Active thrusting, landscape evolution, and late Pleistocene sector collapse of Baru Volcano above the Cocos-Nazca slab tear, southern Central America

摘要

In this paper, stratigraphy, geochronology, and geologic mapping are used to characterize a sequence of Quaternary deposits associated with edifice failure of Baru 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 Costena inner forearc fold-and-thrust belt and the exhumed Cordillera de Talamanca terminate along strike to the southeast. The Fila Costena 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 Baru Volcano at its southeastern termination. These units, constrained in age by radiocarbon dating (? = 11), soil color, and ~(40)Ar/~(39)Ar in = 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 Costena 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.