2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 16
Presentation Time: 1:30 PM-5:30 PM


MAHER Jr, Harmon D., BERRIGAN, John, DERVIN, Renee, FIRKINS, Jon, HART, Bryce, MARSHALL, Lucas, MOAK, William, REYNA, Layla and SCHIELE, Philip, Geography and Geology, Univ of Nebraska - Omaha, Omaha, NE 68182-0199, harmon_maher@mail.unomaha.edu

A suite of veins and normal faults are well exposed in the badlands of Toadstool Geologic Park, and are being mapped into a GIS database as an undergraduate research exercise. Many gradations between smaller tensile veins and larger faults occur. At horizontal lengths > ca. 10 m slip is often evident along vein walls or along calcite median zones. Fault surfaces commonly change laterally into vein tips, steepening as they do so. Other fault tips transition into en echelon vein arrays. Fault zones are segmented into individual fault surfaces that link via tip curls. The same geometry is observed with the veins. Most faults have a zoned vein in their core. Both faults and veins are associated with wall rock alteration. These relationships, plus common orientations, indicate the faulting and veining are part of one deformation event. Similar veins and faults occur 30 miles to the W and are known from the Big Badlands (S.D.), indicating a more regional extent. Assuming these geometries represent brittle structures arrested at different stages of development, the following model for vein-to-fault evolution is proposed. Failure starts as a vertical tensile feature with local fluid migration depositing silica gels. Once the tensile fracture achieves a critical length, normal slip nucleates on the vein surface or along a calcite interior, producing striae or slickensides, respectively. Despite a sub-vertical orientation the vein surface may be weak enough to resolve sufficient shear stress to slip in the extensional regime. Continued vertical shear propagation at the fracture margin is in a mixed mode along a dipping surface, while horizontally is along a sub-vertical tensile surface. Brecciation, veining, and wall rock alteration with continued slip causes fault zone widening. What determines the threshold for shear to initiate on the veins is presently uncertain. The vein-to-fault transition eventually causes local strain hardening (by dilation and fluid pressure decrease, and/or mineralization), eventual strain migration, and a more distributed strain pattern.