Models for intraplate strain.

Context: This is a document for students engaged in an undergraduate research project at the University of Nebraska at Omaha looking at the faults and veins of Toadstool Geological Park in NW Nebraska.

Most geoscientists attribute intraplate seismicity, faulting and deformation to the reactivation of a basement weakness. Basement rocks are the metamorphic and plutonic rocks that underlying the younger sedimentary cover strata, and which compose most of the continental crust. Basement weaknesses are often associated with old rift zones, and sometimes with buried suture zones, now often buried out of view. The New Madrid seismic zone is the primary example, but the Charleston, South Carolina area is another. In Nebraska, the primary example is the Humboldt fault zone, in the southeastern corner of the state. Faulting seen in Paleozoic strata is associated with the southern, subsurface extension of the much older, Precambrian Keweenawan rift. The basic idea is that these old structures, especially old faults, are distinctly weak surfaces that reactivate when forces within the plate surpass their strength. When they reactivate, any younger strata that were deposited since the last episode of movement are deformed. Because of this repeated history of movement these are also known as long-lived lineaments.

The architecture for the fault geometry of basement reactivation can be thought of as thick-skinned. Surface faults continue to depth in some manner, and retain a relatively steep dip, and deformation is concentrated along the underlying basement lineament. This can be contrasted with a thin-skinned architecture, where the surface faults and deformation may form above a subhorizontal or shallowly dipping surface, and the underlying rocks may not be affected. A vigorous debate exists in the literature regarding the occurrence of these two different types of architecture.

Simple diagram illustrating the difference for a thin and thick skinned geometry of normal faulting. In the thin-skinned geometry, somewhere the normal fault does cut down into the basement.

Very early in the history of geology it was recognized that vertical movements occur. Sunken temples and marine limestones on the top of mountains speak for themselves. Thick-skinned types of geometries, where uplift was associated with steep faults, made sense. Continental drift and plate tectonics focused on the idea that horizontal movements are primary and the vertical movements are secondary. As part of the plate tectonic revolution ideas on thin-skinned geometries developed. With more detailed mapping, drilling and geophysical exploration it became clear that fold-thrust belts at the margins are thin-skinned. Shallowly sloping detachments exist such that above the rocks are highly deformed by faults and folds, but below the rocks are undeformed or deformed in a significantly different way. The detachment can be traced to deeper and deeper depths in one direction, and the overlying material can be thought of a wedge of deforming material sliding over an undeformed 'floor'. Large scale regional dips of the detachments could be only a few degrees. For the leading edge of fold-thrust belts the detachments often occur in shales or evaporites, very weak layers. It was soon realized that many normal fault complexes had a similar geometry, although the associated detachment was often deeper, often at the brittle-ductile transition in the earth's crust. The critical insight is that the earth is mechanically laminated, and that different geometries and amounts of deformation occur at different levels. Delamination, where some layers separate from others and behave differently is a crucial concept here.

In early definitions of plate tectonics plate interiors were considered rigid. Intraplate seismicity and tectonism suggests that that criteria needs to be at least locally relaxed, especially for continental crust (less so for the stronger oceanic crust). However, many still assume that vertical movements dominate plate interiors. Related to this has been the discovery that strain from plate margins can penetrate much deeper into continental plates than thought. Craddock and others used microscopic deformation features within the mineral calcite to document low levels of wide spread, pervasive strain associated with the time the Appalachian mountains formed hundreds of kilometers into the continental interior beyond the visible front of folds and thrusts. The fact that the fold-thrust belt is thin skinned, and that the pattern of strain seen in the calcite is regionally pervasive may suggest that this more interior deformation is also thin-skinned in some manner. One broad objective of the Toadstool project is to explore the possibility of a thin-skinned style of intra-plate tectonism.

It is a challenge to detect such a phenomena of widely-distributed but low levels of strain. In Nebraska, there are two tantalizing suggestions that strain may be more pervasive. First, small earthquakes do not cluster on basement weakness zones. Instead, they are dispersed in a broad band across the top of the state and down into the southeast. Second, small scale faulting is know from three widely separated localities; Toadstool Geologic Park in northwest Nebraska , along the Niobrara River in northeast Nebraska, and the from Harlan reservoir in south central Nebraska. What all these areas have in common is that exposures are quite good. Other areas with faulting are known (e.g. at Agate Springs). The question arises, what additional faulting would we see if more areas were similarly exposed and not mantled by young Quaternary deposits and other forms of cover? Interestingly, there are some strong similarities in the character of faults between the three widely separated localities mentioned above. In addition, similar faulting to that seen at Toadstool has been documented in the Big Badlands of South Dakota, and elsewhere.

In this endeavor it is useful to make a list of some models for intraplate strain (bulleted items below), and one of the project objectives is to test these for the Toadstool area. Some of the list below can be readily ruled out.

Some of these can be quickly ruled out for Toadstool (e.g. impact related deformation) Distinguishing between many of the others is possible.