Discussion: In the analog models, the edges of the rubber sheets represent the rheological transition zones at the margins of the brittle-ductile regions. These regions are determined by anomalies in the geothermal gradient caused by the intrusion of basaltic dikes (fig.4). The width of these zones are controlled by the location and frequency of the rising dikes. Localized clustering of dike placement results in multiple zones of ductility, while the distribution of dikes controls the concentration of heat in the region, which in turn affects the ductility of crustal materials. At the same time, thinning of the crust (fig.5) results in reduction of pressure, which allows partial melting of some components in the crust, also increasing ductile behavior. Although the materials used in the analog models do not reflect the ductility gradient seen in nature (fig.6), the location of border faults and formation of basins do mimic those found in natural systems. Conclusions: Each of our experiments clearly highlights that border faults form directly above (following fault angles) the rheological transition zones. The experiments also confirm that grabens are confined to the area directly above the subsurface thermal anomaly. The final experiment exhibited that independent basins form over each individual ductile zone. This conclusion refutes the popular misunderstanding that basin down-drop occurs randomly across extensional stress regions. This establishes the use of surface topography as a viable indicator of magmatic activity/history at depth. References: 1. Lister, L,. And Davis, G, The origin of metamorphic core complexes and detachment faults formed during tertiary continental extension in the northern Colorado River region U.S.A., Journal of Structural Geology, vol. 11 pp Weinberg, R,. And Regenauer-Lieb, K, Mantle detachment faults and the breakup of cold continental lithosphere, Geology, vol. 35, pp Analog Modeling of Tectonic Rifting: Extensional Structures Based on Crustal Rheological Boundaries Trevor Ellis, Allen Hooper, and Scott Melby Missouri University of Science and Technology Department of Geological Sciences and Engineering Initial Observations: It is observable that in regions where normal faulting has occurred, such as in basin-and- range topography, grabens created by dropped blocks tend to be restricted in width, regardless of the distance between stress margins. Basalt dikes are often associated with these regions, indicating a causational relationship with the ductile zone. Hypothesis: When extensional stress is applied over large scale flat lying areas of sedimentary rocks, border faults will initially form at the edge of the brittle-ductile transition zone at depth. The location of these faults will spatially confine subsequent faulting and the resulting basin formations. Experiment Setup: The sandbox apparatus was adapted from earlier experiments. Sand scales remarkably well to behavioral properties of sedimentary rocks found in basins in the upper crust. Latex sheeting is used to mimic the ductile layer. On the experiment photos (figs. 1A-3B), the location of the rubber sheets are shown in yellow, and the incremental scale is in centimeters. Experiment 1 This was used as the control experiment. A single rubber sheet representing the “ductile zone” is 12 cm wide initially. Figure 1A shows the initial setup for Experiment 1. Figure 1B shows border faults forming at the edges of the “ductile” rubber sheet. Figure 1B Experiment 2 Experiment 2 used a narrower rubber sheet than Experiment 1. This narrower sheet was 4 cm wide. Figure 2B Figure 2A shows the initial setup of Experiment 2. Figure 2B shows the border faults that form at the edges of the “ductile” region. Figure 2A Experiment 3 Experiment 3 used two rubber sheets of about 4 cm in width. A metal strip was place between the sheets. Figure 3A Figure 3A shows the initial setup of Experiment 3. Figure 3B Figure 3B shows two sets of border faults that form at the edges of the “ductile” region. Figure 1A Figure 6 Figure 4 Three dimensional block diagram showing dikes in a magma-assisted extensional environment. Shows controlling features leading to rift formations. (Ref. 1) Figure 5 Thinning of the crust above an elevated transition zone. Analog models depict similar basin formation. (Ref. 2) Schematic showing typical ductility with depth. This gradient is not represented in analog models. (Ref. 1) Initial setup of Experiment 3 showing two ductile zones (orange rubber). Acknowledgements: Special thanks to Dr. John Hogan and Devon Rumbaugh for their valuable input.