PHANEROZOIC STRIKE-SLIP FAULTING IN THE CONTINENTAL INTERIOR PLATFORM OF THE UNITED STATES

The continental-interior platform of the USA is that part of North America's craton where a veneer of Phanerozoic strata covers Precambrian basement. N- to NE-trending and W- to NW-trending fault zones, many of which initiated during Proterozoic/Cambrian rifting, break the crust of the platform into roughly rectilinear blocks. These fault zones reactivated during the Phanerozoic, most notably in the late Paleozoic Ancestral Rockies event (which not only affected regions west of the Rocky Mountain front, but also the entire Midcontinent), and the Mesozoic/Cenozoic Laramide orogeny. Some of the zones remain active today. Dip-slip reactivation can be recognized in cross section by offset stratigraphic horizons and monoclinal fault-propagation folds, but strike-slip displacements are hard to document because of poor exposure. Though offset paleochannels, horizontal slip lineations, and strain at map-view fault bends locally confirm strike-slip, most reports of strike-slip for interior-platform faults are based on occurrence of map-view belts of en echelon faults and anticlines. These belts overlie basement-penetrating master faults, which typically splay upwards to define flower structure. Both strike-slip and dip-slip components occur in the same fault zone, so some belts of en echelon structures occur on the flanks of monoclinal folds. Thus, strike-slip displacement represents the lateral component of oblique fault reactivation. Most dip-slip and strike-slip components are the same order of magnitude (10s of m to a few km). The largest strike-slip components are only 10s of km. Effectively, faults with strike-slip components act as transfers accommodating regional jostling of rectilinear crustal blocks. In this context, the sense of strike slip on an individual fault depends on block geometry, not on the trajectory of regional principal stresses during movement. Strike-slip faulting in the North American interior differs from that of south/central Eurasia, possibly because of a contrast in lithosphere strength. Weak Eurasia strained significantly during the Alpine-Himalayan collision, forcing crustal blocks to undergo lateral escape. The strong North American craton strained relatively little during collisional/convergent orogeny, so crustal blocks underwent only small displacements.