic basement show the greatest impact on seafloor heat flow to the upper basement aquifer; aquifer temperatures increase, increasing within ~20 km landward and seaward of the deformation front. To better un- the conductive heat flow through the overlying sediment. This mechanism derstand the potential roles of hydrothermal circulation in this setting, closely is proposed as an explanation for a broad (~150 km wide) low-amplitude spaced heat flow data from both sides of the deformation front are needed. (~20 mW m–2) heat flow anomaly observed across the outer rise seaward of As seen with examples from the Nankai Trough and the south-central Chile the Japan Trench (Kawada et al., 2014; Yamano et al., 2014). For the global data Trench, hydrothermal circulation may transition from ventilated to insulated set, Stein (2003) observed no significant difference between heat flow data conditions seaward of the deformation front. Where a system shifts from ven- near trenches and away from trenches on lithosphere of the same age. Thus, tilated to insulated conditions (e.g., where sediment accumulation on a plate if plate-bending normal faults enhance hydrothermal circulation in either of approaching a trench is sufficient to isolate the basement aquifer from the the two ways presented here, the results of Stein (2003) indicate that neither ocean), heat flow may display a landward increase. Heat flow data on the in- the ventilated scenario nor the insulated scenario dominate the global data coming plate are essential to identify and understand this transition. set. Testing these hypotheses for effects of plate-bending normal faults on Deviations in seafloor heat flow 20–100 km landward of the deformation seafloor heat flow may require detailed studies of closely spaced heat flow front are both less apparent in observational studies and more difficult to use measurements that are collocated with complementary data, and bottom sur- to distinguish between underthrust plates dominated by conductive versus veys to locate potential venting sites where advective heat extraction from the advective thermal processes. Data from these areas are subject to numerous plate is hypothesized to occur. environmental perturbations, including sediment slumping and changes in In spite of the progress made over the past decade on understanding tem- the temperature of bottom water; the latter becomes especially common in perature along the subduction thrust, there is a lack of knowledge for some key oceanic depths <2 km. In addition, the relatively thick wedge of sediment com- parameters. A fundamental uncertainty in these models is the permeability monly found above the underthrust plate tends to obscure spatial variations in structure of the igneous crust and how the permeability evolves with depth. thermal properties at and below the subduction thrust, because of both con- The evolution of permeability with depth is poorly known but is likely a func- ductive refraction and fluid flow within the wedge. tion of pressure, temperature, and diagenetic and/or metamorphic effects. To In some cases hydrothermal circulation in the oceanic crust may occur date, the surface heat flow distribution near the deformation front has proved without producing obvious anomalies in seafloor heat flow. These cases in- to be a somewhat blunt instrument in constraining the details of permeabil- clude weak ventilated circulation (resulting in a small fraction of advective ity evolution within the subducting aquifer (e.g., Spinelli and Wang, 2008). To heat extraction), and cases of insulated convection where the sediment cover better define this permeability distribution and/or evolution may require (or is thick enough to attenuate local variations at depth, as may be happening at least, benefit from) a multipronged approach. For example, surface heat at Cascadia. These environments may still lead to significant perturbations of flow has been used in conjunction with the seismically inferred location of temperature on the plate interface relative to conductive predictions. slab alteration to constrain the vigor and extent of hydrothermal circulation in Stein (2003) used the global heat flow data set to assess whether heat flow Nankai (Spinelli and Wang, 2008) and Cascadia (Cozzens and Spinelli, 2012). associated with trenches show a bias toward low values that could be linked Future studies may benefit from also including geochemical constraints on to enhanced heat extraction from the oceanic lithosphere by hydrothermal cir- the sources of fluids within the basement aquifer (e.g., Solomon et al., 2009). culation. The core hypothesis of that study is that plate-bending normal faults, Distinguishing between the impacts of hydrothermal circulation on sur- induced as the lithosphere traverses the outer rise, enhance ventilated hydro- face heat flow from other processes remains a challenge. At margins ac- thermal circulation relative to oceanic lithosphere of the same age away from creting sediment these processes include the sediment thickening and de- trenches (and not affected by outer rise faulting). In this scenario, the normal watering due to porosity loss as sediment is accreted to the margin (Wang

