Ten-million-year history of a thrust sheet

Abstract

The final, deformed state of a fold-and-thrust belt may be reached by an infinite number of kinematic paths. Two end-member kinematic paths are due to continuous or discontinuous rates of deformation. We have used a new magnetostratigraphic section from the Spanish Pyrenees to calibrate the emplacement history, over ≈10 m.y., of a major thrust sheet (the Sierras Marginales thrust sheet) and the deformation of both its hanging wall and footwall. Six time windows from before 36.5 Ma until after 24.7 Ma were recognized on the basis of structural and stratigraphic relationships between syntectonic strata and major structures. Footwall deformation of the Sierras Marginales thrust sheet occurred continuously on a detachment within the foreland-basin sequence (4.5 km net shortening; shortening rates steadily increased from 0.14 to 1.5 mm/yr with time). Although the detachments at the base of the foreland and the detachment at the base of the Sierras Marginales thrust sheet were active coevally, the rate of displacement on the Sierras Marginales detachment decayed with time and shows considerable variability. Emplacement of the thrust sheet may be divided into three distinct periods: a rapid 13.8 km translation from 37.0 to 36.5 Ma (27.6 mm/yr), a gradual climb of the toe of the thrust sheet up a 4.3-km-long ramp across the foreland-basin succession from 36.5 to 32.0 Ma (0.95 mm/yr), and a final 8.9 km translation from 32.0 to 29.5 Ma (3.56 mm/yr). Internal deformation of the thrust sheet occurred only after it reached its present position at some time before 29.5 Ma. Shortening rates steadily decreased from 0.6 mm/yr between 29.5 and 27.8 Ma to 0.26 mm/yr from 27.8 until after 24.7 Ma. Only ≈1 km of shortening accumulated during each of the two periods; the last shortening localized on the most northerly thrust in the study area. Folding and subsequent faulting above the detachment beneath the foreland suggest that slip was transmitted to its tip point continuously throughout the deformation. In contrast, translation followed by internal deformation on the hinterland side of the toe of the Sierras Marginales thrust sheet indicates a successive deactivation of the southern parts of the detachment with time. Rate of deformation on both detachments was discontinuous and shows substantial variability about the mean. In general, the spatial and temporal pattern of deformation was distributed and continuous. In detail, however, shortening was spatially and temporally discontinuous above each detachment, and structures related to each one display distinctly different deformational patterns, rates, and styles.