CRUSTAL MECHANICAL HETEROGENEITY CONTROLS RIFT BASIN ASYMMETRY DURING EARLY CONTINENTAL EXTENSION

Contrasting rock strengths in the Earth’s upper crust is known to influence the plan-view structure of continental rift basins. However, long-standing questions remain on the influence of contrasting pre-rift rock strength on the cross-sectional rift basin asymmetry and rift-bounding border fault geometry in early-stage continental rifts. Here, we use the finite element, geodynamic modeling software, ASPECT, with viscoplastic rheology and a Drucker-Prager yield criterion to test the contributions of key material properties and velocity boundary conditions on the shapes of rift basin profiles. The results are compared to the San Luis Basin (SLB), Rio Grande Rift, as a natural example.

The SLB is a half-graben (asymmetric in profile view) bounded to the east by the west-dipping Sangre de Cristo border fault. The basin, which hosts a 4 - 9 km-thick sedimentary-dominated syn-rift deposit, sits on a basement that is composed of Precambrian igneous and metamorphic rocks to the east, with overlying Late Paleozoic-Early Cenozoic (i.e., pre-rift faulting) sedimentary and volcanic sequences thickening to the west. We test the possible influence these pre-rift rock units with contrasting strengths have on the SLB geometry.

Model results suggest that in an extending multi-compositional upper crust, the border fault with the greatest amount of cumulative slip occurs in thicker rock units with greater internal angles of friction, creating an asymmetric basin profile. To reproduce the SLB cross-sectional geometry, Precambrian basement must be stronger than adjacent volcaniclastic domain to the west. Also, inclusion of a deep crustal batholith in the model reproduced the buried intra-basinal Alamosa Horst. Further, these results allude to mechanisms for lateral migration of border faulting that may explain observations in some natural multiphase rift basins where later phases of extension record strain migration following temporal re-distribution of upper-crustal rock strength due to significant earlier rift-phase volcanism and fault damage. Overall, we suggest that pre-rift mechanical heterogeneity and syn-rift thermo-mechanical modification of the upper-crust exerts significant influence on the temporal evolution of rift asymmetry during early-stage continental extension.