Controls on the deformation produced by oblique inversion of rift basins: Which structures reflect the paleostrain state?

摘要

Many inverted rift basins have undergone oblique deformation, making determination of the strain state during inversion problematic. Scaled experimental clay models show that obliquity during both tectonic episodes influences the final deformation pattern. The angles between the displacement direction and the rift trend, or preexisting zone of weakness, for both the extensional phase and the contractional phase are alphae and alpha c, respectively. The extensional phase of deformation produces a fault zone that parallels a preexisting zone of weakness, with secondary normal faults subperpendicular to the displacement direction. With initial orthogonal extension, all major through-going faults reactivate, regardless of the displacement direction during inversion. Most new structures are parallel to the rift trend, making their orientations unreliable strain-state indicators.; With initial oblique extension, en-echelon normal faults show varying degrees of reactivation, depending on their orientation relative to the displacement direction. New structures are more likely to form (relative to experiments with initial orthogonal extension) that reflect the strain state during inversion, primarily for alphac≤45°. For alphac≤30°, both new and reactivated faults have a large strike-slip component and no rift-trend-parallel folding occurs. For alphac≥45°, both new and reactivated faults have a large reverse-slip component and large-scale, rift-trend-parallel folding occurs. The clay models suggest that the inversion deformation pattern is more sensitive to variations in alphae than to variations in alphac.; Seismic mapping in the obliquely inverted Fundy rift basin in Maritime Canada demonstrates that most inversion-related folds trend subparallel to the preexisting border faults, regardless of fault strike. Therefore, most inversion-related fold geometries are controlled by the preexisting extensional fault geometries and do not reflect the inversion strain state. However, a few anomalously trending folds suggest that major fault hanging-wall blocks were displaced to the northeast relative to the footwall blocks. Field evidence from the Fundy basin supports this strain state.; Comparing these models to previous studies involving dry sand highlights the rheological differences between wet clay and dry sand. The differences suggest that wet clay may be a more appropriate modeling material for studies of basin inversion. Comparison to the Fundy and Newark rift basins constrains the strain state during inversion of these basins. This strain state suggests that, during the initial stages of seafloor spreading, the rifted portion of present-day northeastern North America moved northeast relative to the more stable North American craton.