Heat Flow and Bending-Related Faulting at Subduction Trenches: Case Studies Offshore of Nicaragua and Central Chile

Earth and Planetary Science Letters 236 (2005) 238–248 www.elsevier.com/locate/epsl

Heat flow and bending-related faulting at subduction trenches: Case studies offshore of Nicaragua and Central Chile

Ingo Grevemeyera,*, Norbert Kaulb, Juan L. Diaz-Naveasc, Heinrich W. Villingerb, Cesar R. Raneroa, Christian Reichertd

aIFM-GEOMAR, Leibniz Institute for Marine Sciences, and SFB 574, Wischhofstrabe 1-3, 24148 Kiel, Germany bDepartment of Earth Sciences, University of Bremen, Klagenfurter Strabe, 28359 Bremen, Germany cSchool of Marine Sciences, Catholic University of Valparaı´so, Av. Altmirano 1480, Valparaı´so, Chile dFederal Institute for Geosciences and Resources, Stilleweg 2, 30655 Hanover, Germany Received 19 July 2004; received in revised form 10 March 2005; accepted 11 April 2005 Available online 1 July 2005 Editor: V. Courtillot

Abstract

Detailed heat flow surveys on the oceanic trench slope offshore Nicaragua and Central Chile indicate heat flow values lower than the expected conductive lithospheric heat loss and lower than the global mean for crust of that age. Both areas are characterised by pervasive normal faults exposing basement in a setting affected by bending-related faulting due to plate subduction. The low heat flow is interpreted to indicate increased hydrothermal circulation by the reactivation and new creation of faults prior to subduction. A previous global approach [1] [Stein C.A., Heat flow and flexure at subduction zones, Geophys. Res. Lett. 30 (2003) doi:10.1029/2003GL018478] failed to detect similar features in the global but sparse data set. Detailed inspection of the global data set suggests that the thickness of the sedimentary blanket on the incoming plate is an important factor controlling the local hydrogeological regime. Areas with a relatively thick sedimentary cover do not show any heat flow anomaly while areas where normal faulting exposes basement suffer from increased hydrothermal activity. Both geochemical data from arc volcanoes and seismological evidence from intra slab events suggest that the flux of water into the deep subduction zone is larger in areas characterised by reactivated hydrothermal circulation. It is reasonable to assume that the larger water flux is caused by serpentinization of the upper mantle, facilitated by bending-related faults cutting into the upper mantle. D 2005 Elsevier B.V. All rights reserved.

Keywords: subduction; normal faulting; heat flow; fluid flow; serpentinization; global water cycle

1. Introduction

* Corresponding author. Tel.: +49 431 600 2337; fax: +49 431 The thermal state and the degree of hydration of 600 2922. the incoming plate influences a wealth of subduction E-mail address: [email protected] (I. Grevemeyer). zone processes, including the location of the seismo-

0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.04.048 I. Grevemeyer et al. / Earth and Planetary Science Letters 236 (2005) 238–248 239 genic zone [2], intermediate-depth earthquakes in the and hence faulting is strongest within 50 km of the Wadati–Benioff zone [3,4], and melt generation under trench. Seafloor mapping [10–12] and earthquake volcanic arcs [5]. Both the thermal state and hydration mechanisms in this area [13] are consistent with of the subducting slab are linked to the hydrogeology bending-related normal faulting, which is suggested and alteration of the incoming oceanic lithosphere. In to provide the pathways for fluids to enter the crust recent years observational evidence is accumulating and mantle. Therefore, it has been speculated that heat that faulting due to bending of the incoming plate in flow seaward of the trench may be low compared to the outer rise seaward of deep sea trenches may that for average crust of the same age, due to in- change the hydrogeological regime and hence the creased hydrothermal circulation [14,15]. However, water flux into the Earth’s interior prior to subduction a recent investigation of a global but sparse heat [6,7]. flow data set shows no significant differences between At subduction zones the oceanic lithosphere bends heat flow near trenches and the global means for the into the deep sea trench, producing a prominent outer same age crust [1]. rise bathymetric bulge. Although uplift starts approx- In this study we use recently collected swath map- imately 300 km from the trench axis [8,9], bending ping bathymetry and new heat flow surveys across

