FAULT PROPAGATION FOLD MODEL PROPOSED TO EXPLAIN ASYMMETRY OF THE BARABOO SYNCLINE

The Baraboo quartzite is one of several super-mature sandstones deposited at ca. 1782–1712 Ma. along the southern margin of the North American continent. In Wisconsin, these were folded and metamorphosed at ca. 1650-1630 Ma during the Mazatzal orogeny. In the Baraboo area, the sandstone was deposited on a 1749 +/- 12 Ma rhyolite unit. Most interpretations fold both units together into an east-west trending, south-verging, doubly plunging synclinal depression with a near-vertical north limb and a south limb shallowly dipping to the north. This research challenges that interpretation with data from detailed mapping and microstructural analysis of samples collected along the contact between these two units.

This research mapped the contact between the rhyolite and quartzite along the northern limb where the contact is exposed best in the northeast of the structure. Along this contact, both units have strong shear fabrics that parallel the contact. This fabric may be a result of movement along a fault contact between the two units, or due to slip between units which is expected during flexural-slip folding. Although the latter is common when two rheologically different units are folded together, the former might better explain the asymmetry and over-steepened north limb. Determining which mechanism is responsible for the shear fabric is possible by studying shear sense indicators. If the shear fabric is a result of slip during folding, shear indicators would show the quartzite moving up relative to the rhyolite, whereas a reverse fault contact would show the rhyolite moving up relative to the quartzite.

Ten samples were studied in thin section, two from the quartzite and eight from the rhyolite. S-C fabric and asymmetric porphyroclasts found in vertically oriented thin sections viewed looking east, from both rock units nearly always show right–lateral shear, supporting the interpretation that the rhyolite moved up relative to the quartzite. Rare evidence for left-lateral shear is likely due to flattening during faulting and folding. This evidence supports the interpretation that the south-vergence and over-steepened north limb in the syncline seen at Baraboo is the result of north-side up reverse faulting leading to fault-propagation folding of the Baraboo Quartzite.