Laterally Heterogeneous Crustal Mechanical Properties Control Early Rift Asymmetry

Continental rifts often localize in crustal blocks with contrasting rock strengths, defined by laterally juxtaposed metamorphic blocks, volcanic sequences, and sedimentary deposits. However, the degree of influence of mechanically heterogeneous pre-rift basement on the evolution and structure of juvenile rift basins remains in question. We use ASPECT, a finite element geodynamic modeling software, to test the influence of bulk mechanical properties and spatial heterogeneities in rock strength on rift basin geometries during the early stages of continental extension. We use a viscoplastic rheology and a Drucker-Prager yield criterion to represent the upper 20 km of the crust. Geodynamic model results suggest that upper crustal strength heterogeneities strongly influence cross-sectional profile geometries of rift basins: 1.) Steep basin-bounding normal faults with large displacement magnitudes tend to localize in strong rocks, whereas, given the same amount of extension, shallow-dipping basin-bounding faults with lesser slip magnitudes develop in softer rocks; 2.) Shear strain magnitudes and resulting basin geometries in these models are strongly controlled by the thickness and internal angle of friction of rock units. Further, we investigate the structural evolution of the San Luis Basin (SLB), northern Rio Grande Rift, where the pre-rift basement is composed of 5 km-thick volcanic sequences with underlying ~5 km-thick granitic batholith flanked to the east by Precambrian gneissic terranes. Our modelling results successfully reproduced the asymmetric basin geometry of the SLB with the main west-dipping boundary fault on the east, an intra-basin horst block, and a flexural margin to the west, similar to subsurface geophysical images of the SLB. In addition, our results suggest that temporal changes in the strength profile of the upper crust during rifting through fault damage and emplacement of thick early-phase syn-rift sedimentary and volcanic sequences would influence later phases of rifting, and may explain the lateral migration of strain observed in many multiphase rift basins.