New Mexico Geological Society Annual Spring Meeting — Abstracts

Recent advances in deep-seismic profiling provide compelling evidence that listric faults occur to depths of 10-12 krn beneath the Albuquerque Basin. The paper considers dynamic deformation and stress analysis of symmetric listric faults in the upper crust, one of which then soles in the upper crust, under an initial compressive pre-stress, followed by extension at constant rates of 100 m/Ma, and uplift at 50 m/Ma. An initial listric fault system creates younging listric faults which steepen inward from the basal fault and break to the soling detachment within 8-14 Ma in the graben, a preferred asymmetric temporal sequence of normal-and-listric faulting younging from west to east across the graben, and asymmetric sub-basins at the surface. A footwall region beneath a westward dipping listric fault and above a eastward dipping soling listric fault is called a "listric tip". This tip causes asymmetric responses to deformation since it is both decoupled by two listric faults and semi-rigid. With extension of 3 km, faulting occurs at horizontal strains <5-10%. Asymmetric sub-basins 23 krn wide form to depths of 750-250 (west-east) meters in 12-20 Ma and show sedimentation rates of 10-40 m/Ma. Without basal uplift, this model essentially undergoes only negative relief, unlike either geodetic or topographic uplift data regarding the Rio Grande Rift.

Sinusoidal basal uplift during creep, produces positive relief, asymmetric sub-basins, and an intrabasin high. Secondary normal faulting reflects early (11-14 Ma) tensile stresses whose axes are -horizontal and fault dips are -vertical, younging from west to east, bpt ignoring the easterm listric fault region. Continued uplift generates a shear stress field whose shear faults occur from 14-17 Ma and dip ± 60° beneath the surface high. First motion microearthquake studies near the Socorro magma body are consistent with predicted fault geometries. Asymmetric uplift shapes, with centroids beneath graben bounding normal faults, indicate the listric tip delays graben faulting from 8-13 Ma (west) to 14-17 Ma (east). Late stage shear stress magnitudes are 20% greater on the west relative (14-17 Ma) to the east (17-20 Ma) without the listric tip's influence.