Testing inverse kinematic models of paleocrustal thickness in extensional systems with high‐resolution forward thermo‐mechanical models

Abstract

Reconstructing continental paleocrustal thickness is important for estimating tectonic accommodation, constraining three‐dimensional basin geometry during early rifting phases of extensional margins and predicting the distribution of thick crustal sills that may block the global ocean and create restricted basins. We test an inverse kinematic method for modeling paleocrustal thickness by inverting the final bulk crustal structure produced from high‐resolution thermo‐mechanical models of lithospheric extension. The inverse kinematic method assumes pure shear, includes simple rules based on geodynamic models and field observations and requires displacement boundary conditions and the prescription of a transition from rigid to nonrigid deformation. The inverse pure‐shear method produces a history of bulk crustal thickness that closely matches the forward models provided that the width of the rift zone is narrow during the later phases of continental extension when crust undergoes hyper‐extension. We also observe that the width and surface trace of large‐scale (LS) shear zones observed in the thermo‐mechanical models coincide with inflection points and large gradients in inverted nonrigid velocity field. Our results demonstrate that if displacement boundary conditions can be constrained and the transition from rigid to nonrigid deformation defines a narrow rift zone during hyper‐extension then relatively simple kinematic rules can be used to invert present‐day bulk crustal structure for paleocrustal thickness, bulk lateral strain and aspects of upper crustal shear zone geometry from extensional systems with nonlinear rheology, structures dominated by simple shear in the upper crust, depth‐dependent extension and asymmetric crustal thinning.