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Thermal Effects of Hydrothermal Circulation and Seamount Subduction: Temperatures in the Nankai Trough Seismogenic Zone Experiment Transect, Japan

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Thermal Effects of Hydrothermal Circulation and Seamount Subduction: Temperatures in the Nankai Trough Seismogenic Zone Experiment Transect, Japan Article Volume 12, Number 12 10 December 2011 Q0AD21, doi:10.1029/2011GC003727 ISSN: 1525-2027 Thermal effects of hydrothermal circulation and seamount subduction: Temperatures in the Nankai Trough Seismogenic Zone Experiment transect, Japan G. A. Spinelli Earth and Environmental Science Department, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA ([email protected]) R. N. Harris College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA [1] We examine the thermal effects of seamount subduction. Seamount subduction may cause transient changes in oceanic crust hydrogeology and plate boundary fault position. Prior to subduction, seamounts provide high-permeability pathways between the basaltic crustal aquifer and overlying ocean that can focus fluid flow and efficiently cool the oceanic crust. As the seamount is subducted, the high-permeability pathway is closed, shutting down the advective transfer of heat. If significant fluid flow occurs, it would be restricted after seamount subduction and would result in a redistribution of heat warming the trench and cooling land- ward parts of the system. Additionally, subducting seamounts can influence the position of the plate bound- ary fault that has thermal consequences by locally controlling the proportions of incoming sediment that subduct and accrete. Shifting the décollement to the seafloor at the trench in the wake of seamount subduction causes limited cooling focused at the toe of the margin wedge. We apply these features of seamount subduc- tion to a thermal model for the Nankai Trough Seismogenic Zone Experiment transect on the margin of Japan. Models with hydrothermal circulation provide an explanation for anomalously high surface heat flux observations near the trench. They yield temperatures of 100°C−295°C for the rupture area of the 1944 Tonankai earthquake. Temperatures in the region of episodic tremor and slip are estimated at 290° C–325°C, 70°C cooler than a model with no fluid circulation. Components: 8200 words, 8 figures, 2 tables. Keywords: Nankai; hydrothermal; seamount; subduction; temperature. Index Terms: 3015 Marine Geology and Geophysics: Heat flow (benthic); 3021 Marine Geology and Geophysics: Marine hydrogeology; 3036 Marine Geology and Geophysics: Ocean drilling. Received 31 May 2011; Revised 25 October 2011; Accepted 25 October 2011; Published 10 December 2011. Spinelli, G. A., and R. N. Harris (2011), Thermal effects of hydrothermal circulation and seamount subduction: Temperatures in the Nankai Trough Seismogenic Zone Experiment transect, Japan, Geochem. Geophys. Geosyst., 12, Q0AD21, doi:10.1029/ 2011GC003727. Theme: Mechanics, Deformation, and Hydrologic Processes at Subduction Complexes, With Emphasis on the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Drilling Transect Guest Editors: D. Saffer, H. Tobin, P. Henry, and F. Chester Copyright 2011 by the American Geophysical Union 1 of 15 Geochemistry Geophysics 3 SPINELLI AND HARRIS: NANKAI SUBDUCTION ZONE TEMPERATURES 10.1029/2011GC003727 Geosystems G 1. Introduction [4] In addition to their influence on the hydro- thermal system, subducting seamounts disrupt the [2] Seamounts approaching and entering a sub- margin, leading to landslides and erosional embay- duction zone likely modify the thermal state of the ments [von Huene et al., 2000] as well as fracturing region. Seamounts are an important control on the the overriding margin [Wang and Bilek, 2011]. thermal state of oceanic lithosphere due to their Importantly for the thermal regime, seamount sub- influence on hydrothermal circulation [Villinger duction may also transiently move the décollement et al., 2002; Fisher et al., 2003b; Fisher and Wheat, upward, locally altering the ratio of accreting and 2010]. Seamounts and other basement outcrops subducting sediments. By moving the décollement provide high-permeability regions of recharge or upward, seamounts may locally increase the vol- discharge, influencing region-scale fluid circulation ume of subducting sediments and erode the upper that can affect the thermal state of surrounding plate from below [Dominguez et al., 2000]. Because oceanic crust [Mottl et al., 1998; Fisher et al., the recently deposited trench sediments are rela- 2003a, 2003b; Harris et al., 2004; Hutnak et al., tively cold, this effect may also cool the shallow 2006, 2008; Wheat and Fisher, 2008]. “Venti- subduction zone. The physical disruption of the lated” fluid circulation, where fluids freely circulate margin wedge by seamount subduction may also between the ocean and the crust, can efficiently