Fig. 1. Multibeam bathymetry offshore Nicaragua and seismic lines BGR99_39 and BGR99_41 of Ranero et al. [6]. Heat flow data (circles) were collected offshore Central Nicaragua with a violin design heat probe where the throw of the faults is largest. Additional heat flow data were obtained using outriggers on gravity corers (diamonds). Magnetic lineations [41] and hence abyssal hill fabric and active normal faults on the incoming plate strike both NW–SE, parallel to the trench axis. Labels like H0202 indicate the name of the heat flow station (see Table 1). 240 I. Grevemeyer et al. / Earth and Planetary Science Letters 236 (2005) 238–248

Fig. 2. Multibeam bathymetry offshore Central Chile and multi-channel seismic lines (white broken lines) [19]. Heat flow data (circles) were collected to the north of the Arauco Peninsula. Sea floor fabric strikes NW–SE. Normal faults created while the plate is bend prior to subduction strike roughly parallel to the trench axis in NNE–SSW direction. Black solid portion along seismic line ENAP6 indicates the location of seismic data shown in Fig. 3. Labels like H0306 indicate the name of the heat flow station (see Table 1). 1234 and 1235 are holes drilled during ODP leg 202 [19]. For color scale see Fig. 1. I. Grevemeyer et al. / Earth and Planetary Science Letters 236 (2005) 238–248 241 normal faults caused by plate bending offshore Nicar- CMP agua and Central Chile to search for evidence indi- 6200 6100 6000 5900 5.5 cating increased hydrothermal mining of heat and possibly hydration of the plate prior to subduction. 6.0

6.5

2. Tectonic framework and setting 7.0 Fault 7.5 2.1. The Nicaraguan subduction zone TWT [s] Moho 8.0

High resolution bathymetric mapping of the in- 8.5 coming plate [6] shows that bending-related faulting (following Ranero et al. [6] we call them bend-faults) 9.0 is pervasive across most of the ocean trench slope 9.5 (Fig. 1). Seafloor spreading anomalies strike approx- 0 2 4 6 8 imately parallel to the trench axis, and the orientation Distance [km] of the tectonic fabric formed at the spreading centre Fig. 3. Seismic image of a normal fault created by plate bending seems to govern the amount of faulting. Some faults prior to subduction. The fault cuts through the crust/mantle bound- can be tracked in the multibeam bathymetry for at ary (Moho) into the uppermost mantle. least 50 km along the trench and multi-channel seismic reflection data suggest that they cut ~20 km into the lithosphere [6]. Ranero et al. [6] hypothesized that (Fig. 2); bend-faults strike approximately parallel to these faults promote fluid flow down to mantle depth the trench axis. Multi-channel seismic reflection data and cause serpentinization of the mantle between the from the incoming plate suggest that the smooth outer rise and the trench axis. Evidence for an in- trench area mapped in the bathymetric data is caused creased degree of hydration and hence serpentiniza- by sediments filling the trench [18,19]. Under the tion is perhaps provided by geochemical data from the trench fill, the multi-channel data reveal bending-re- volcanic arc, which suggests that mafic magmas in lated faults cutting across the crust–mantle boundary Nicaragua have water concentrations among the high- into the upper mantle (Fig. 3). est world-wide [16]. In addition, seismological data suggest that regional P-waves from intraslab events at 100–150 km depth show high-frequency late arrivals, 3. Data acquisition apparently trapped in a 2.5–6 km thick low-velocity waveguide at the top of the downgoing plate. Such The Nicaraguan data are from the cruise M54-2 low velocities can best be explained by N5 wt.% of carried out aboard the German research vessel Meteor water in the subducted crust, 2–3 times the hydration in summer 2002 and the data from Central Chile were inferred for other slabs [17]. Existing data therefore obtained aboard the Chilean Navy research vessel suggest that the Nicaraguan slab may contain unusu- Vidal Gormaz in March 2003. Two different heat ally high amounts of water. probes of violin bow design were used to acquire the heat flow data. The instruments have 11 and 22 2.2. The Central Chilean subduction zone thermistors, which are spaced in 0.27 m intervals and mounted inside an oil filled hydraulic tube that pene- In Central Chile the seafloor spreading fabric of the trates into a sedimented seafloor. Thus, the probes are incoming plate strikes approximately 458 obliquely to able to obtain thermal gradient over a length of 3 and the trend of the deep sea trench. In the outer rise area 6 m, respectively. Both probes are equipped with the seafloor fabric generated at the spreading centre online data transmission for operation control and and cross-cutting normal faults caused by plate bend- independent data storage inside the instrument for ing are well imaged in multibeam bathymetric data double data security. At every other station, in situ 242 I. Grevemeyer et al. / Earth and Planetary Science Letters 236 (2005) 238–248

