By Caroline Fernandez, Kathryn Boardman, and Dr. John Hogan Advanced Structural Geology Course

1. By Caroline Fernandez, Kathryn Boardman, and Dr. John Hogan Advanced Structural Geology Course Department of Geological Sciences and Engineering Missouri University of Science and Technology Introduction: During continental collision (i.e. compression), reactivation of preexisting faults may influence the development of geologic structures that lead to the formation of mountain chains. For example, during crustal rifting normal faults develop in the basement as it is extended. During continental collision (i.e., compression), we predict that these normal faults will reactivate as reverse faults and change the shape and evolution of the accretional wedge. The structures that develop commonly form important hydrocarbon traps. Thus their origin is of considerable interest to petroleum companies. Goals: The goal of this project is to use analog models to help us better understand the role of inherited structures, especially normal faulting, of basement rock in the formation of orogenic wedges. We hope that our models will correlate with real world regions so that we can perhaps contribute to the better understanding of their geologic history. Experimental Design: The compression in the analog models is achieved by the incremental movement of one of the walls of the box by a hand crank. The results are viewed from plexiglass or glass windows on either side of the sandbox. Each sandbox will have two distinct colored sand layers so that the evolution of the wedge can be tracked. The shortening will be conducted in 2 centimeter intervals. At each interval the following data will be collected; top and side view pictures, fault characteristics (dip, length, sequence), and maximum wedge thickness The first model is a simple compressional sandbox to represent continental collision where the basement rock is detached, and therefore does not affect the accretionary wedge. The other models incorporate a rigid board section at the base of the sand which will have cuts at 60 degree angles to simulate preexisting normal faults in the basement rock. Proposed Modifications: Based on the results from model 2, we feel that certain changes to the experimental design will improve the chances of fault reactivation in the basement board. Additional models will incorporate one or more of the following proposed modifications to the sandbox model design;1) Lubricate the faults 2) Add an additional board section to move the pre-existing faults closer to the moving wall of the sandbox. 3) Decrease the thickness of the sand 4) Use low-angle faults , thrust faults, as the pre-existing faults in the basement. In addition to the above modifications, we will also have an analog model with “Horst” and “Graben” structures in the basement board . These structures are formed by normal faults that dip in opposite directions. The reactivation of these faults will produce pop-up structures that may influence the formation of the accretionary wedge. Preliminary Results: The Role of Preexisting Basement Structures During Continental Collision:Insights from Analog “Sandbox” Modeling Model 1: simple model without basement structures Model 2: model with three 60 degree normal faults (dipping in the same direction) in the basement