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

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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 seis- mic 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
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