Table 1 New heat flow data presented in this study Station Longitude Latitude Depth Grad k HF N Type (m) (K/km) (W/mK) (mW/m2) H0201 À87.3058 11.3351 1205 28.2 0.88 24.8 5 OR H0202 À87.5107 11.2334 4138 51.2 0.72 36.7 11 3 m H0202 À87.5043 11.2400 4080 51.0 0.72 37.1 11 3 m H0202 À87.498 11.2464 4071 56.2 0.72 41.1 11 3 m H0202 À87.4917 11.2529 3976 52.1 0.72 35.4 11 3 m H0202 À87.4788 11.2660 3471 40.6 0.72 29.2 10 3 m H0202 À87.4724 11.2724 3337 43.9 0.72 31.4 11 3 m H0202 À87.466 11.2789 3143 38.7 0.73 28.0 8 3 m H0202 À87.4596 11.2853 2991 38.2 0.72 28.5 11 3 m H0203 À87.5152 11.1168 5320 15.5 0.72 11.2 9 OR H0204 À87.7187 11.0204 3882 40.7 0.73 29.9 10 3 m H0204 À87.7124 11.0270 3879 39.9 0.72 28.6 11 3 m H0204 À87.7060 11.0334 3886 35.0 0.72 26.3 11 3 m H0204 À87.6996 11.0399 4026 37.1 0.71 26.5 10 3 m H0204 À87.6932 11.0465 4054 19.6 0.72 15.4 11 3 m H0206 À87.3392 11.4087 941 25.2 0.75* 18.9 8 6 m H0206 À87.3328 11.4152 905 23.7 0.75* 16.1 19 6 m H0206 À87.3264 11.4217 877 32.6 0.75* 24.4 9 6 m H0206 À87.3200 11.4282 845 33.7 0.75* 25.0 20 6 m H0206 À87.3136 11.4346 811 41.0 0.75* 28.1 21 6 m H0206 À87.3072 11.4411 776 25.1 0.75* 18.8 9 6 m H0206 À87.3007 11.4476 743 53.2 0.75* 39.3 21 6 m H0208 À87.6342 11.1068 4895 17.2 0.72* 12.4 9 6 m H0208 À87.6278 11.1134 4861 18.5 0.72* 16.0 9 6 m H0208 À87.6215 11.1199 4817 21.0 0.72* 15.1 10 6 m H0306 À73.7377 À36.2118 1759 59.0 0.89 52.5 9 3 m H0306 À73.7481 À36.2104 1817 47.1 0.89* 41.9 9 3 m H0306 À73.7596 À36.2087 1890 47.7 0.89* 42.4 9 3 m H0306 À73.7702 À36.2077 1957 47.0 0.89* 41.8 9 3 m H0306 À73.7805 À36.2058 2046 49.1 0.89* 42.9 9 3 m H0306 À73.7922 À36.2040 2103 48.0 0.89* 42.7 9 3 m H0306 À73.8039 À36.2026 2144 37.8 0.89 33.8 8 3 m H0306 À73.8268 À36.1994 2218 41.4 0.90 37.1 10 3 m H0306 À73.8363 À36.1980 2294 44.6 0.89* 39.7 8 3 m H0308 À74.1783 À36.1507 4983 22.7 0.85* 19.3 4 OR H0308 À74.1537 À36.1520 4986 23.7 0.85* 20.2 4 OR H0308 À74.1307 À36.1562 4979 25.6 0.85* 21.8 4 OR H0308 À74.1112 À36.1605 4966 24.5 0.85* 20.8 4 OR H0308 À74.0713 À36.1654 4447 39.0 0.85* 33.2 4 OR H0308 À74.0568 À36.1674 4485 48.6 0.85* 41.3 4 OR H0308 À74.0373 À36.1689 4295 69.9 0.85* 59.4 4 OR H0308 À74.0170 À36.1723 3894 53.3 0.85* 45.3 4 OR H0308 À74.0054 À36.1746 3700 64.7 0.85* 55.0 4 OR H0308 À73.9942 À36.1755 3589 57.4 0.85* 48.8 4 OR H0308 À73.9853 À36.1782 3427 68.6 0.85* 58.3 4 OR H0308 À73.9722 À36.1799 3217 57.9 0.85* 49.2 4 OR H0309 À74.5873 À36.0214 4831 20.8 0.82* 17.1 3 OR H0309 À74.5977 À36.0198 4942 18.2 0.82* 15.5 3 OR H0309 À74.6197 À36.0173 4869 13.7 0.82* 11.6 3 OR Grad is the thermal gradient, k the thermal conductivity, HF the heat flow anomaly, N the number of sensors used to obtain the thermal gradient, and Type indicates the instrument used to obtain the heat flow (OR: outriggers; 3 m: 3 m long violin bow design probe; 6 m: 6 m long violin bow design probe). Thermal conductivity marked by an asterisk has been estimated from adjacent measurements or ODP Leg 202 measurements [19]. I. Grevemeyer et al. / Earth and Planetary Science Letters 236 (2005) 238–248 243 conductivity measurements were made by applying a 4.1. The Nicaraguan subduction zone 20 s pulse of electric current along heater wires within the lance. The thermal decay of this calibrated heat Offshore Nicaragua, heat flow data and deep-tow pulse allows to estimate the conductivity at the loca- video observations across normal faults are available. tion of in situ temperature measurements [20]. Heat flow determinations on the incoming plate cover In addition to measurements with the heat probes, a 25 km wide area of the trench, where the seafloor thermal gradients were measured by outriggers [21] morphology indicates active normal faulting (Fig. 1). mounted on gravity corers. Thermal conductivities Deep-tow video