CRUSTAL MECHANICAL HETEROGENEITY CONTROLS RIFT BASIN ASYMMETRY DURING EARLY CONTINENTAL EXTENSION
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.