Simple-shear conjugate rift margins of the Argentine Precordillera and the Ouachita embayment of Laurentia
The Argentine Precordillera was rifted from the Ouachita embayment of Laurentia during Cambrian time. The Ouachita rifted margin along the Texas promontory of Laurentia exhibits a narrow zone of transitional crust, a lack of synrift rocks, and a thin passive-margin succession, indicating slow and limited postrift subsidence. In contrast, data from the western margin of the Precordillera suggest more extensive synrift sediment accumulations and document a thick passive-margin succession, indicating more rapid and relatively greater postrift subsidence than along the Ouachita margin of Laurentia. Passive-margin deposition began in latest Early Cambrian time around the Precordillera, but it did not begin before latest Middle Cambrian time around the Ouachita embayment. The contrasts in structure and stratigraphy are best explained by an asymmetric rift system in the context of simple-shear, low-angle–detachment models for continental rifting. Differences in subsidence rates and inferred crustal structure suggest that the rifted margin of the Ouachita embayment represents an upper-plate configuration, whereas the conjugate margin on the western Precordillera represents a lower-plate configuration.