Phanerozoic strike-slip faulting in the continental interior platform of the United States: examples from the Laramide Orogen, Midcontinent, and Ancestral Rocky Mountains

Abstract

The continental interior platform of the United States is that part of the North American craton where a thin veneer of Phanerozoic strata covers Precambrian crystalline basement. N-to NE-trending and W-to NW-trending fault zones, formed initially by Proterozoic/Cambrian rifting, break the crust of the platform into rectilinear blocks. These zones were reactivated during the Phanerozoic, most notably in the late Palaeozoic Ancestral Rockies event and the Mesozoic-Cenozoic Laramide orogeny -some remain active today. Dip-slip reactivation can be readily recognized in cross section by offset stratigraphic horizons and monoclinal faultpropagation folds. Strike-slip displacement is hard to document because of poor exposure. Though offset palaeochannels, horizontal slip lineations, and strain at fault bends locally demonstrate strike-slip offset, most reports of strike-slip movements for interior-platform faults are based on occurrence of map-view belts of en echelon faults and anticlines. Each belt overlies a basement-penetrating master fault, which typically splays upwards into a flower structure. In general, both strike-slip and dip-slip components of displacement 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; dip-slip and strikeslip components are the same order of magnitude (tens of metres to tens of kilometres). Effectively, faults with strike-slip components of displacement act as transfers accommodating jostling of rectilinear crustal blocks. In this context, the sense of slip on an individual strike-slip fault depends on block geometry, not necessarily on the trajectory of regional ~r~. Strike-slip faulting in the North American interior differs markedly from that of southern and central Eurasia, possibly because of a contrast in lithosphere strength. Weak Eurasia strained significantly during the Alpine-Himalayan collision, forcing crustal blocks to undergo significant lateral escape. The strong North American craton strained relatively little during collisional-convergent orogeny, so crustal blocks underwent relatively small displacements.

Tectonic evolution of the Cape and Karoo basins of South Africa

“…Furthermore, the complex association of structures, which includes thrust-related ramps and flats, backthrusts, hangingwall folds, and transtensional faults that post-date folding (de Beer, 1990;Shone and Booth, 1993) may have resulted from the jostling and rotation of blocks as a result of regional strain (cf. Marshak et al, 2003). These two structural complexes are contemporaneous (de Beer, 1990) mirror images.…”

“…Macroscopic criteria which allow us to recognize strike-slip displacement include displacement markers, en echelon faults and folds, flower structures, the vertical components of displacement at fault bends, and the distribution of extensional and compressional tracts that served to accommodate translation (Marshak et al, 2003). Displacement markers are the least reliable because it is difficult to prove that they were once contiguous.…”

Tectonics of strike-slip restraining and releasing bends

“…Strike-slip faults form in continental and oceanic transform plate boundaries; in intraplate settings as a continental interior response to a plate collision; and can occur as transfer zones connecting normal faults in rift systems and thrust faults in fold-thrust belts Sylvester 1988;Yeats et al 1997;Marshak et al 2003). Strike-slip faults also are common in obliquely convergent subduction settings where interplate strain is partitioned into arc-parallel strike-slip zones within the fore-arc, arc or back-arc region (Beck 1983;Jarrard 1986;Sieh & Natawidjaja 2000).…”

Paleozoic rocks structure versus Cenozoic cuesta relief along the Baltic Shield–East European Platform transect

“…whether it was formed in a compressional or ten sional stress field. The pos si ble strike-slip com po nent, which is rarely seen in the plat form cover at this kind of struc tures (Marshak et al, 2003), has been so far en tirely ne glected at the LZD. In this sense, the LZD or groups of LZD that ex press fea tures of a regional struc tural hinge-line must be more care fully ana lysed.…”