2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 179-8
Presentation Time: 10:25 AM


PAVLIS, Terry, Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79902 and AMATO, Jeffrey, Geological Sciences, New Mexico State University, P.O. Box 30001/MSC 3AB, Las Cruces, NM 88003, tlpavlis@utep.edu

The mixing process in mélange zones has been debated for decades but kinematic modeling experiments suggest that a simple mechanism for this process is simultaneous motion on multiple faults in the subduction channel. The experiments arise from theoretical studies of out-of-sequence thrust systems described in Pavlis (2013, J. Structural Geology). We use Move software to analyze structural geometries resulting from three active faults along the channel: a floor, mid-channel, and roof thrust. Structurally higher faults are warped by motion on structurally lower faults and we assume that new faults break through warped fault surfaces to form horse blocks; the building blocks for what Pavlis (2013) called an upper plate duplex. Experiments demonstrate that variations in even a few parameters can produce a bewildering array of possible geometries, consistent with the complexity of mélange zones. Most importantly, however, modeling of this type can lead to testable hypotheses for geometry of rock bodies and age distribution constrained by detrital zircon analyses in conjunction with traditional fossil dating. Examples of some of these complex geometries can be recognized in the Chugach mélange, southern Alaska, but our reconnaissance work is too preliminary to fully evaluate hypotheses raised by the models. Important observation from models include: 1) young rocks carried into the subduction channel can move into the hanging-wall, above the subduction channel, or older rocks can be plucked from the hanging-wall, depending on geometry producing mixed assemblages of melanges; and 2) fault juxtapositions include both younger on older and older on younger, despite a fully thrust-related system, partially explaining the apparent lack of age stacking in most mélanges. Rock bodies formed by this process are a series of lensoidal horses, some with distinctive fault cut-offs. It seems likely that a process of this type, at a range of scales, is responsible for the characteristic lensoidal phacoid fabric of mélanges. The common occurrence of young-on-older faults in these models suggests great caution should be used in inferring extensional structure from stratigraphic juxtapositions in mélanges.