surveying (H. Sahling, personal com- from the cores have been measured using needle munication) indicates that some fault offsets are large probes [22], which were inserted into undisturbed enough to expose basement at the seafloor. Values of areas of split cores recovered from the seafloor. Out- 27–30 mW/m2 occur 25 km seaward of the trench axis riggers have also been used for the deep water stations and decrease systematically to ~11 mW/m2 near the during the Vidal Gormaz cruise, where the ship’s trench axis (Fig. 4a). The decrease in heat flow corre- coaxial cable was limited to a length of 2800 m. sponds to an increase in the number of faults [6] Here, constraints from Ocean Drilling Program towards the trench axis (Fig. 5), indicating that new (ODP) leg 202 [19] have been used to assess the faults continue to form as the plate approaches the thermal conductivity to yield heat flow from the gra- trench. Furthermore, seismic data suggest that faults dients measured with the outriggers. remain active until the incoming plate is subducted Heat flow stations are clustered locations of mea- [6]. The measured heat flow anomaly is well below surements, consisting of 5 to 12 positions with a the theoretically expected conductive heat loss of 100 spacing of 0.5 to 1 nautical mile. Multi penetration mW/m2 of a 24 Myr old lithosphere [23] and a bglobal mode (pogo style) is the most effective way of heat flow averageQ of 69 mW/m2 for oceanic crust of advancing along a station while the probe is lifted that age defined using limited observations [1]. Over above sea floor some hundred metres. After penetra- the marine fore-arc, however, a simple analytical tion into the sedimented seabed, the sensors record a model of the advection of heat into the subduction pulse of frictional heating, which decays while the zone [24] suggests that heat flow recovers rapidly tools remains for 7–10 min in the seafloor. The from depressed geotherms due to hydrothermal min- calculation of equilibrium temperatures follows the ing of heat in the incoming plate to conductive values same procedure for all instruments and equilibrium as the circulation system is shut off by the overlying temperatures are calculated by extrapolating the margin wedge (Fig. 4a; Table 2). decay of the heating pulse [20]. Individual temperature and conductivity measurements were inverted to 4.2. The Central Chilean subduction zone obtain surface heat flow. The complete processing sequence is described elsewhere [20]. The heat flow Offshore Central Chile near Concepcio´n, heat data and the number of temperature measurements flow data were collected on the incoming plate and used to derived the thermal gradient are summarised on the continental slope. Heat flow determinations in Table 1. approximately 40 km off the trench axis cross a major fault clearly visible in the swath mapping bathymetry, which cross cut the trend of the abyssal 4. Data description and results hill fabric inherited at the spreading centre (Fig. 2). Very low heat flow values of only 12 mW/m2 char- In both study areas, new heat flow data were acterise the fault area. Values tend to increase away acquired along transects that cover the incoming from the fault to 17 mW/m2 (Fig. 4b). The age of the oceanic lithosphere seaward of the trench axis and incoming plate is ~32 Myr, corresponding to a con- the continental slope. These data allow us to describe ductive heat loss of 90 mW/m2 [23] and a global the hydrogeological regime of the incoming plate mean average of 61 mW/m2 [1]. Low values of 20 and heat transfer trough the continental margin mW/m2 characterise the heat flow pattern immedi- wedge. ately seaward of the deformation front. High sedi- 244 I. Grevemeyer et al. / Earth and Planetary Science Letters 236 (2005) 238–248

