Heat flow in the Southern Chile Forearc controlled by large-scale tectonic processes

From north to south, the Southern Chile forearc is affected by the subduction of the aseismic Juan Fernandez Ridge, a number of major oceanic fracture zones on the downgoing Nazca Plate, the active Chile Ridge spreading center, and underthrusting of the Antarctic Plate. The tectonic structure is characterized by intense deformation of the lower continental slope within a variably wide accretionary wedge. In places the middle and upper slope is affected by out-of-sequence overthrusting and by normal faulting. In the area of the Chile Triple Junction at 46°S latitude most of the forearc is destroyed by subduction erosion, to be rebuilt further south by sediment offscraping and accretion from the Antarctic Plate. The Southern Chile forearc has been intensively explored by reflection seismic surveys, and has been drilled by the Ocean Drilling Program during two expeditions (ODP Leg 141 - Behrmann et al., 1992; ODP Leg 202 – Mix et al., 2003). The widespread occurrence of gas hydrates has been known for some time. Regarding the analysis of reflection seismic sections we have used data of R/V SONNE Expeditions 101 and 161, R/V VIDAL GORMAZ Expeditions VG02 and VG06, and R/V ROBERT CONRAD Cruises RC2901 and RC2902. Using bottom water temperature data obtained from the World Ocean Data Base (NOAA) and an acoustic velocity model constrained from the seismic sections, and measurements of temperature, thermal conductivity and acoustic velocity from ODP boreholes, we use the position of the Bottom Simulating Reflector (BSR) in reflection seismic sections to estimate the heat flow through the forearc in an area between 32°S and 47°S latitude. Heat flow in most of the upper and middle continental slope is on the order of 50-80 mWm-2. This is normal for continental basement and overlying slope sediments, and is true also for those parts in the south of the area that are being underthrusted by hot, young oceanic crust. The middle and lower slopes, however, in some places display up to 50% increased heat flow. Here the sea floor is underlain by zones of active deformation and accretionary wedge building. This observation cannot be easily reconciled with models of conductive heat transfer, but is an indication that advecting pore fluids from deeper in the subduction zone may transport a substantial part of the heat there. The size of the anomalies indicates that fluid advection and outflow at the sea floor is diffuse rather than being restricted to individual fault structures, or mud volcanoes and mud mounds, as is the case in other convergent margins. A large area with higher heat flow correlate in space with tectonic phenomena, however. On the lower slope above the subducting Chile Ridge at 46°S, values of up to 280 mWm-2 indicate that the overriding South American Plate is effectively heated by subjacent zero-age oceanic plate material on a regional scale. References Behrmann, J.H., Lewis, S.D., Musgrave R., et al. (1992) Proceedings of the Ocean Drilling Program, Initial Reports, 141. Ocean Drilling Program. College Station, TX Mix, A.C., Tiedemann, R., Blum, P., et al. (2003) Proceedings of the Ocean Drilling Program Initial Reports, 202. Ocean Drilling Program, College Station, TX.