AN ANALYSIS OF FAULT SYSTEMS IN AN EXPERIMENTAL STRATIGRAPHIC BASIN, WITH A COMPARISON TO THE MID-CONTINENT RIFT SYSTEM

The Experimental Stratigraphy Basin ("Jurassic Tank") at the University of Minnesota St. Anthony Falls Laboratory models sedimentation in a subsiding basin. In this tank, sediment supply, base level, and subsidence are independently controlled. Preliminary experiments were performed in a prototype basin 2 meters long, 1.5 m wide, and 1.5 m deep, and with a floor that consists of ten independent hexagonal cells forming a honeycomb pattern. Subsidence in each cell is controlled to within ~0.12 mm. After each run, the basin is drained and longitudinal cross-sections are cut at 2.5 mm intervals. Acetate peels are made of the cross-sections; these are later converted to digital images.

We analyzed over 1000 faults in seventeen sections through the prototype basin. It is important to note that the basin is neither extended nor compressed during an experiment; these faults form solely as the result of subsidence. Spatially, there are two populations of faults: one near the sediment input zone, in the region of minimal subsidence, and the other in the region of maximum subsidence. These two populations are separated by a fault-free zone. The faults in the region of minimal subsidence are steeper, averaging greater than 60 degrees dip at the surface, and are more likely to dip toward the edge of the basin than away from it. Faults in the region of maximum subsidence tend to be shallower, with dips averaging about 45 degrees, and more commonly dip into the basin than toward the edges. Notably, some of these faults have reverse displacement. In addition, in each set of faults the average dip for "left-dipping" faults is significantly different than for "right-dipping" faults.

We have compared the geometry of fault systems in the experimental stratigraphy basin to those in the Mid-Continent Rift System. Late-stage reverse motion on the rift-bounding faults, including the Douglas, Lake Owen, and Hastings-Atkins Lake Faults, has generally been interpreted as compressional. The existence of reverse faults in the experimental stratigraphy basin suggests the possibility that reverse faulting in the Mid-Continent Rift System could have been caused, instead, by subsidence. This could correspond to the thermal subsidence that is known to have occurred as a result of the cessation of rifting.