cool alter the permeability structure of the wedge. oceanic lithosphere, locally decreasing the heat The permeability of subduction zone faults [Bekins content of the plate. This ventilated circulation et al., 1995; Fisher et al., 1996; Saffer and Bekins, occurs as fluids circulate laterally between basement 1998; Screaton et al., 2000] is typically 4–8 orders outcrops that serve as regions of recharge and dis- of magnitude lower than the permeability of the charge. If fluid flow velocities are sufficient for basaltic basement aquifer of oceanic crust [Becker efficient heat extraction, the oceanic lithosphere is and Davis, 2004]. However, transient increases cooled. Sediment cover acts as a low-permeability in fault zone permeability are possible [e.g., Bekins cap to flow within the underlying basaltic basement et al., 1995; Saffer and Bekins, 1998], and associ- of oceanic crust [Spinelli et al., 2004]. Where sedi- ated transient pulses of fluid flow can advect heat mentary cover inhibits the advective heat exchange [e.g., Fisher and Hounslow, 1990]. between the basaltic basement aquifer and the [5] We develop general thermal models to examine ocean, “insulated” flow conditions in the underlying some of the thermal effects of seamount subduc- basaltic basement exist [Fisher, 2004]. tion. We examine the potential influence on ocean [3] Individually, both ventilated and insulated pat- crust hydrogeology of a seamount approaching and terns of heat transfer in oceanic lithosphere can entering the subduction zone. We also examine the have a profound impact on the thermal state of a thermal effect of moving the décollement position subduction zone. Offshore Cape Muroto on the in response to seamount subduction. Finally, we Nankai margin of southern Japan, insulated hydro- model temperatures in the subduction zone in the thermal circulation in the incoming and subducting NanTroSEIZE transect, including potential thermal oceanic crust efficiently transfers heat from the effects of the recent subduction of a seamount in the deeper subduction zone to the trench and incoming region. plate [Spinelli and Wang, 2008]. Offshore Costa Rica, the thermal state of the subduction zone 2. Geological Setting of the in adjacent regions with and without prominent NanTroSEIZE Transect seamounts on the incoming plate are affected by ventilated and insulated hydrothermal circulation, [6] Along the Nankai margin, the Philippine Sea respectively [Harris et al., 2010]. As a fairly iso- plate subducts beneath Japan at 4.6 cm yr−1 [Seno lated seamount approaches then enters a subduction et al., 1993]. Subduction along this margin has been zone, the regional fluid flow regime may transition active for the past 15 Ma [Okino et al., 1994]. from ventilated to insulated circulation as the high- Magnetic lineations and oceanic lithosphere iso- permeability conduit (i.e., seamount) between the chrons are roughly parallel to the convergence basaltic basement aquifer and ocean is eliminated direction, perpendicular to the trench axis [Okino (i.e., subducted) from the system. This scenario has et al., 1994]. The Nankai Trough Seismogenic Zone yet to be explored, but may apply to the well-studied Experiment (NanTroSEIZE) transect is 200 km NanTroSEIZE transect on the Nankai margin of northeast of the axis of a fossil spreading center; Japan. currently, 20 million year old oceanic lithosphere 2of15 Geochemistry Geophysics 3 SPINELLI AND HARRIS: NANKAI SUBDUCTION ZONE TEMPERATURES 10.1029/2011GC003727 Geosystems G is subducting in this transect. The oceanic litho- 2005], estimates from the depth to a gas hydrate sphere subducting in the NanTroSEIZE transect has related bottom simulating reflector [Kinoshita et al., aged from 5 Ma at the initiation of subduction to 2011], and temperature gradients in boreholes on 20 Ma today [Okino et al., 1994]. At the defor- land [Yamano et al., 2003]. mation front, sediment on the incoming plate is [8] Along the entire Nankai margin, an anomalously 2400 m thick [Ike et al., 2008; Moore et al., 2009]. “hot trench” has been a long-standing observation Despite the thick incoming sediment section, cur- [Yamano et al., 1984]. Similar to other parts of the rently all of the incoming sediment is subducting margin, the incoming plate slightly seaward of the [Moore et al., 2009]. In a global analysis, Clift trench in the NanTroSEIZE transect is characterized and Vannucchi [2004] found that sedimentary by high heat flux (Figure 2) [Watanabe et al., 1970; accretion is favored in regions of slow conver- − Yamano et al., 1984, 2003; Kinoshita et al., 2008]. gence (<7.6 cm yr 1) and thick sediments (>1 km). Heat flux decreases landward of the trench, consis- This complete sediment subduction is likely a recent tent with the subduction of low-temperature oce- phenomenon, as young sediment comprises the anic lithosphere relative to the overriding plate. margin wedge in
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