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Abstract. During the Chile triple junction (CTJ) cruise (March-April 1997), EM12 bathymetry and seismic reflection data were collected in the vicinity of the Chile triple junction (45-48øS), where an active spreading ridge is being subducted beneath the Andean continental margin. Results show a continental margin development shaped by tectonic processes spanning a spectrum from subduction-erosion to subduction-accretion. The Andean continental margin and the Chile trench exhibit a strong segmentation which reflects the slab segmentation and the Chile triple junction migration. Three segments were identified along the Andean continental margin: the presubduction, the synsubduction, and the postsubduction segments, from north to south. Both climate-induced variations of the sediment supply to the trench and the tectonic reorganization at the Nazca-Antarctica plate boundary involving postsubduction ridge jump are the two main factors that control the tectonic regime of this continental margin. Along the survey area we infer the succession of two different periods during the last glacial-interglacial cycle: a glacial period with ice-rafted detrital discharges restricted to the shoreline area and low river output and a warmer period during which the Andean ice cap retreat allowed the Andes to be drained off. During these warm periods, rapid increase in trench deposition caused the margin to switch from subductionerosion or nonaccretion to subduction-accretion: (1) along the presubduction segment after the last deglaciation and (2)

Abstract. The heat flow map derived from 550 measurements collected in a the southern portion of the sedimented rift in Middle Valley, northern Juan de Fuca Ridge, displays kilometersized quasi-circular regions of very high heat flow. Some of these domains, explored during Ocean Drilling Program (ODP) leg 139, are thought to be discharge zones of large-scale hydrothermal plumes. To understand this unique data set, we modeled the kilometer-scale hydrothermal circulation within both the sedimentary and the igneous crust, using a set of two-and threedimensional models that use a numerical technique based on horizontal spectral decomposition of the flow equations. These models include variations in the viscosity and density of the hydrothermal fluids with temperature. We examine the variations in flow patterns due to different permeability-versus-depth distribution within sediment and pillow layers. Models with the same permeability in both layers do not match the seafloor heat flow field in Middle Valley. When the permeability decreases from the bottom to the top of the simulation domain by a factor greater than 20, convection assumes a plume pattern to produce surface heat flow comparable to that observed in Middle Valley. Within the models the ratio of the heat flux above the recharge and discharge domains is directly related to the vertical harmonic mean of the permeability field. A value of 7 X 10 -16 m 2 provides a good match to the heat flow observations. The Darcy velocities of the hydrothermal fluids in the discharge areas approach 16 crn/yr while in the recharge areas they are lower than 3 crn/yr. These rates and the temperature inside the plumes are sufficiently high to produce the observed massive sulfide deposits and mineral alterations in 1-2 X 105 years.The dynamic pressure produced by the hydrothermal flow matches the pressure measured in drill sites. This process may play a major role in compaction, fracturing, and uplift of the sediment cover. For example, the dynamic pressure in the ascending plume equals the lithostatic pressure at a depth of 50 m. Resulting hydrofracturing could explain the genesis of the vent fields associated with the hydrothermal discharge

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