a) b)

125 125 expected heat flow ] ] 100

100 -2

-2 expected heat flow

75 global mean heat flow 75 global mean heat flow 10 50 50 10 5 5 0 Heat flow [mW m Heat flow [mW m 25 25 0 ODP site 1234 + 1235 0 0 -60 -40 -20 0 20 40 60 80 -60 -40 -20 0 20 40 60 80 Distance [km] Distance [km] c) d)

125 125 ] 100 expected heat flow ] -2 100 -2

global mean heat flow 75 75 expected heat flow 10 50 50 5 10 0 5 Heat flow [mW m 25 Heat flow [mW m 25 0 0 0 -60 -40 -20 0 20 40 60 80 -60 -40 -20 0 20 40 60 80 Distance [km] Distance [km]

Fig. 4. Heat flow transects on the incoming plate and across the trench axis in a) Nicaragua (locations of heat flow measurements are shown in Fig. 1), b) Central Chile (locations of heat flow measurements are shown in Fig. 2), c) Peru [14,29], d) Honshu and Hokkaido, Japan [29,30]. The numbers give the percentage of the lithostatic pressure P, which is turned into shear stress s (s =0–10% P). Model parameters are given in Table 2. mentation rates due to sediment transport through the 5. Discussion and conclusions adjacent Bio-Bio canyon [25] may lower the surface heat flow by 10–30% [26]. However, even corrected The global heat flow data compilation provides values would be much lower than the mean of little evidence for lower heat flow seawards of the world-wide surface heat flow of 32 Myr old litho- trench related to increased hydrothermal circulation sphere. At the continental slope, over a young ac- from bend-faulting [1]. However, the paucity of data cretionary prism [18,27], high heat flow values may results in the problem that on average only one suggest that sediments are tectonically dewatered value every 20 km of trench exists in the global while they are incorporated into the accretionary data set. The already sparse data are not evenly complex. Landwards, heat flow values rapidly de- distributed. Thus, a statistical analysis might be crease and may reflect the advective transport of heat biased by a few detailed study areas, providing by the downgoing slab, as indicated by the good fit the majority of data. Our two detailed study areas of a simple thermal model of fore-arc heat flow [24]. in Nicaragua and Central Chile fill in gaps in the The trend of landward decreasing heat flow is sup- world-wide data set. For example, along the 5000 ported by downhole temperature measurements in km long Chilean margin, only two data points have the adjacent drill sites 1234 and 1235 of the ODP been found seaward of the trench axis in the global [19]. database. I. Grevemeyer et al. / Earth and Planetary Science Letters 236 (2005) 238–248 245

a) are very likely related to increased hydrothermal cir- 35 culation of cold seawater in the upper portion of the incoming plate caused by bend-faulting. However, ] 30 -2 like on the flanks of mid-ocean ridges, fluid circula- 25 tion should cause both fluid recharge and discharge [28]. Recharge is characterised by low heat flow 20 values while discharge is indicated by high heat 15 flow values. We believe that high-permeability pathways support low-temperature vent sites in the area of

