FORMATION OF BASEMENT-INVOLVED FORELAND ARCHES: NEW RESULTS FROM THE EarthScope BIGHORN PROJECT
Minor faults predominantly show sub-horizontal, ENE Laramide shortening and compression. In sedimentary strata near the arch culmination, late Laramide thrusts show radial slip away from the arch crest, suggesting an additional component of gravitational spreading. This is supported by the dominance of strike-slip faulting in the crystalline rocks of the arch core, which allowed both ENE shortening and axis-parallel extension.
The BASE active source experiment consisted of 20 shots recorded by ~1800 Texan seismometers deployed at 100-500 m spacing along an E-W and a N-S profile. The BASE passive-source experiments, with short-period (850 instruments), intermediate period (172 instruments) and broadband (38 instruments) arrays, also recorded these shots and other more distant seismic sources.
2D tomographic P-wave velocity models from the active source experiment give mantle velocities (>7.8 km/s) at ~45-50 km depth below surface elevations. This is consistent with a Moho defined using PmP and Pn phase arrivals. Low crustal velocities show shallow basin structures on either side of the arch. Deeper low-velocity zones within the upper 20 km of the crust locally coincide with known Laramide fault zones, suggesting velocity reduction due to cataclasis.
The discordance between the upper crustal arch geometry and the more gently undulating Moho indicates that a listric master fault caused thickening of the upper crust and did not involve the Moho. This is consistent with initial 2D structural restorations using trishear and fault-parallel flow on a listric fault that merges into a detachment at 30 km depth. This suggests that the Laramide arches of the Rockies formed by lower crustal detachment rooting under the Sevier orogenic belt to the west.