COLORADO’S ROLE IN THE DEVELOPMENT OF MODERN 4D THRUST TECTONICS

Colorado’s excellent Rocky Mountain exposures and diverse geoscience communities have helped advance thrust tectonics from 2D to 4D (3D space + time). While Laramide geometric complexities in Colorado are made even more puzzling by post-Laramide overprinting, reactivation, and magmatism, even dead-ends, like vertical tectonics, forced valuable revisions of thrust tectonic “rules” and added a new folding mechanism.

Outcrop and subsurface data from the Canadian thin-skinned thrust belt allowed the flowering of cross-section restoration techniques, adding time to our 2D profiles of the earth. The large shortening in thin-skinned belts made the direction perpendicular to slip of lesser importance in these orogens. But this is not the case for the highly 3D Laramide geometries of the Rockies, so the vertical school of tectonics invoked near-vertical slip on high-angle faults which defined discrete basement blocks. The advantage of their 4D models is that since all vertical planes contain a vertical line, basement blocks can be easily restored using a single vertical slipline without opening up gaps or causing overlaps.

Unfortunately, vertical tectonic models can’t restore the fold-shortened overlying strata, and are inconsistent with field, well and seismic data indicating large thrust faults in basement. This has forced a new set of 4D structural models invoking a combination of thrust and strike-slip faults that share low-angle sliplines. The problem of synchronous basement faulting and stratal folding led to the concept of trishear, a fault-propagation-fold mechanism which was then successfully applied to thin-skinned thrust belts.

New seismic data from the NSF/EarthScope Bighorn Project show that the Moho is not cut by the Bighorn master fault and thus requires models invoking lower-crustal detachment at ~30 km depth. Complex interactions with Precambrian weaknesses, discrete zones of internal transpression, and a component of gravitational spreading are responsible for many complex 3D Laramide geometries without necessitating changes in the direction of regional shortening. Future Laramide contributions from Colorado will probably explain the origins of the ever enigmatic Colorado Mineral Belt as well as how Laramide thrust geometries impact post-Laramide extension and magmatism.