et al., 1993). Sediment thickening can decrease margin heat flow while marizes thermal evidence for hydrothermal circulation in subducting oceanic sediment dewatering can increase it. The impact on surface heat flow for crust. Heat flow data from Nankai, Costa Rica, south-central Chile, and Haida these processes is likely to be smaller than the impact of hydrothermal Gwaii are best fit by models incorporating hydrothermal circulation in sub- circulation. These processes act over similar length scales, and the lack of ducting oceanic crust. This result is consistent with the young basement ages data regarding these processes makes it difficult to separate these effects (35–8 Ma) of the incoming igneous crust and results from global heat flow from hydrothermal circulation in the subducting aquifer. analysis showing a significant heat flow deficit for crustal ages younger than Another key process influencing the thermal regime of the shallow sub- 65 Ma (e.g., Stein and Stein, 1992) interpreted in terms of advective fluid flow. duction zone is frictional heating along the plate interface. Temperature In ridge flank settings, buoyancy driven hydrothermal circulation is perva- along the subduction thrust is extremely sensitive to the coefficient of fric- sive in the basaltic basement aquifer. Where sediment is thin and discontinu- tion, whereas surface heat flow is much less sensitive to this parameter. Sev- ous and basement is exposed, the oceanic crust tends to be well ventilated. eral studies have explored the impact of frictional heating and hydrother- Surface heat flow associated with subducting oceanic crust that is well venti- mal circulation on surface heat flow (e.g., Rotman and Spinelli, 2014; Hass lated just seaward of the deformation front is characterized by low magnitudes and Harris, 2016; Wang et al., 2015). These studies are consistent in finding relative to conductive predictions and large variability. Surface heat flow just that hydrothermal circulation and frictional heat have opposing effects on landward of the deformation front may increase as the crustal aquifer becomes plate interface temperatures and surface heat flow above the subduction insulated. Subducting crust that includes thick and continuous sediment cover thrust. In general, hydrothermal circulation decreases temperatures on the may also host vigorous fluid flow. In this environment, surface heat flow val- subducting thrust. The magnitude of cooling on the subduction thrust gen- ues may approach conductive predictions on a regional basis, but variability is erally increases with depth until the aquifer loses sufficient permeability. larger than can be explained with conductive refraction. Near the deformation In contrast, frictional heating increases plate boundary temperatures. Be- front, heat flow may be much higher than conductive predictions and decrease cause frictional heating is proportional to the normal stress (Equation 2), landward at a greater rate than can be explained by plate convergence and warming increases with depth until ~350 °C, where the rheology changes geometry and landward thickening of the margin. If hydrothermal circulation from brittle to ductile. Figure 7 shows that these processes work over a is present within the incoming crust, it is likely to continue with subduction as similar length scale, but as the age of the incoming basement ages and the long as permeability remains sufficiently high to allow convection to occur. brittle-ductile transition deepens, peak frictional heating shifts landward. The presence of hydrothermal circulation within subducting oceanic crust Studies evaluating both hydrothermal circulation and frictional heating in- has a large influence on plate boundary temperatures. In general, hydrother- dicate that frictional heat can be efficiently extracted from the system by mal circulation leads to plate boundary temperatures that are cooler than hydrothermal circulation. As a result, in order to use heat flow observations those where fluid flow does not occur. The magnitude of cooling depends on above a subduction thrust to constrain the amount of frictional heating on the permeability structure of the incoming plate and the evolution of permea- the fault, independent knowledge of the vigor of hydrothermal circulation bility with depth and time. Resolving complex relationships between subduc- or the lack of hydrothermal circulation is required. In Hass and Harris (2016) tion processes, the permeability structure in the ocean crust, and the dynamics the root mean square misfit between thermal models and heat flow data of hydrothermal circulation remains an interdisciplinary frontier. at Costa Rica were evaluated as a function of the initial permeability and coefficient of frictional heating; for Costa Rica, it was found that the initial ACKNOWLEDGMENTS permeability is relatively well resolved and that the coefficient of friction is Harris was supported by National Science Foundation (NSF) grants OCE 1458211 and OCE 1355878. less well resolved. Spinelli was supported by NSF grant OCE 1551587. Fisher was supported by NSF grants OIA- Abundant evidence for high pore fluid pressure (i.e., pressure greater 0939564, OCE-1260408, and OCE-1355870. We thank Guest Associate Editor Laura Wallace and re- then hydrostatic) along the subduction thrust seems at odds with a high-per- viewers Carol Stein and Michael Underwood for providing constructive comments that improved this study. This is C-DEBI (Center for Dark Energy Biosphere Investigations) contribution 373. meability aquifer immediately below it. Sediments between the basaltic aquifer and subduction thrust must be relatively impermeable to maintain high pore pressure along the fault, but little is known about the permeability REFERENCES CITED or continuity of this seal. Abers, G.A., MacKenzie, L.S., Rondenay, S., Zhang, Z., Wech, A.G., and Creager, K.C., 2009, Imag- ing the source region of Cascadia tremor and intermediate depth earthquakes: Geology, v. 37, p. 1119–1122, doi:10 .1130 /G30143A .1. 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