COSEISMIC SIGNATURE OF FOLD GROWTH IN STRATA AND TERRACES, SIERRA DE VILLICUM, SAN JUAN, ARGENTINA

New evidence for fold growth during individual earthquakes is documented with growth strata and deformed geomorphic surfaces on the eastern flank of Sierra de Villicum; an active structure in San Juan Province, Argentina situated between the Andean thin-skinned fold-and-thrust belt and the thick-skinned Sierras Pampeanas structural province to the east. Accumulated deformation in repeated earthquakes is recorded by the structural geometry of deformed geomorphic surfaces. Structural style of active folding was characterized via geologic mapping and topographic surveying of six strath terraces beveled into the eastern flank of Sierra de Villicum (T1-6). A comparison between profiles of modern channels and terraces indicates that recent deformation is restricted to the eastern half of the study area and is characterized by monoclinal folding of each terrace. A ~3° east-dipping western limb and a ~ 15-18° east-dipping eastern limb comprise each monocline. Fold axes of sequentially younger terraces are located systematically to the east of the axis formed in the penultimate surface. This fold style has created a stepped topography that is interpreted to reflect eastward migration of an active axial surface relative to growth axial surfaces represented by fold axes in individual terraces. Correlation of the terraces of this study with 18.7, 6.8, and 1.5 ka BP surfaces preserved to the south of the study area dated using cosmogenic radionuclides allows for calculation of fold growth and shortening rates. Differential height between strath elevation and modern channels and horizontal offset between each growth axial surface and the active axial surface were measured for each terrace level. Minimum and maximum uplift and shortening rates are 0.7 to 1.25 mm/yr and 2.6 to 4.6 mm/yr, respectively, and depend on whether T1 or T2 correlates with the 18.7 ka surface. T6, the youngest surface, is tilted ~18° to the east and the strath is uplifted ~0.4 m, roughly the same amount as coseismic surface rupture of the nearby La Laja flexural slip fault in the 1944 Ms. 7.4 San Juan earthquake. The fold style in the youngest surface is the same as that in each of the older terraces and thus represents one of the few examples in which the stratigraphic and geomorphic consequences of coseismic fold growth can be tied to the long-term evolution of a fold.