CONTROLS ON STRUCTURAL STYLES IN THE EASTERN SIERRA MADRE ORIENTAL

Analysis of ASTER and SRTM data from the eastern portion of the Sierra Madre Oriental fold belt between longitudes of 99º W and 100º W depicts two contrasting regions in their structural trends. The western portion of the fold belt is characterized by salients and recesses while the eastern portion of the fold belt is characterized by north-northeast trending linear structures. The boundary between these two regions is clearly defined on regional geologic maps, ASTER, and SRTM images. This boundary terminates against volcanics to the south and the frontal portion of the fold belt to the north. Field work conducted along the northern portion of the boundary zone near Zaragosa shows the contrasting styles of deformation are controlled in part by their stratigraphic sequences involved in the fold belt. West of the boundary, the fold belt is detached along Jurassic evaporite deposits. To the east of the boundary, no evaporite deposits were observed. Absence of evaporites, and the linear nature of fold belt trends east of the boundary suggest that contractional deformation occurred over no or substantially thinner evaporites.

Previously published Bouguer gravity data shows a gradient of increasing values towards the east suggestive of crustal thinning. In order to explain the contrasting structural trends and gravity data we propose a model that incorporates a late Triassic-Early Jurassic rift basin to the west of the boundary. We interpret thick sequences of evaporite deposits accumulated within the basin. During contractional deformation a rift bounding fault was reactivated as a footwall ramp to an east-directed thrust fault. Structures formed below the ramp developed curvilinear geometries, while structures formed above the ramp developed linear geometries. Future work includes gravity surveys and structural studies along the northern portion of the boundary to constrain subsurface character of the fold belt.