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A 220-km-long, single-channel seismic reflection profile crosses the northern Sumatra margin and presumed rupture zone of the December 2004 Mw9.2 tsunamigenic earthquake and images active deformation across the forearc. At the largest wavelength (tens of kilometers), the forearc surface is defined by a steep, 55-km-wide outer slope, a 110-km-wide upper slope forming a broad depression between two forearc highs, and a 25-km-wide steep inner slope between the landward high and forearc basin. Superimposed on these prism-wide variations are anticlinal ridges spaced ~13 km apart; the inner and outer slopes are characterized by landward and seaward fold vergence, respectively. Between anticlines, growth strata deposited in slope basins are folded at ~2-3 km wavelengths. These small folds deform the seafloor and increase in amplitude with depth, verging toward anticlinal hinges. We suggest that long-wavelength variations are consistent with variations in strength across the forearc. The ~13 km anticline spacing implies deformation of a slope apron that deforms independently of a stronger wedge interior. Growth strata geometries indicate ongoing deformation within individual basins. Our model for prism architecture suggests that the wedge interior advances during great earthquakes like the 2004 M!9.2 event, peeling up shallower and less competent trench fill, deforming the toe and the upper slope of the forearc, and producing seabottom uplift responsible for the tsunami. [ABSTRACT FROM AUTHOR]

In this paper, stratigraphy, geochronology, and geologic mapping are used to characterize a sequence of Quaternary deposits associated with edifice failure of Barú Volcano, which, together with balanced cross sections, illustrate the upper plate's response to along-strike variations in subduction properties that occur across the tear in the subducting slab located at the Panama fracture zone. The subducting Panama fracture zone is an active transform that separates disparate styles of subduction between the thick, rapid, and flat subduction of the Cocos plate to the west and the thinner, more oblique, and steeper subduction of the Nazca plate to the east. We focus on the arc-forearc region above the Cocos-Nazca slab tear, where both the Fila Costeña inner forearc fold-and-thrust belt and the exhumed Cordillera de Talamanca terminate along strike to the southeast. The Fila Costeña thrust belt imbricates an Eocene-late Miocene forearc basin sequence with up to 40 km of shortening inboard of Cocos plate subduction and crosscuts a sequence of Pleistocene and younger volcano-sedimentary units on the southwestern flanks of the active Barú Volcano at its southeastern termination. These units, constrained in age by radiocarbon dating (n = 11), soil color, and 40Ar/39Ar (n = 5) data, include a sequence of late Pleistocene to Holocene debris-avalanche and lahar deposits. Landscape morphology, the areal distribution of units, and new shortening estimates from balanced cross sections collectively suggest that the southeastern termination of the Fila Costeña thrust belt ac- tively propagates to the southeast coeval with migration of the Panama triple junction. The prevailing map pattern suggests that the forearc's response to southeastward migration of the slab tear varies with distance to the trench. In contrast to deformation patterns in the outermost forearc, plate-boundary tractions associated with shallow subduction and Cocos-Caribbean convergence are more influential in the development of arc-forearc deformation than either oblique subduction of the ~2 km bathymetric scarp, or right-lateral shear, which both occur astride the subducting Panama fracture zone. [ABSTRACT FROM AUTHOR]