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The Case for a Continuous Heart Mountain Allochthon: Abstract
Two fundamentally different models describing the geometry, kinematics, and mechanism of emplacement of the allochthon of the Heart Mountain detachment have been advocated. The traditional "tectonic denudation model" comprises catastrophic gravity sliding of numerous detached blocks, with widespread subaerial exposure of the detachment by tectonic denudation. In this view, catastrophic volcanism covered the allochthonous blocks and the denuded detachment immediately after implacement of the slide blocks. The alternative "continuous allochthon model" comprises non-catastrophic gravity spreading of a continuous allochthon, without tectonic denudation of the detachment. In this view, volcanism was noncatastrophic and coeval with, rather than subsequent to faulting; and volcanic rocks constitute a large volume of the allochthon. Recent field studies indicate that the continuous allochthon model is geometrically and kinematically in accord with field relationships, analog models, geometric models, and relationships characterizing other detachment complexes; whereas the tectonic denudation model is not similarly substantiated. In particular, the tectonic denudation model is disproved by field relationships which demonstrate that volcanic rocks once thought to postdate faulting are instead tectonically emplaced. This is demonstrated at specific localities where volcanic rocks have previously been interpreted as in depositional contact with the detachment.
Theoretical studies indicate that emplacement of the allochthon envisioned in the continuous allochthon model is mechanically plausible, whereas the catastrophic emplacement of detached blocks envisioned in the tectonic denudation model is mechanically implausible. Despite a broad range of proposed mechanisms [volcanic explosions, earthquake vibrations, acoustic fluidization, abnormal fluid pressure ("beer can," "hovercraft," "air-cushion," and "phreatomagmatic-hydraulic" versions), and "pneumatic-hydraulic plastic-wedge" models], no plausible mechanism has been found to allow detachment of numerous large slide blocks (areas of up to 10s of km2) and long-distance displacement of such blocks (up to 30 miles (50 km)), as is required by the tectonic denudation model. In contrast, the mechanics of gravitational spreading of continuous allochthon, both compressional and extensional, is well understood both theoretically and empirically. In the context of the continuous allochthon model, gravitational spreading explains the emplacement of the Heart Mountain allochthon.
Acknowledgments and Associated Footnotes
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Copyright © 2005 by the Wyoming Geological Association