10-MILLION-YEAR HISTORY OF A THRUST SHEET

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 u sed a new magnetostratigraphic section from the Spanish Pyrenees to ca librate the emplacement history, over approximate to 10 m.y., of a maj or thrust sheet (the Sierras Marginales thrust sheet) and the deformat ion of both its hanging wall and footwall. Six time windows from befor e 36.5 Ma until after 24.7 Ma were recognized on the basis of structur al and stratigraphic relationships between syntectonic strata and majo r structures. Footwall deformation of the Sierras Marginales thrust sh eet occurred continuously on a detachment within the foreland-basin se quence (4.5 km net shortening; shortening rates steadily increased fro m 0.14 to 1.5 mm/yr with time). Although the detachments at the base o f the foreland and the detachment at the base of the Sierras Marginale s thrust sheet were active coevally, the rate of displacement on the S ierras 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 (2 7.6 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, Shortenin g 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 M.a. Only approximate to 1 km of shortening accumulated during each of the two periods; the last short ening localized on the most northerly thrust in the study area. Foldin g and subsequent faulting above the detachment beneath the foreland su ggest that slip was transmitted to its tip point continuously througho ut the deformation. In contrast, translation followed by internal defo rmation on the hinterland side of the toe of the Sierras Marginales th rust sheet indicates a successive deactivation of the southern parts o f the detachment with time. Rate of deformation on both detachments wa s discontinuous and shows substantial variability about the mean. In g eneral, the spatial and temporal pattern of deformation was distribute d and continuous. In detail, however, shortening was spatially and tem porally discontinuous above each detachment, and structures related to each one display distinctly different deformational patterns, rates, and styles.