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BSRs at the Peruvian convergent margin: their regional distribution
and constraints for the thermal field and seismogenic zone
Nina Kukowski 1 (+49 331 288 1318; nina@gfz-potsdam.de) Ingo A Pecher 2 (ingo@ig.utexas.edu) Carolyn Ruppel 3 (cdr@piedmont.eas.gatech.edu) 1Geoforschungszentrum Potsdam, Telegrafenberg, Potsdam D-14473, Germany 2Univ. of Texas Inst. for Geophysics, 4412 Spicewood Springs Rd., Bldg. 600 , Austin, TX 78759-8500, United States 3School of Earth and Atmospheric Sciences, Georgia Tech, Atlanta, GA 30332-0340, United States The availability of abundant methane produced by degradation of organic carbon favor the formation of gas hydrate across large portions of the Peruvian active margin. However, seismic data across the margin indicate a complex distribution of bottom simulating reflectors (BSRs), which are believed to mark the top of the gas zones in gas hydrate provinces. BSRs are present on the lower slope but largely absent in the upper-slope Lima Basin. Earlier work has shown that rapid subsidence of Lima Basin may have caused a downward movement of the base of the gas hydrate stability zone (BGHS) into the underlying free gas zone. This should lead to absorption of free gas into the gas hydrate zone (GHZ) and consequent destruction of BSRs. At a few locations in Lima Basin, however, BSRs are present beneath an apparently downward-migrating BGHS. This can be explained by elevated methane flux into the GHZ. Our quantitative models for the evolution of the GHZ during subsidence of the Lima Basin indicate that methane flux into the GHZ from below would have to increase by $\sim$25\% during subsidence for a BSR to form. While the distribution of BSRs in Lima Basin appears to be related to subsidence history and methane flux, variations in BSR depth along the lower slope reflect heat flow variations. We simulated the heat flow pattern offshore Peru with numerical models of coupled heat and fluid flow. Local shoaling of the BSR on the slope can be explained by focused fluid flow along fault zones. Our models require significant fricitional heating along the d\'{e}collement in order to simulate the regional thermal field. This is in agreement with results of a mechanical analysis of this margin and has important implications for the nature of the seismogenic zone. The predicted 150$\rm ^o$ C isotherm, which is often assumed to be the upper boundary of the seismogenic zone, intersects the plate boundary $\sim$30-35 km arc-ward of the trench. This location roughly coincides with the seaward boundary of aftershocks from the 1974 Mw 8 seismic event. Hence, our numerical models of coupled heat and fluid flow are apparently successful in predicting not only BSR distribution, but also the upper boundary of the seismogenic zone on the Peruvian convergent margin. |
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Last Modified: October 8, 1999
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