Tectonostratigraphic model of the Fila Costeña in southern Central America based on new biostratigraphic data: implications of a two-thrust-fault hypothesis

Abstract

Biostratigraphic determinations in the surroundings of the Térraba River, Fila Costeña, Costa Rica, lead to a two-thrust-fault model to explain the distribution of sedimentary rocks. Eocene limestones of the Fila de Cal Formation appear as a single succession with a NE-SW predominant strike, and a thrust fault is required to explain their presence above other Oligocene rocks. Another thrust fault explains the outcrop of Oligocene rocks above presumable Miocene strata. A structural model based on surface data suggests a shallower basement to the southwest and deeper to the northeast; both parts are connected by a frontal thrust ramp, an oblique ramp (close to frontal) and a lateral ramp. In that sense, the superficial trend of thrust faults is interpreted as the result of slip along these thrust ramps. Based on these assumptions, minimum NE-SW shortening estimations vary between 8.3 and 22.2 km in the study area. This underlines the importance of geologic fieldwork and biostratigraphic determinations for structural modeling and minimum shortening estimations of the Fila Costeña and other sedimentary successions for which these approaches are possible.