Experimental Modeling of Orthogonal and Oblique Basin Inversion: Identifying Structures That Reflect the Paleostrain State

Abstract

Basin inversion involves two phases of deformation: an initial extensional phase followed by a shortening phase. Scaled experimental models show that obliquity during both phases of deformation influences the final deformation pattern. In our models, wet clay covers two overlapping metal plates: one fixed and one mobile. The edge of the fixed plate represents a preexisting zone of weakness (PZW). The angles between the displacement direction and the PZW for the extensional and shortening phases are aE and aS, respectively. The extensional phase produces a fault zone that parallels the PZW, with secondary normal faults subperpendicular to the displacement direction. With initial orthogonal extension (αE=90°), all major through-going faults are reactivated, regardless of αS during inversion. Most new faults and folds produced during inversion are parallel to the PZW; thus, their orientations are unreliable strain-state indicators. However, with highly oblique shortening (αS≤30°), extensional fault-displacement folds are amplified and rotated; their sense of rotation, along with minor pull-apart and restraining structures, are consistent with the sense of shear during inversion. With initial oblique extension (αE=45°), en-echelon normal faults show varying degrees of reactivation, depending on their orientation relative to the displacement direction during shortening. Compared to models with αE=90°, new faults (parallel or normal to displacement direction) are more likely to form that constrain the strain state, especially for αS≤30°. For αS≥45°, the first-order deformation pattern is similar to that observed in the models with initial αE=90°; both new and reactivated faults have a large reverse-slip component and large-scale folds form parallel to the PZW. We conclude that inversion-related fault geometries in map view are more sensitive to variations in aE than to variations in αS. In contrast, fault geometries in cross-section view are more sensitive to variations in αS than to variations in αE; specifically, the apparent dip angles of faults exhibiting reverse separation increases as aS decreases. Comparison of the modeling results to the Mesozoic Fundy and Newark rift basins of eastern North America constrains the strain state during inversion of these basins. This strain state suggests that, during post-rift deformation, the rifted part of present-day northeastern North America moved northeastward relative to the more stable North American craton.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016