Heat flow [mW m 10 bending related faulting which may occur elsewhere. 5 In order to understand differences in the pattern observed in our case studies off Nicaragua and Central b) Chile and the constraints from the global data set, we 10 investigated two other areas using the global data base 1000 [29] and previously published geothermal measure- 8 ments from the Peru trench [14] and Japan trench 800 [30]. Off Peru, low heat flow in the trench and sea- 6 600 ward of the trench axis are characteristic features (Fig. 8 4 4c). The low heat flow near 13 S offshore Peru was # Faults 400 already investigated by Yamano and Uyeda [14]. They 2 200 suggested that water migrating along normal faults may act as cooling agent and were the first to propose 0 0 Cumulative fault offset [m] that bend-faulting at trenches may reactivate hydro- 30 25 20 15 10 5 0 Distance from trench [km] thermal circulation in the oceanic lithosphere prior to subduction. In the Japan trench offshore Honshu and Fig. 5. a) Heat flow data from the incoming plate off Central the Kurile trench offshore Hokkaido, detailed multi- Nicaragua (see Figs. 1 and 4a) and b) statistics of active normal beam bathymetric surveys show that bending causes faults on the incoming plate [6]. The histogram displays the fault density as a function of distance from the trench axis. The number pervasive normal faulting in the incoming plate [11]. of faults increases towards the axis, indicating that new faults Although only a few heat flow values are available, continue to form as the plate approaches the trench axis. Lines they support the results from the Stein [1] study. Thus, indicate the cumulative fault offset [6] measured at the top basement heat flow is not significantly different from the global _ of multi-channel seismic line BGR99 39 (black line) and mean or the prediction of the plate cooling model for BGR99_41 (grey line). There is a tendency to larger fault offsets towards the trench. Note that the measured heat flow decreases with crust older than 100 Myr. Consequently, bend-faulting increasing number of faults and fault throw. We interpret this is not the sole factor governing the reactivation of phenomenon in terms of increased hydrothermal activity when the hydrothermal circulation at subduction zones. plate approaches the deep sea trench. It has long been recognised that hydrothermal circulation systems are inherently affected by the sedi- In both detailed study areas chosen for this case mentary blanket [31]. The forces available to drive study the measured heat flow is only one-third of the hydrothermal flow through the oceanic crust are mod- expected global mean for crust of that age (Fig. 4a and est, being limited mainly to pressure differences at the b); thus, much lower than suggested by the global base of recharging and discharging columns of water approach of Stein [1]. Our closely spaced, carefully [32]. These forces are insufficient to drive fluids at located measurements show low values that are clear- thermally significant rates through N200 m of sedi- ly linked to reactivated or new faults created in the ments. Thus, basement outcrops are required to allow outer rise. Off Nicaragua, a trenchward decrease in hydrothermal fluids to bypass the sediment and mine heat flow on the incoming plate correlates well with lithospheric heat efficiently [28,33]. In the NW Pacific an increase in the number and offset of faults cutting about 600 m of sediments cover the incoming plate. into the igneous basement (Fig. 5). Thus, low values Although escarpment heights on the incoming plate 246 I. Grevemeyer et al. / Earth and Planetary Science Letters 236 (2005) 238–248

