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The combination of logging, and downhole temperature measurements at Site 859 (and to a lesser extent Site 861) suggest that the composition of the hydrates forming the regionally distributed bottom simulating reflector in the Chile Triple Junction region may be close to a seawater/methane composition. Direct calibration of the pressure-temperature condition at the BSR has allowed us to extend our downhole measurements laterally. We focus much of our discussion in this paper on the transect of holes (Site 859, 860, and 861) along seismic Line 745. This transect characterizes the thermal regime existing in the wedge just prior to ridge subduction. Temperature gradients and heat flows were found to be lower, and the temperature profiles dominantly linear, in the older regions of the wedge (i.e., Site 861). In contrast, higher temperature gradients and highly non-linear profiles are found at the toe of the wedge (i.e., Sites 859 and 863). The major regional increase in the geothermal gradient and heat flow towards the toe of the wedge is also clearly shown by the shallowing in depth to the BSR across the transect. This is proposed to stem from a combination of the greater heat input to the base of the toe of the wedge, due to its closer proximity to the subducting spreading ridge, and the lateral expulsion of warm fluids. The laterally expelled fluids cause disturbances in both temperature and pore water chemistry profiles and discrepancies in shallow and deep heat flow estimates. Hydraulic disturbances are particularly evident within 5 km of the toe of the wedge where much of the fluid expulsion is proposed to be focused. Near the toe, the hydrogeologic system is also spatially heterogeneous, may be effected by both wedge and oceanic basement components, and appears to change episodically on a short time scale. This makes quantitative estimates of heat budgets difficult to constrain. It is possible that a significant amount of the heat input to the base of the wedge from the oceanic basement is subsquently expelled at the accretionary toe; carried there by the fluids migrating along fault zones. INTRODUCTION Tectonic and Thermal Environment The Chile Triple Junction (CTJ) is one of only two world-wide examples where an oceanic/transform spreading axis is currently being subducted beneath continental lithosphere. Ridge subduction has, however, formed a part of the history of most of the western Pacific subduction zones and the study of this currently active system can be used to constrain on land investigations of the effects of these paleo-ridge subduction events. Subduction of the ridge beneath South America began in the south near Tierra del Fuego and has migrated north to its current position near the Taitao peninsula. Hiatuses in ridge subduction occur during the underthrusting of fracture zones that offset the ridge axis (Cande et al., 1987). Ocean Drilling Program (ODP) Leg 141 studied the thermal structure of the portion of the wedge just to the north of the triple junction. This section is currently overthrusting the segment of the Chile oceanic spreading center situated between the Darwin and Taitao fracture zones from 45°45' to 46°18'S(Fig. 1). Significant accretion and tectonic thickening has not occurred south of the Darwin Fracture Zone (DFZ) in the region drilled during ODP Leg 141 since Pleistocene times (Fig. 1). Here it seems the thin cover of sediments (-100 m) on the subducted plate bypass the toe and become thrust deep beneath the wedge. Indeed, substantial subduction erosion is probably occurring in this region (Behrmann et al., in press.; Cande, et al., 1987). The transect of drilling sites (Sites 859, 860, and 861) provides a characterization of the tectonic environment just prior to spreading ridge subduction and are located within the Lewis, S.D., Behrmann, J.H., Musgrave, R.J., and Cande, S.C. (Eds.), 1995. Proc. ODP, Sci. Results, 141: College Station, TX (Ocean Drilling Program). 2 Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0220, U.S.A. 3 Institute for Geophysics, University of Texas at Austin, Austin, TX 78759-8397, U.S.A. accretionary wedge predominantly along Conrad Line 745 (Fig. 1). The line is approximately 10 km south of the DFZ and 18 km north of the present triple junction location. On Line 745 the axis of the spreading center lies ~5 km west of the accretionary toe and is expected to be thrust beneath the toe of the wedge in ~ 100 to 200 k.y. Very high heat flows (greatly in excess of 250 mWm~) were expected from this practically zero age crust. In contrast, Leg 141 Site 863, located 18 km south of Line 745, is drilled into the toe of the accretionary wedge, directly over the subducting spreading center (Fig. 1). As will be shown herein, there is evidence that the heat flow data obtained during ODP Leg 141 are strongly influenced by fluid migration. Oceanic spreading centers are noted for their generally high heat flows and vigorous convective hydrogeologic systems that are driven by the cooling of the new basaltic material (e.g., Sclater et al., 1980). Accretionary wedges also are hydrogeologically active regions. In accretionary wedges, warm fluids having exotic geochemistries commonly migrate laterally long distances along faults as they are expelled out of the deep regions of the subducted sedimentary pile (Carson et al., 1990; Gieskes et al., 1990; Moore et al., 1991). The common association of thermal and geochemical anomalies with faults suggests that faults form preferred pathways due to their higher permeabilities (Moore et al., 1991). Our data indicates that the progressive underthrusting of a vigorously convective hydrothermal system, associated with the spreading ridge beneath a hydrogeologically active accretionary wedge, produces a complex, dynamic thermal, and hydrogeologic regime in the Leg 141 region. Gas Hydrates Bottom Simulating Reflectors (BSRs) are frequently seen in seismic sections several hundred meters below the seafloor. The BSR is taken to mark the base of the pressure temperature (PT) stability field for methane clathrate hydrate because BSRs are thought to relate to the impedance contrast between hydrate-bearing and hydrate-free sediments, which lie correspondingly above and below the hydrate stability field (Bangs et al, 1993; Hyndman et al., 1992; Kvenvolden, K.M. BROWN, N.L. BANGS

Thermal Regime of the Chile Triple Junction: Constraints Provided by Downhole Temperature Measurements and Distribution of Gas Hydrate