Table 2 List of parameters used to calculate the conductive reference models shown in Fig. 4 Plate age Plate dip Convergence rate Thermal conductivity Thermal diffusivity Fore-arc density (Myr) (8) (cm/yr) (W/mK) (m2/s) (kg/m3) Nicaragua 24 13 9.1 1.6 1.1 10À 6 2700 Chile 33 10 8.0 1.6 1.1 10À 6 2700 Peru 30 13 7.8 1.6 1.1 10À 6 2700 NE Japan 100 12 9.1 1.6 1.1 10À 6 2700 offshore Japan often exceed 300 m, most faults may reflected in the geochemistry of the Nicaraguan vol- not breach the sedimentary blanket. Thus, except at canic arc and the seismic structure of the slab. Com- seamounts, basement outcrops are generally absent pared to other arcs, volcanoes in Nicaragua show and hence seafloor heat flow matches conductive some of the highest concentrations of geochemical predictions. Offshore Central Chile and Nicaragua, tracers for oceanic crustal fluid, including the maxi- however, some faults have offsets of N500 m and mum concentration of 10Be [36] and among the high- may expose basement rocks. Offshore Nicaragua, on est B/La ratios [37,38]. Numerical modelling the incoming plate prominent fault escarpments and suggests that high Be and B concentrations in lavas outcropping basement were observed in deep-tow could be caused by a wetter subducting plate under video surveys. Basement outcrops facilitate the effec- Nicaragua [5]. Seismological data support this view. tive exchange of fluids, heat and solutes between the Intraslab events at 100–150 km depth often have early crust and ocean. low-frequency arrivals followed by later high frequen- In hydrogeology of the flanks of oceanic spreading cy and high amplitude signals, interpreted to indicate centres researchers divide the regimes in the so-called dispersion caused by interaction with a low velocity bopenQ and bclosedQ systems [31]. In the open system waveguide [39]. In Honshu and Kuriles, for example, the seawater can easily enter the permeable igneous the wave groups are 5–8% slower than the surround- lava pile through outcropping basement highs, while in ing medium for both P- and S-waves. The low veloc- the closed system a thick sedimentary cover hinders ities may represent a mixture of eclogite and interaction between the ocean and the basement. untransformed gabbro, or more likely, partially hy- The water–rock ratio changes significantly between drated blueschist-facies metagabbro containing 1.5 to an open and closed system. The Nicaraguan trench 3.5 wt.% chemically bound water [39]. In Nicaragua, slope system can be interpreted as an analogue to the however, the low-velocity waveguide at the top of the open system, while the trench of NE Japan may rep- downgoing slab is ~14% slower than the surrounding resent a closed circulation system. However, although mantle. Thus, the seismic velocity anomaly of the the water–rock ratio might be higher in an open sys- Nicaragua slab is about two times the anomaly of tem, even in a closed system fluids are expected to other slabs and hence the plate is significantly wetter reside in the permeable lava pile of the oceanic litho- than the slabs entering the Honshu and the Kurile sphere. Consequently, bend-faulting may allow fluids trench [17]. The differences between the slabs might trapped within crustal pore spaces to enter the lower be related to the more extensive hydrogeological sys- crust and perhaps even the upper mantle. This mech- tem offshore Nicaragua. As a consequence, it seems anism may explain why blueschist-facies metabasalts, reasonable to hypothesize that the larger transport of which represents mid-ocean ridge basalts (MORB) water into the deep subduction zone is caused by metamorphosed under temperature–pressure condi- serpentinization when fluids migrate along faults tions of subduction zones, commonly contain more down to mantle depth and alter mantle peridotites chemically bound water than observed in altered [6,7]. Serpentinization at the outer rise may therefore basalts recovered in the oceanic crust [7,34,35]. affect the global water cycle. The importance of the reactivation of the hydro- Recent evidence from subduction zones clearly thermal circulation system and hydration of the suggest that different slabs show different pattern of oceanic lithosphere prior to subduction is perhaps arc volcanism [5] and hydration [17,39], which is to I. Grevemeyer et al. / Earth and Planetary Science Letters 236 (2005) 238–248 247 some extent related to the input and hence subduction [7] S.M. Peacock, Insight into the hydrogeology and alteration of erosion or accretion [40], outer rise bend-faulting, and oceanic lithosphere based on subduction zones and arc volcanism, in: E.E. Davis, H. Elderfield (Eds.), Hydrogeology of plate hydration [6,7]. However, the interrelation be- Oceanic Lithosphere, Cambridge University Press, 2004, tween outer rise faulting, reactivation of hydrothermal pp. 659–676. circulation, sedimentary cover, age, and the interac- [8] J.H. Bondine, A.B. Watts, On lithospheric flexure seaward of tion between the upper and lower plate (accretion the Bonin and Mariana trenches, Earth Planet. Sci. Lett. 43 versus erosion) is not well understood. We therefore (1979) 132–148. [9] D.A. Levitt, D.T. 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